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Britannica Concise Encyclopedia:
biochemistry |
For more information on biochemistry, visit Britannica.com.
McGraw-Hill Science & Technology Encyclopedia:
Biochemistry |
The study of the substances and chemical processes which occur in living organisms. It includes the identification and quantitative determination of the substances, studies of their structure, determining how they are synthesized and degraded in organisms, and elucidating their role in the operation of the organism.
Substances studied in biochemistry include carbohydrates (including simple sugars and large polysaccharides), proteins (such as enzymes), ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), lipids, minerals, vitamins, and hormones. See also Carbohydrate; Enzyme; Protein.
Metabolism and energy production
Many of the chemical steps involved in the biological breakdown of sugars, lipids (fats), and amino acids are known. It is well established that living organisms capture the energy liberated from these reactions by forming a high-energy compound, adenosine triphosphate (ATP). In the absence of oxygen, some organisms and tissues derive ATP from an incomplete breakdown of glucose, degrading the sugar to an alcohol or an acid in the process. In the presence of oxygen, many organisms degrade glucose and other foodstuff to carbon dioxide and water, producing ATP in a process known as oxidative phosphorylation.
Structure and function studies
The relationship of the structure of enzymes to their catalytic activity is becoming increasingly clear. It is now possible to visualize atoms and groups of atoms in some enzymes by x-ray crystallography. Some enzyme-catalyzed processes can now be described in terms of the spatial arrangement of the groups on the enzyme surface and how these groups influence the reacting molecules to promote the reaction. It is also possible to explain how the catalytic activity of an enzyme may be increased or decreased by changes in the shape of the enzyme molecule. An important advance has been the development of an automated procedure for joining amino acids together into a predetermined sequence. This technology will permit the synthesis of slightly altered enzymes and will improve the understanding of the relationship between the structure and the function of enzymes. In addition, this procedure permits the synthesis of medically important polypeptides (short chains of amino acids) such as some hormones and antibiotics.
Molecular genetics
A subject of intensive investigation has been the explanation of genetics in molecular terms. It is now well established that genetic information is encoded in the sequence of nucleotides of DNA and that, with the exception of some viruses which utilize RNA, DNA is the ultimate repository of genetic information. The sequence of amino acids in a protein is programmed in DNA; this information is first transferred by copying the nucleotide sequence of DNA into that of messenger RNA, from which this sequence is translated into the specific sequence of amino acids of the protein.
The biochemical basis for a number of genetically inherited diseases, in which the cause has been traced to the production of a defective protein, has been determined. Sickle cell anemia is a striking example; it is well established that the change of a single amino acid in hemoglobin has resulted in a serious abnormality in the properties of the hemoglobin molecule.
Regulation
Increased understanding of the chemical events in biological processes has permitted the investigation of the regulation of these proceses. An important concept is the chemical feedback circuit: the product of a series of reactions can itself influence the rates of the reactions. For example, the reactions which lead to the production of ATP proceed vigorously when the supply of ATP within the cell is low, but they slow down markedly when ATP is plentiful. These observations can be explained, in part, by the fact that ATP molecules bind to some of the enzymes involved, changing the surface features of the enzymes sufficiently to decrease their effectiveness as catalysts. It is also possible to regulate these reactions by changing the amounts of the enzymes; the amount of an enzyme can be controlled by modulating the synthesis of its specific messenger RNA or by modulating the translation of the information of the RNA molecule into the enzyme molecule. Another level of regulation involves the interaction of cells and tissues in multicellular organisms. For instance, endocrine glands can sense certain tissue activities and appropriately secrete hormones which control these activities. The chemical events and substances involved in cellular and tissue “communication” have become subjects of much investigation.
Photosynthesis and nitrogen fixation
Two subjects of substantial interest are the processes of photosynthesis and nitrogen fixation. In photosynthesis, the chemical reactions whereby the gas carbon dioxide is converted into carbohydrate are understood, but the reactions whereby light energy is trapped and converted into the chemical energy necessary for the synthesis of carbohydrate are unclear. The process of nitrogen fixation involves the conversion of nitrogen gas into a chemical form which can be utilized for the synthesis of numerous biologically important substances; the chemical events of this process are not fully understood.
