Histamine h3n—c ch2
Figure 2-3. Examples of posttranslationally modified and less conventional amino acids. (A), Some of the modifications are the result of specific enzymatic reactions wherein the amino acid to be modified is first incorporated into a given peptide or protein and then altered by an enzyme-catalyzed reaction. Examples of the products of a methylation (t-n-trimethyllysine residue formation), sulfation (tyrosine sulfate residue formation), hydroxylation (5-hydroxylysine residue formation), and a y-carboxylation (y-carboxyglutamate residue formation) are shown. (B), The structures for citrulline, ornithine, homocysteine, histamine, and taurine are shown. These amino acids and related compounds are derived from amino acids, but in contrast to the amino acids shown in A, these amino acids and amino acid-derived products are produced in a step-wise fashion in metabolic pathways. Arginine and glutamate are precursors of citrulline and ornithine. Homocysteine is the demethylation product of methionine. Histamine is formed by decarboxylation of histidine. Taurine is formed by oxidative catabolism of cysteine. It is the free amino acid rather than an amino acid residue in a protein that is substrate for the metabolic pathways that produce these amino acids and amino acid derivatives.
more base eventually causes the amino group to lose its proton, and alanine becomes negatively charged because of the loss of the positively charged amino group. Compare the titration curve for alanine to the more complex titration curve for histidine, which contains three titratable functional groups (Fig. 2-4).
As is the case with organic acids and amine compounds, amino acid titration curves can be described by constants called pA^oS that help to define characteristics of the associated titratable groups. A pKa is the nega tive log of the dissociation constant Ka for an acid. When the associated (protonated) and dissociated (nonprotonated) species are present in equal concentrations, the pKa is equal to the pH. The pA^ of carboxylic acid groups are relatively low, usually around 2 to 4. Amino groups have pKas that are relatively high, usually 9 to 11 (see Table 2-1). Accordingly the classification of an amino acid as acidic or basic depends on the pKa of a titratable group on its side chain (Darby 1993).
Why is it important to have a knowledge
of the titration characteristics? First, inter- and intramolecular ionic interactions are very important to protein structure. For example, if it is essential for a protein to be electrically or ionically neutral to function, then the protein must be designed with a combination of amino acids that result in a zero net charge. Cell communication, as articulated through specific proteins, is often dependent on the ionic characteristics of given amino acids.
Amino acids and proteins also have the ability to act as buffers because of their acid and base characteristics.
Information about the ionic properties of proteins also has a practical significance. When a protein is titrated to the point that corresponds to neutrality (i.e., to a pH where a protein has no net charge), this pH is referred to as the isoelectric point (p/) of the protein. Many proteins are insoluble at their isoelectric points. Thus, protein isolation and concentration can sometimes be achieved by adjusting the pH. A good example is the isolation of the major milk protein fractions, casein and whey (Darby, 1993).
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