|MadSci Network: Biochemistry|
Actually, some of the givens in your question are slightly misleading: first, although histine has a net positive charge under physiological conditions (around pH 7.4), most of the histidine under physiological conditions is uncharged; second, while histidine may be the best amino acid to buffer a solution at pH 6, it is not the best amino acid for buffering solutions at other pH's closer to the pKa's of the other charged amino acids. All of this gets to the point of this questions, which is "how does a buffer work?"
A buffer is defined chemically as any compound that minimizes the change in pH of a solution when other compounds are added to the solution. The ability of a compound to act as a buffer at a given pH is determined by how readily it will accept and donate protons at that pH. For this reason, any compound that will both accept and donate a lot of protons at a given pH will be an excellent buffer at that pH. The action of a compound releasing protons is referred to as acid dissociation (the protons dissociate from the compound), and every compound dissociates at a rate that is specific for that compound. This rate is called the dissocation constant, and for acid dissociation is abbreviated as Ka. It can be demonstrated mathematically as well as chemically that, at the point where the concentration of protons (actually hydronium ions) in solution is equal to the Ka of a compound, that compound can act as a buffer. Just as pH is defined as the negative logarithm of the proton concentration, one can take the negative log of the Ka of a compound and refer to it as the pKa. Since a compound will act as a buffer when the proton concentration is equal to its Ka, we can rewrite this to say that a compound will buffer a solution if the pH is the same as its pKa.
Most compounds can actually act as reasonably good buffers across a whole range of pH's centered around their pKa's and not just at their pKa's. The pKa of histidine is 6.0, so histidine is best at buffering at pH 6.0. The acidic amino acids have pKa's below histidine's, and the basic amino acids have pKa's far above histidine's, such that the pKa of histidine is the closest to pH 7.4 of any of the amino acids. That is to say that histidine is the only amino acid with pH 7.4 within its buffering range, such that histidine is the best amino acid buffer under physiological conditions. It is certainly not the best biological buffer under these conditions, and it is not the best amino acid buffer under either very acidic or very basic conditions, but it is the best amino acid buffer within its buffering range.
As an aside: you can calculate the amount of charged histidine (or whatever) at a given pH. First subtract the pH from the pKa, and then take ten to this difference. The amount of charged histidine will be equal to ten to the difference divided by one plus ten to the difference.
10(pKa - pH) Proportion charged = -------------- 1 + 10(pKa - pH)
For histidine under physiological conditions, this works out as: 6.0 - 7.4 = -1.4; 10^-1.4 = 0.0398; 0.0398/1.0398 = 0.0383. So at physiological pH, about 3.8% of histidine is charged: that is a net positive charge, but not much of one.
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