Oxford Companion to the Body:
biochemistry |
The science of biochemistry, nowadays regarded as one of the fundamental pillars upon which the study of medicine rests, is something of a newcomer. It has its origins almost equally in chemistry and physiology, and indeed what we would today call biochemistry was commonly referred to as physiological chemistry a hundred years ago. Looking further back we can trace early ideas about the make-up of living things to the birth of organic chemistry, the scope of which seems originally to have been a good deal wider than would be admitted today. In 1806 Berzelius referred to organic chemistry as ‘the part of physiology which describes the composition of living bodies, and the chemical processes which occur in them. In the early nineteenth century there was a good deal of debate as to whether the chemical substances found in living things were fundamentally different in character from the ‘inorganic’ constituents of inanimate matter, and the issue was only resolved (in favour of no difference) with the chemical synthesis of urea by Wöhler in 1828 and by subsequent syntheses of molecules hitherto only associated with living organisms. Thereafter organic chemistry became confined to the study of carbon compounds, and knowledge of the transformations undergone by such compounds in the course of metabolism was left to be re-born as biochemistry decades later. A major influence in that re-birth was the concept of catalysis and the realization that catalysts must play a vital part in living processes. Here the studies of Pasteur and his contemporaries in the mid nineteenth century played an indispensable part, and led to the broad unifying concept that the nature of life processes must be very similar in disparate organisms, including man, and that catalytic enzymes (the word literally means ‘in yeast’) are responsible for directing and controlling chemical transformations in the living cell.
Given the acceptance of the concept of oxidation, and the demise of the phlogiston theory, thanks to the work of Lavoisier in the late 1700s, it was natural that the early study of metabolism should be preoccupied with understanding the processes of respiration, breakdown of sugars, and energy generation. There was also much interest in nutrition and the chemical processes underlying the digestion of food. The identification of enzymes as proteins arose naturally from these efforts and spawned the science of enzymology, which remains a major division of biochemistry to the present day. The question of how enzymes work engaged the attention of many of the finest biochemical brains in the 1990s and will continue to do so for the foreseeable future. Moreover, the day is not far away when enzymologists will astonish us all by creating more or less de novo enzymes endowed with hitherto-unknown catalytic properties. Already ‘catalytic antibodies’ have been described, that bind small molecules with exquisite specificity, producing chemical change, and as knowledge of fundamental mechanisms of catalysis emerges from the efforts of physical organic chemists, the practical applications of that knowledge will not be far behind.
It was with the arrival of the twentieth century that biochemistry came of age, so to speak, and made such a major impact on medicine that it was recognized as a formidable science indispensable to the understanding of the human body. Those were the days of vitamin and hormone research. The pioneering work of Sir Frederick Gowland Hopkins (1861-1947) and his colleagues, which led to the discovery of vitamins, had a lasting influence on generations of biochemists and underpinned the unravelling of intermediary metabolism. The isolation, identification, and eventual production of hormones in sufficient quantity for therapeutic use likewise illuminated some of the most perplexing medical problems, and transformed endocrinology into an important branch of clinical science.
Biochemistry has long boasted of its roots in exact physical sciences and has never been afraid to divert the attentions of practitioners of those sciences to the study of life. By that route some of the most spectacular advances of knowledge in the twentieth century have been achieved, perhaps none more so than the birth of the enfant terrible, molecular biology, which nowadays dominates the subject. Molecular biology, rooted in structural studies on proteins and nucleic acids, owes much to the contributions of far-sighted crystallographers and geneticists (aided and abetted by a cohort of physicists and even mathematicians) who built upon the bed-rock of biochemistry to produce a veritable revolution in biology that is still evolving apace. It is sometimes hard to imagine how abstract the concept of a gene was prior to the discovery of the structure of DNA by Watson and Crick in 1953, since nowadays the precise identification of genes and expectations of their manipulation (for good or ill) can be read about in newspapers intended for the man in the street. Biochemistry is no longer the academic tool of medical researchers but, having embraced its sister disciplines in the physical as well as biological sciences, has taken on new meaning as the huge promise of biotechnology looms before us.
Never has it been more evident how the pace of scientific discovery is driven by technical advances in experimentation, the invention of new techniques, and the application of ideas imported from cognate disciplines. The twin sciences of molecular and cell biology have adapted the foundations laid by the painstaking ‘bucket’ experiments of the early biochemists to illuminate the marvels and mysteries of molecules and cells in a fashion which can only be described as spectacular. Even philosophers and theologians can no longer ignore the prospects of bio-revolution introduced into our daily lives: genetically engineered foodstuffs; super-athletes; new approaches to treating infertility; eradication of diseases. How many more triumphs (or horrors) attributable to the application of biochemically-based technology await us? And how are we going to cope with them?
— M. J. Waring
Oxford Dictionary of Sports Science & Medicine:
biochemistry |
The study of the chemistry of living organisms.
Gale Encyclopedia of US History:
Biochemistry |
Biochemistry, the chemical investigation and explanation of biological processes. American biochemistry acquired its institutional base as a result of the medical reform movement during the Progressive Era and was characterized until World War II by its emphasis on applied research and close association with medicine. American biochemists have been involved in the testing of foods and drugs, the development of diagnostic tests and medical treatments, and the production of consumer goods ranging from synthetic fibers and biological detergents to vitamin supplements and the contraceptive pill. Since the 1970s, biochemists have been actively engaged in biotechnological enterprises.
