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has a pI of 3.22, considerably lower than that of glycine. This is due to the presence of two carboxyl groups, which, at the average of their pKa values (3.22), contribute a net charge of — 1 that balances the +1 contributed by the amino group. Similarly, the pI of histi-dine, with two groups that are positively charged when protonated, is 7.59 (the average of the pKa values of the amino and imidazole groups), much higher than that of glycine.

Finally, as pointed out earlier, under the general condition of free and open exposure to the aqueous environment, only histidine has an R group (pKa = 6.0) providing significant buffering power near the neutral pH usually found in the intracellular and extracellular fluids of most animals and bacteria (Table 3-1).

SUMMARY 3.1 Amino Acids

■ The 20 amino acids commonly found as residues in proteins contain an a-carboxyl group, an a-amino group, and a distinctive R group substituted on the a-carbon atom. The a-carbon atom of all amino acids except glycine is asymmetric, and thus amino acids can exist in at least two stereoisomeric forms. Only the l stereoisomers, with a configuration related to the absolute configuration of the reference molecule l-glyceraldehyde, are found in proteins.

■ Other, less common amino acids also occur, either as constituents of proteins (through modification of common amino acid residues after protein synthesis) or as free metabolites.

■ Amino acids are classified into five types on the basis of the polarity and charge (at pH 7) of their R groups.

■ Amino acids vary in their acid-base properties and have characteristic titration curves. Monoamino monocarboxylic amino acids (with nonionizable R groups) are diprotic acids (+H3NCH(R)COOH) at low pH and exist in several different ionic forms as the pH is increased. Amino acids with ionizable R groups have additional ionic species, depending on the pH of the medium and the pKa of the R group.

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