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You mean "why is a protein least soluble when the pH of aqueous media matches the value of its isoelectric point?".
The answer is that at its isoelectric point the protein surface carries no net charge. As protein solubility is based upon favourable electrostatic interactions between the charges (negative or positive) on the protein surface and the delta-negative or delta-positive dipoles on water molecules, when there is LEAST charge on the protein surface you can also expect their to be the least favourable interactions between water molecules and the protein. As the protein-water interaction is in competition with protein-protein interactions, being at the pI most favours protein-protein interactions, which leads to precipitation, compared to protein-water interaction which lead to solvation.
Isoelectric. "equal electric [charge]"
The previous answer is incorrect in multiple ways and correct in some. It states "protein solubility is based upon electrostatic interactions between the charges (negative or positive) on the protein surface and the ...dipoles of water". Firstly, there cannot exist an electrostatic interaction (coulombic) with one charged molecule and an uncharged water molecule/induced dipole. An electrostatic force of a charged particle must have an electrolyte in solution for interaction.
What the explanation aimed to describe is the decreased ion-induced dipole forces from a charged amino acid side chain to molecules of water at the pI of the protein. The singular force of which is stronger than H-bonding (and much stronger than van der waals, london/dispersion forces). Collectively H-bonding of the exterior amino acids is a much stronger force than total of the ion-induced dipoles.
...However, if we were talking about "solvation" of the molecule, then his answer would be completely correct where at the isoelectric point the least solvation would occur, largely due to the absence of charged outer residues...
Why is the solubility not dependent on ion-induced dipole? We only have 5 natural occurring amino acid R groups that can be charged around neutral pH: aspartate, glutamate - whose side chains behave as any other organic acid in the physiological range, reacting with other amine side chains, going through esterification with alcohols, chelation of divalent metal ions, etc.
The basic residues - lysine and arginine, lysine will undergo a number of reactions with or without a protonated nitrogen. Arginine however is likely going to carry a net charge at any protein pI and is the least reactive A.A. (however its' frequency in proteins is 5.7%, of this percent a fraction of which will go toward ion-dipole surface interaction with water. The same goes for histidine but it is even less frequent in proteins (2.2%).
The pI of most any protein will be nearer to the physiological range than to either extreme, so we can rule out mass protonation/deprotonation of side chains at its pI, which is the only way ion-induced dipole forces would participate in such solubility changes anyhow.
So what does make a protein soluble/insoluble in water?
A protein's 'solubility' comes first from the specific volume of water being much smaller than the specific volume of the protein. Meaning although our protein may be more dense than water, it has a tendency to be suspended in solution because of the much larger volume occupied (this of course is not true for many types of proteins, seen more in globular proteins but the theory is the same).
The major factor of protein solubility according to pI is hydrophilic residues (vs hydrophobic) on the exterior. As the pH of the solution nears an equilibrium of oxidation/reduction of the hydrophobic exterior residues, less water molecules are forming h-bonds, dipoles, van der waals, etc on the protein's exterior. You could imagine the H2O 'dispersing' away from the membrane, decreasing the volume occupied by H2O molecules near the protein exterior. This action allows the protein to 'fall' from solution and form a precipitate at the pI of the solution. The physical chemistry behind this action is much more involved, but this should give some idea of the biochemistry of the event.
Wiki User
∙ 12y ago
Wiki User
∙ 11y agoBecause it is polae
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The isoelectric point of an amino acid is?
An amino acid is considered to be at its isoelectric point when the positive charges on the molecule exactly balance its negative charges. At this point, the amino acid carries no net charge and is therefore immobile in an electric field. Isoelectric points of amino acids widely vary accoriding to their side chains and polarity characteristics.
What is a mutation called that has no effect on an organism?
A point mutation, in which one nitrogen base in a codon is substituted for another, may have no effect on an organism. This is true if the base substitution does not change the amino acid that the codon represents, or if the mutation occurs in a non-critical location in the protein so that the protein's structure is not changed significantly and the protein is still able to function.
According to the graph below at which point is the plant performing the least photosynthesis?
For apex--- Point a.
Why does an insertion mutation usually cause defects during protein synthesis than a point mutation?
Insertion mutations can affect many amino acids in the protein.An insertion mutation usually causes more defects during protein synthesis than point mutation because an insertion mutation will affect many amino acids in the protein.
Why does an insert mutation usually cause more defects during protein synthesis than a point mutation?
Insertion mutations can affect many amino acids in the protein.An insertion mutation usually causes more defects during protein synthesis than point mutation because an insertion mutation will affect many amino acids in the protein.
What is the pI of a protein?
pI is the isoelectric point. This is a pH value where a protein has no net charge. NOTE: Proteins may have multiple pI's.
What is meant by isoelectric point of protein?
It is the pH at which a particular molecule or surface carries no net electrical charge
Protein electrophoresis and how its carried out?
An example of protein electrophoresis is SDS-PAGE ( sodium do-decyl sulpahate-polyacrrlamide gel electrophoresis).Another example includess " isoelectric focusing".In isoelectric focusing the protein is separated on the basis of its net charge.The main principle lies on the basis of finding isoelectric point i.e. at which the net charge on the protein is zero.The protein is loaded in the gel and then it separates itself on the basis of the charge.NEgatively charged on the negative side and positively gharged on the positive side and the neutral ones in the centre.
How do you calculate the isoelectric point of all 20 amino acid?
by isoelectric focusing appratus
What is the isoelectric point of HBsAg?
4.5 to 5.5 . its acidic.
What is meant by isoelectronic point of protein?
Isoelectric point of a protein or amino acid is defined as the pH value at which the molecule has equalpositive charges on protonized basic (amino) groups as negative charges on protolized acid (carbonic) groups, so the net charge is neutral (zero).
What is isoelectric pH?
it is the pH at which a particular molecule or surface carries no net electrical charge(or contain both electric charch, negative as wall as positive).
How can you calculate the pH of NaHCO3 at its isoelectric point?
(ka1*ka2)^(1/2)
The isoelectric point of an amino acid is?
An amino acid is considered to be at its isoelectric point when the positive charges on the molecule exactly balance its negative charges. At this point, the amino acid carries no net charge and is therefore immobile in an electric field. Isoelectric points of amino acids widely vary accoriding to their side chains and polarity characteristics.
What is point between qrs and st segment on ekg called?
what does isoelectric line represent
Are proteins electrically charged?
Proteins carries both positively and negatively charged amino acids on them. so they are known as Amphoteric molecules that contain both charges (Zwitterions). An isoelectric pH or point is a pH where the net charge of the protein molecule is zero.
What metals or compounds can inhibit cellulase action?
See US patent 6083581 - they use a combination of a protein having an acidic isoelectric point (ex: beta-lactoglobuin) and kymene. Should work for paper or cellulose. The patent is still in effect so you will need to work on a project outside of this area.