Biochemistry's antecedents lie in nineteenth-century Europe, where the rise of organic chemistry and experimental physiology generated much investigation into the chemical constituents of living organisms and the chemical changes associated with physiological functions. The many American scientists who trained in European laboratories imported these practices into the United States, where research in animal chemistry, agricultural chemistry, medical chemistry, and physiological chemistry gained a firm foothold in agricultural research stations, hospitals, colleges, and universities.
In the early 1900s, investigators in Europe and America sought to unite the diverse fields dealing with the chemistry of life under name of "biochemistry" or "biological chemistry" (then the preferred term in the United States). Among the first journals expressing this aim was the Journal of Biological Chemistry, founded in the United States in 1905. The American Society of Biological Chemists was constituted in 1906. In the same decade, many American medical schools, newly under university control, began to teach biochemistry as part of a nationwide reorganization of preclinical education. By 1920, most American medical schools had established departments of biochemistry where research had a predominantly clinical orientation.
Early American biochemists led in the development of new analytical methods for determining chemicals in the body that were used to diagnose specific diseases and monitor physiological states. Otto Folin at Harvard, Donald D. Van Slyke at the Rockefeller Institute Hospital in New York, and Stanley Benedict at Cornell acquired international renown. The widespread use of techniques they developed led simultaneously to redefinitions and reclassifications of diseases in chemical terms.
A second prominent stream of American biochemistry, that of nutritional investigation, built on established strengths in agricultural research, especially at experimental stations in Connecticut and Wisconsin. Recognition of the importance for health of vitamins stimulated much American research into the distribution of vitamins in foods, their chemical properties, and their role in metabolism. Diseases identified as resulting from vitamin deficiencies in the diet, such as rickets, scurvy, and pellagra, were cured and prevented by specific dietary changes. Commercially produced vitamin preparations and vitamin-fortified foods were widely promoted among the public from the 1920s and became an important source of profit for the American food and pharmaceutical industries.
In the 1930s, some American centers began to develop biochemistry as a broad, fundamental biological science, after the model of leading German, English, and Scandinavian schools. A similar vision was promoted by Warren Weaver, who, as manager of its Natural Sciences Division, turned the Rockefeller Foundation into the major international funding body for basic research in bio-chemistry and biophysics. In this decade, the biochemistry department at Columbia University in New York, headed by Hans T. Clarke, became the largest and most influential American school of basic biochemical research.
Clarke gave place in his department to an exceptionally high number of biochemists who had escaped national socialist regimes in Europe and who brought their distinctive research styles with them. Among them was Rudolf Schoenheimer, who, at Columbia, was responsible for a milestone in twentieth-century biochemistry: he introduced the use of isotopes as labels that allow biochemists to follow in detail how, and at what rate, specific molecules undergo change in metabolic reactions. His re-search with David Rittenberg and Sarah Ratner not only heralded the use of what has since become an indispensable tool in the life sciences but showed that all cell constituents are in constant flux: molecules are continuously being broken down and rebuilt from the foods organismsingest.
During World War II, biochemists participated in the war effort in major ways. For example, American biochemists were involved in the large-scale production of penicillin, other antibacterial drugs, and blood fractionation products for use in transfusion. These war-related projects involved complex translations between basic and applied research, managed through close collaborations between scientists, government, and industry. The blood fractionation project, organized by Edwin Cohn of the physical chemistry department at Harvard, was one of the wartime successes that stimulated a massive expansion of public funding for basic biochemical research in postwar America.
One manifestation of this new focus was the foundation of institutes dedicated to basic biochemistry, the first being the Enzyme Institute at the University of Wisconsin, opened in 1950. By the 1960s, fundamental biochemical research was firmly entrenched institutionally, and American biochemists were making ever more inter-nationally renowned contributions to all areas of bio-chemistry. Indicative of this trend is the rapid increase in Nobel laureates among American biochemists.
Between 1901 and 1950, only three Nobel Prizes were awarded for American biochemical research: the 1946 chemistry prize awarded to James B. Sumner, John H. Northrop, and Wendell M. Stanley for work on enzymes and virus proteins; and two prizes in physiology or medicine (shared with others abroad), awarded to Edward A. Doisy in 1943 for work on vitamin K, and to Carl F. Cori and Gerty Radnitz Cori in 1947 for work on glycogen metabolism. (Gerty Cori was the third woman, and the first woman biochemist, to win a Nobel Prize.) In the second half of the twentieth century, by contrast, approximately forty Nobel Prizes were awarded for American research with a biochemical dimension, to some seventy American laureates.
