The different levels of protein structure are primary,
secondary, tertiary, and quaternary.
Primary structure is its unique sequence of amino acids. Both
the length of the chain and the order of amino acids are decisive
in the sequence.
Secondary structure is the localized, repetitive coiling or
folding of the polypeptide due to hydrogen bond formation between
the amino acids. The two secondary structure are the alpha helix
and beta pleated sheet. The alpha helix is a coil formed by the
hydrogen bonding between every fourth amino acid. Some fibrous
proteins are composed mostly of alpha helixes, like alpha keratin,
the structural protein of hair. Likewise, some globular proteins
may also be made up of alpha helixes, like hemoglobin. The beta
pleated sheet occurs when two or more regions of the polypeptide
lying side by side are connected by hydrogen bonds between parts of
the two parallel polypeptide portions. Beta pleated sheets make up
the core of many globular proteins, like transthyretin. They can
also make up some fibrous proteins like the silk proteins of
spiders.
Tertiary structure is the culmination of irregular contortions
due to the interactions of the side chains which may have
hydrophobic interaction, ionic bonds, hydrogen bonds, and disulfide
bridges. As the polypeptide begins to fold into its functional
shape, the hydrophobic (nonpolar) side chains of certain amino
acids, proline or methionine for example, end up clustering in the
center of the protein, away from the polar water. Because they are
so close and clustered together, van der Waals interactions fortify
their clumping. Oppositely, the hydrogen bonds between polar side
chains and ionic bonds between positively and negatively charged
side chains aid in stabilizing the tertiary structure.The formation
of disulfide bridges from pairs of cysteine monomers (amino acids
with sulfhydryl groups) also serve to reinforce the structure.
Quaternary structure is the overall protein structure after two
or more polypeptides join together. They can take on many various
shapes, each of which is extremely specific to serve their
function. For example, collagen is a fibrous protein which serve as
girders of connective tissue in skin, bones, tendons, ligaments,
and other body parts. It must be pretty strong - and it is. It's
made up of helical subunits intertwined into a larger triple helix,
giving the long fibers strength and durability.
Quaternary structure might not always be applicable. In the
cases where one polypeptide is a protein by itself, quaternary
structure is not necessary.