In this later period, biochemistry became increasingly intertwined with molecular biology and cell biology, partly through the development of new chemical, physical, and morphological techniques used in all three fields and through much traffic of biochemists across the boundaries between them. For biochemistry, these new developments made it possible to locate particular biochemical reactions in specific structures of the cell. Moreover, its institutional strength and practical flexibility enabled bio-chemistry to withstand challenges to its status as a fundamental science of life when these were issued in the 1950s and 1960s by molecular biologists seeking autonomy for their own science. In practice, there has been continuous overlap, and in 1987 the American Society of Biological Chemists renamed itself the American Society for Biochemistry and Molecular Biology.
Bibliography
Apple, Rima D. Vitamania: Vitamins in American Culture. New Brunswick: Rutgers University Press, 1996.
Bud, Robert. The Uses of Life: A History of Biotechnology. New York: Cambridge University Press, 1993.
De Chadarevian, Soraya, and Harmke Kamminga, eds. Molecularizing Biology and Medicine: New Practices and Alliances, 1910s–1970s. Amsterdam: Harwood Academic Publishers, 1998.
Kohler, Robert E. From Medical Chemistry to Biochemistry: The Making of a Biomedical Discipline. Cambridge, U.K.: Cambridge University Press, 1982.
—Harmke Kamminga
Columbia Encyclopedia:
biochemistry |
Bibliography
See L. Stryer, Biochemistry (3d ed. 1988); C. K. Mathews and K. E. van Holde, Biochemistry (1990); G. Zubay, Biochemistry (3d ed. 1993).
Biology Q&A:
What is biochemistry? |
As a field of scientific study, chemistry
may be divided into various subgroups. One major subgroup is organic chemistry,
which refers to the study of carbon-based compounds, including carbohydrates
and hydrocarbons such as methane and butane. Within the field of organic
chemistry there is a discipline that focuses solely on the study of the organic
molecules that are important to living organisms; this branch is known as
biochemistry.
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Dictionary of Cultural Literacy: Science:
biochemistry |
The study of the structure and interactions of the complex organic molecules found in living systems.
Wiley Dictionary of Flavors:
Biochemistry |
Oxford Dictionary of Biochemistry:
biochemistry |
| biochemical oxygen demand, biochemical marker, biochemical lesion | |
| biochip, biocide, biocompatible |
Saunders Veterinary Dictionary:
biochemistry |
The chemistry of living organisms and of their chemical constituents and vital processes.
Mosby's Dental Dictionary:
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Wikipedia on Answers.com:
Biochemistry |
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This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2011) |
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes. By controlling information flow through biochemical signalling and the flow of chemical energy through metabolism, biochemical processes give rise to the incredible complexity of life.
Much of biochemistry deals with the structures and functions of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules —although increasingly processes rather than individual molecules are the main focus. Among the vast number of different biomolecules, many are complex and large molecules (called biopolymers), which are composed of similar repeating subunits (called monomers). Each class of polymeric biomolecule has a different set of subunit types.[1] For example, a protein is a polymer whose subunits are selected from a set of 20 or more amino acids. Biochemistry studies the chemical properties of important biological molecules, like proteins, and in particular the chemistry of enzyme-catalyzed reactions.
The biochemistry of cell metabolism and the endocrine system has been extensively described. Other areas of biochemistry include the genetic code (DNA, RNA), protein synthesis, cell membrane transport and signal transduction.
Over the last 40 years biochemistry has become so successful at explaining living processes that now almost all areas of the life sciences from botany to medicine are engaged in biochemical research. Today the main focus of pure biochemistry is in understanding how biological molecules give rise to the processes that occur within living cells, which in turn relates greatly to the study and understanding of whole organisms.
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It once was generally believed that life and its materials had some essential property or substance distinct from any found in non-living matter, and it was thought that only living beings could produce the molecules of life.[citation needed] Then, in 1828, Friedrich Wöhler published a paper on the synthesis of urea, proving that organic compounds can be created artificially.[2][3]
The dawn of biochemistry may have been the discovery of the first enzyme, diastase (today called amylase), in 1833 by Anselme Payen. Eduard Buchner contributed the first demonstration of a complex biochemical process outside of a cell in 1896: alcoholic fermentation in cell extracts of yeast. Although the term “biochemistry” seems to have been first used in 1882, it is generally accepted that the formal coinage of biochemistry occurred in 1903 by Carl Neuberg, a German chemist. Previous to this time, this area would have been referred to as physiological chemistry[citation needed]. Since then, biochemistry has advanced, especially since the mid-20th century, with the development of new techniques such as chromatography, X-ray diffraction, dual polarisation interferometry, NMR spectroscopy, radioisotopic labeling, electron microscopy, and molecular dynamics simulations. These techniques allowed for the discovery and detailed analysis of many molecules and metabolic pathways of the cell, such as glycolysis and the Krebs cycle (citric acid cycle).
Another significant historic event in biochemistry is the discovery of the gene and its role in the transfer of information in the cell. This part of biochemistry is often called molecular biology. In the 1950s, James D. Watson, Francis Crick, Rosalind Franklin, and Maurice Wilkins were instrumental in solving DNA structure and suggesting its relationship with genetic transfer of information. In 1958, George Beadle and Edward Tatum received the Nobel Prize for work in fungi showing that one gene produces one enzyme. In 1988, Colin Pitchfork was the first person convicted of murder with DNA evidence, which led to growth of forensic science. More recently, Andrew Z. Fire and Craig C. Mello received the 2006 Nobel Prize for discovering the role of RNA interference (RNAi), in the silencing of gene expression.
Around two dozen of the 94 naturally-occurring chemical elements are essential to various kinds of biological life. Most rare elements on Earth are not needed by life (exceptions being selenium and iodine), while a few common ones (aluminum and titanium) are not used. Most organisms share element needs, but there are a few differences between plants and animals. For example ocean algae use bromine but land plants and animals seem to need none. All animals require sodium, but some plants do not. Plants need boron and silicon, but animals may not (or may need ultra-small amounts).
Just six elements—carbon, hydrogen, nitrogen, oxygen, calcium, and phosphorus—make up almost 99% of the mass of a human body (see composition of the human body for a complete list). In addition to the six major elements that compose most of the human body, humans require smaller amounts of possibly 18 more.[4]
The four main classes of molecules in biochemistry are carbohydrates, lipids, proteins, and nucleic acids. Many biological molecules are polymers: in this terminology, monomers are relatively small micromolecules that are linked together to create large macromolecules, which are known as polymers. When monomers are linked together to synthesize a biological polymer, they undergo a process called dehydration synthesis.
Carbohydrates are made from monomers called monosaccharides. Some of these monosaccharides include glucose (C6H12O6), fructose (C6H12O6), and deoxyribose (C5H10O4). When two monosaccharides undergo dehydration synthesis, water is produced, as two hydrogen atoms and one oxygen atom are lost from the two monosaccharides' hydroxyl group.
Lipids are usually made from one molecule of glycerol combined with other molecules. In triglycerides, the main group of bulk lipids, there is one molecule of glycerol and three fatty acids. Fatty acids are considered the monomer in that case, and may be saturated (no double bonds in the carbon chain) or unsaturated (one or more double bonds in the carbon chain).
Lipids, especially phospholipids, are also used in various pharmaceutical products, either as co-solubilisers (e.g., in parenteral infusions) or else as drug carrier components (e.g., in a liposome or transfersome).
Proteins are very large molecules – macro-biopolymers – made from monomers called amino acids. There are 20 standard amino acids, each containing a carboxyl group, an amino group, and a side-chain (known as an "R" group). The "R" group is what makes each amino acid different, and the properties of the side-chains greatly influence the overall three-dimensional conformation of a protein. When amino acids combine, they form a special bond called a peptide bond through dehydration synthesis, and become a polypeptide, or protein.
In order to determine whether two proteins are related, or in other words to decide whether they are homologous or not, scientists use sequence-comparison methods. Methods like Sequence Alignments and Structural Alignments are powerful tools that help scientists identify homologies between related molecules.
The relevance of finding homologies among proteins goes beyond forming an evolutionary pattern of protein families. By finding how similar two protein sequences are, we acquire knowledge about their structure and therefore their function.
Nucleic acids are the molecules that make up DNA, an extremely important substance that all cellular organisms use to store their genetic information. The most common nucleic acids are deoxyribonucleic acid and ribonucleic acid. Their monomers are called nucleotides. The most common nucleotides are adenine, cytosine, guanine, thymine, and uracil. Adenine binds with thymine and uracil; Thymine binds only with adenine; and cytosine and guanine can bind only with each other.
The function of carbohydrates includes energy storage and providing structure. Sugars are carbohydrates, but not all carbohydrates are sugars. There are more carbohydrates on Earth than any other known type of biomolecule; they are used to store energy and genetic information, as well as play important roles in cell to cell interactions and communications.
The simplest type of carbohydrate is a monosaccharide, which among other properties contains carbon, hydrogen, and oxygen, mostly in a ratio of 1:2:1 (generalized formula CnH2nOn, where n is at least 3). Glucose, one of the most important carbohydrates, is an example of a monosaccharide. So is fructose, the sugar commonly associated with the sweet taste of fruits.[5][a] Some carbohydrates (especially after condensation to oligo- and polysaccharides) contain less carbon relative to H and O, which still are present in 2:1 (H:O) ratio. Monosaccharides can be grouped into aldoses (having an aldehyde group at the end of the chain, e.g. glucose) and ketoses (having a keto group in their chain; e.g. fructose). Both aldoses and ketoses occur in an equilibrium (starting with chain lengths of C4) cyclic forms. These are generated by bond formation between one of the hydroxyl groups of the sugar chain with the carbon of the aldehyde or keto group to form a hemiacetal bond. This leads to saturated five-membered (in furanoses) or six-membered (in pyranoses) heterocyclic rings containing one O as heteroatom.
Two monosaccharides can be joined together using dehydration synthesis, in which a hydrogen atom is removed from the end of one molecule and a hydroxyl group (—OH) is removed from the other; the remaining residues are then attached at the sites from which the atoms were removed. The H—OH or H2O is then released as a molecule of water, hence the term dehydration. The new molecule, consisting of two monosaccharides, is called a disaccharide and is conjoined together by a glycosidic or ether bond. The reverse reaction can also occur, using a molecule of water to split up a disaccharide and break the glycosidic bond; this is termed hydrolysis. The most well-known disaccharide is sucrose, ordinary sugar (in scientific contexts, called table sugar or cane sugar to differentiate it from other sugars). Sucrose consists of a glucose molecule and a fructose molecule joined together. Another important disaccharide is lactose, consisting of a glucose molecule and a galactose molecule. As most humans age, the production of lactase, the enzyme that hydrolyzes lactose back into glucose and galactose, typically decreases. This results in lactase deficiency, also called lactose intolerance.
Sugar polymers are characterised by having reducing or non-reducing ends. A reducing end of a carbohydrate is a carbon atom that can be in equilibrium with the open-chain aldehyde or keto form. If the joining of monomers takes place at such a carbon atom, the free hydroxy group of the pyranose or furanose form is exchanged with an OH-side-chain of another sugar, yielding a full acetal. This prevents opening of the chain to the aldehyde or keto form and renders the modified residue non-reducing. Lactose contains a reducing end at its glucose moiety, whereas the galactose moiety form a full acetal with the C4-OH group of glucose. Saccharose does not have a reducing end because of full acetal formation between the aldehyde carbon of glucose (C1) and the keto carbon of fructose (C2).
When a few (around three to six) monosaccharides are joined together, it is called an oligosaccharide (oligo- meaning "few"). These molecules tend to be used as markers and signals, as well as having some other uses. Many monosaccharides joined together make a polysaccharide. They can be joined together in one long linear chain, or they may be branched. Two of the most common polysaccharides are cellulose and glycogen, both consisting of repeating glucose monomers.
Glucose is the major energy source in most life forms. For instance, polysaccharides are broken down into their monomers (glycogen phosphorylase removes glucose residues from glycogen). Disaccharides like lactose or sucrose are cleaved into their two component monosaccharides.
Glucose is mainly metabolized by a very important ten-step pathway called glycolysis, the net result of which is to break down one molecule of glucose into two molecules of pyruvate; this also produces a net two molecules of ATP, the energy currency of cells, along with two reducing equivalents in the form of converting NAD+ to NADH. This does not require oxygen; if no oxygen is available (or the cell cannot use oxygen), the NAD is restored by converting the pyruvate to lactate (lactic acid) (e.g., in humans) or to ethanol plus carbon dioxide (e.g., in yeast). Other monosaccharides like galactose and fructose can be converted into intermediates of the glycolytic pathway.
In aerobic cells with sufficient oxygen, as in most human cells, the pyruvate is further metabolized. It is irreversibly converted to acetyl-CoA, giving off one carbon atom as the waste product carbon dioxide, generating another reducing equivalent as NADH. The two molecules acetyl-CoA (from one molecule of glucose) then enter the citric acid cycle, producing two more molecules of ATP, six more NADH molecules and two reduced (ubi)quinones (via FADH2 as enzyme-bound cofactor), and releasing the remaining carbon atoms as carbon dioxide. The produced NADH and quinol molecules then feed into the enzyme complexes of the respiratory chain, an electron transport system transferring the electrons ultimately to oxygen and conserving the released energy in the form of a proton gradient over a membrane (inner mitochondrial membrane in eukaryotes). Thus, oxygen is reduced to water and the original electron acceptors NAD+ and quinone are regenerated. This is why humans breathe in oxygen and breathe out carbon dioxide. The energy released from transferring the electrons from high-energy states in NADH and quinol is conserved first as proton gradient and converted to ATP via ATP synthase. This generates an additional 28 molecules of ATP (24 from the 8 NADH + 4 from the 2 quinols), totaling to 32 molecules of ATP conserved per degraded glucose (two from glycolysis + two from the citrate cycle). It is clear that using oxygen to completely oxidize glucose provides an organism with far more energy than any oxygen-independent metabolic feature, and this is thought to be the reason why complex life appeared only after Earth's atmosphere accumulated large amounts of oxygen.
In vertebrates, vigorously contracting skeletal muscles (during weightlifting or sprinting, for example) do not receive enough oxygen to meet the energy demand, and so they shift to anaerobic metabolism, converting glucose to lactate. The liver regenerates the glucose, using a process called gluconeogenesis. This process is not quite the opposite of glycolysis, and actually requires three times the amount of energy gained from glycolysis (six molecules of ATP are used, compared to the two gained in glycolysis). Analogous to the above reactions, the glucose produced can then undergo glycolysis in tissues that need energy, be stored as glycogen (or starch in plants), or be converted to other monosaccharides or joined into di- or oligosaccharides. The combined pathways of glycolysis during exercise, lactate's crossing via the bloodstream to the liver, subsequent gluconeogenesis and release of glucose into the bloodstream is called the Cori cycle.[citation needed]
Like carbohydrates, some proteins perform largely structural roles. For instance, movements of the proteins actin and myosin ultimately are responsible for the contraction of skeletal muscle. One property many proteins have is that they specifically bind to a certain molecule or class of molecules—they may be extremely selective in what they bind. Antibodies are an example of proteins that attach to one specific type of molecule. In fact, the enzyme-linked immunosorbent assay (ELISA), which uses antibodies, is currently one of the most sensitive tests modern medicine uses to detect various biomolecules. Probably the most important proteins, however, are the enzymes. These molecules recognize specific reactant molecules called substrates; they then catalyze the reaction between them. By lowering the activation energy, the enzyme speeds up that reaction by a rate of 1011 or more: a reaction that would normally take over 3,000 years to complete spontaneously might take less than a second with an enzyme. The enzyme itself is not used up in the process, and is free to catalyze the same reaction with a new set of substrates. Using various modifiers, the activity of the enzyme can be regulated, enabling control of the biochemistry of the cell as a whole.
In essence, proteins are chains of amino acids. An amino acid consists of a carbon atom bound to four groups. One is an amino group, —NH2, and one is a carboxylic acid group, —COOH (although these exist as —NH3+ and —COO− under physiologic conditions). The third is a simple hydrogen atom. The fourth is commonly denoted "—R" and is different for each amino acid. There are twenty standard amino acids. Some of these have functions by themselves or in a modified form; for instance, glutamate functions as an important neurotransmitter.
Amino acids can be joined together via a peptide bond. In this dehydration synthesis, a water molecule is removed and the peptide bond connects the nitrogen of one amino acid's amino group to the carbon of the other's carboxylic acid group. The resulting molecule is called a dipeptide, and short stretches of amino acids (usually, fewer than thirty) are called peptides or polypeptides. Longer stretches merit the title proteins. As an example, the important blood serum protein albumin contains 585 amino acid residues.
The structure of proteins is traditionally described in a hierarchy of four levels. The primary structure of a protein simply consists of its linear sequence of amino acids; for instance, "alanine-glycine-tryptophan-serine-glutamate-asparagine-glycine-lysine-…". Secondary structure is concerned with local morphology (morphology being the study of structure). Some combinations of amino acids will tend to curl up in a coil called an α-helix or into a sheet called a β-sheet; some α-helixes can be seen in the hemoglobin schematic above. Tertiary structure is the entire three-dimensional shape of the protein. This shape is determined by the sequence of amino acids. In fact, a single change can change the entire structure. The alpha chain of hemoglobin contains 146 amino acid residues; substitution of the glutamate residue at position 6 with a valine residue changes the behavior of hemoglobin so much that it results in sickle-cell disease. Finally, quaternary structure is concerned with the structure of a protein with multiple peptide subunits, like hemoglobin with its four subunits. Not all proteins have more than one subunit.
Ingested proteins are usually broken up into single amino acids or dipeptides in the small intestine, and then absorbed. They can then be joined together to make new proteins. Intermediate products of glycolysis, the citric acid cycle, and the pentose phosphate pathway can be used to make all twenty amino acids, and most bacteria and plants possess all the necessary enzymes to synthesize them. Humans and other mammals, however, can synthesize only half of them. They cannot synthesize isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These are the essential amino acids, since it is essential to ingest them. Mammals do possess the enzymes to synthesize alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine, the nonessential amino acids. While they can synthesize arginine and histidine, they cannot produce it in sufficient amounts for young, growing animals, and so these are often considered essential amino acids.
If the amino group is removed from an amino acid, it leaves behind a carbon skeleton called an α-keto acid. Enzymes called transaminases can easily transfer the amino group from one amino acid (making it an α-keto acid) to another α-keto acid (making it an amino acid). This is important in the biosynthesis of amino acids, as for many of the pathways, intermediates from other biochemical pathways are converted to the α-keto acid skeleton, and then an amino group is added, often via transamination. The amino acids may then be linked together to make a protein.
A similar process is used to break down proteins. It is first hydrolyzed into its component amino acids. Free ammonia (NH3), existing as the ammonium ion (NH4+) in blood, is toxic to life forms. A suitable method for excreting it must therefore exist. Different strategies have evolved in different animals, depending on the animals' needs. Unicellular organisms, of course, simply release the ammonia into the environment. Likewise, bony fish can release the ammonia into the water where it is quickly diluted. In general, mammals convert the ammonia into urea, via the urea cycle.
The term lipid comprises a diverse range of molecules and to some extent is a catchall for relatively water-insoluble or nonpolar compounds of biological origin, including waxes, fatty acids, fatty-acid derived phospholipids, sphingolipids, glycolipids, and terpenoids (e.g., retinoids and steroids). Some lipids are linear aliphatic molecules, while others have ring structures. Some are aromatic, while others are not. Some are flexible, while others are rigid.
Most lipids have some polar character in addition to being largely nonpolar. In general, the bulk of their structure is nonpolar or hydrophobic ("water-fearing"), meaning that it does not interact well with polar solvents like water. Another part of their structure is polar or hydrophilic ("water-loving") and will tend to associate with polar solvents like water. This makes them amphiphilic molecules (having both hydrophobic and hydrophilic portions). In the case of cholesterol, the polar group is a mere -OH (hydroxyl or alcohol). In the case of phospholipids, the polar groups are considerably larger and more polar, as described below.
Lipids are an integral part of our daily diet. Most oils and milk products that we use for cooking and eating like butter, cheese, ghee etc., are composed of fats. Vegetable oils are rich in various polyunsaturated fatty acids (PUFA). Lipid-containing foods undergo digestion within the body and are broken into fatty acids and glycerol, which are the final degradation products of fats and lipids.
A nucleic acid is a complex, high-molecular-weight biochemical macromolecule composed of nucleotide chains that convey genetic information. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are found in all living cells and viruses. Aside from the genetic material of the cell, nucleic acids often play a role as second messengers, as well as forming the base molecule for adenosine triphosphate, the primary energy-carrier molecule found in all living organisms.
Nucleic acid, so called because of its prevalence in cellular nuclei, is the generic name of the family of biopolymers. The monomers are called nucleotides, and each consists of three components: a nitrogenous heterocyclic base (either a purine or a pyrimidine), a pentose sugar, and a phosphate group. Different nucleic acid types differ in the specific sugar found in their chain (e.g., DNA or deoxyribonucleic acid contains 2-deoxyriboses). Also, the nitrogenous bases possible in the two nucleic acids are different: adenine, cytosine, and guanine occur in both RNA and DNA, while thymine occurs only in DNA and uracil occurs in RNA.
Researchers in biochemistry use specific techniques native to biochemistry, but increasingly combine these with techniques and ideas from genetics, molecular biology and biophysics. There has never been a hard-line between these disciplines in terms of content and technique. Today, the terms molecular biology and biochemistry are nearly interchangeable. The following figure is a schematic that depicts one possible view of the relationship between the fields:
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Book: Biochemistry: An introduction |
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a. ^ It should be noted that fructose is not the only sugar found in fruits. Glucose and sucrose are also found in varying quantities in various fruits, and indeed sometimes exceed the fructose present. For example, 32 % of the edible portion of date is glucose, compared with 23.70 % fructose and 8.20 % sucrose. However, peaches contain more sucrose (6.66 %) than they do fructose (0.93 %) or glucose (1.47 %).[6]
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biochemical families: proteins (amino acids/intermediates) · nucleic acids (constituents/intermediates) · carbohydrates (glycoproteins, alcohols, glycosides)
lipids (fatty acids/intermediates, phospholipids, steroids, sphingolipids, eicosanoids) · tetrapyrroles/intermediates
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Translations:
Biochemistry |
Français (French)
n. - biochimie
Deutsch (German)
n. - Biochemie
Ελληνική (Greek)
n. - βιοχημεία
Português (Portuguese)
n. - bioquímica (f)
Español (Spanish)
n. - bioquímica
Svenska (Swedish)
n. - biokemi
中文（简体）(Chinese (Simplified))
生物化学
中文（繁體）(Chinese (Traditional))
n. - 生物化學
日本語 (Japanese)
n. - 生化学, 生化学的組成
العربيه (Arabic)
(الاسم) علم الكيمياء الحيويه
עברית (Hebrew)
n. - ביוכימיה
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