Aspartate ionizing groups

The first hint that two active-site carboxyl groups—one proto-nated and one ionized—might be involved in the catalytic activity of the aspartic proteases came from studies of the pH dependence of enzymatic activity. If an ionizable group in an enzyme active site is essential for activity, a plot of enzyme activity versus pH may look like one of the plots at right. 

Some amino acids have additional ionizable groups in their side-chains. These may be acidic or potentially acidic (aspartic acid, glutamic acid, tyrosine, cysteine), or basic (lysine, arginine, histidine). We use the term potentially acidic to describe the phenol and thiol groups of tyrosine and cysteine respectively under physiological conditions, these groups are unlikely to be ionized. It is relatively easy to calculate the amount of ionization at a particular pH, and to justify that latter statement. 

When the R group contains another ionizable group, the amino acid will have more than two dissociation constants. The carboxylic acid gronps of aspartic acid and glutamic acid, the amine of lysine, and the guanidino group of arginine will all... 

Titration curve of /3-lactoglobulin. At very low values of pH (<2) all ionizable groups are protonated. At a pH of about 7.2 (indicated by horizontal bar) 51 groups (mostly the glutamic and aspartic amino acids and some of the histidines) have lost their protons. At pH 12 most of the remaining ionizable groups (mostly lysine and arginine amino acids and some histidines) have lost their protons as well. 

Some amino acids, such as aspartic acid and lysine, have acidic or basic side chains. These additional ionizable groups complicate somewhat the acid—base behavior of these amino acids. Table 28.1 lists the pAT values for these acidic and basic side chains as well. 

This is an important misconception that seriously complicates the efforts of students to understand the properties of proteins, but it is not as crazy as it may appear. If the free aminoacids are prepared in their pure forms they are indeed acids—aspartic acid and glutamic acid. However, when they are incorporated into proteins they lose two of their ionizable groups completely, and the third, the one that justifies calling these free forms acids, loses its proton in neutral solution it then no longer has a proton to donate, so it is no longer an acid but it can accept one, so it can act as a base. 

The pK values of ionizable groups are sensitive to their environment. The change in the environment of His 146 in deoxyhemoglobin increases its affinity for protons as a result of the electrostatic attraction between the negative charge of the aspartate and the proton. 

Write the form of aspartic acid most prevalent at pH 1.0, and then show its reaction with the following (consult Table 18.2 for p/Q values of the ionizable groups in aspartic acid) (See Example 18.2) ... 

Anticorrosion activity could be much more effectively influenced by the acyl moiety than by the amino acid, although amino dicarboxylic acids (aspartic acid, glutamic acid) provide an additional ionized group, which may strengthen the linkage between the inhibitor molecule and the metal surface covered by oxide/hydroxide, oxy-hydroxide, and salts. 

Breakdown of the amide dihydrate occurs by a mechanism similar to its formation. The ionized aspartate carboxyl (Asp in Figure 16.27) acts as a general base to accept a proton from one of the hydroxyl groups of the amide dihydrate, while the protonated carboxyl of the other asparate (Asp in this case) simultaneously acts as a general acid to donate a proton to the nitrogen atom of one of the departing peptide products. 

The pH dependence of the rate of modification shows that the pKa of Asp-32 is less than 3.165 It is seen in the high-resolution crystal structures that the carboxyl groups of the two aspartate residues are hydrogen-bonded to each other. This is similar to the ionization of maleic acid, which has pKa values of 1.9 and 6.2 (equation 16.30). 

Similar calculations as above for the basic side-chain groups of arginine pK 12.48) and lysine pK 10.52), and the acidic side-chains of aspartic acid (pATa 3.65) and glutamic acid (pAfa 4.25) show essentially complete ionization at pH 7.0. However, for cysteine (pATa of the thiol group 10.29) and for tyrosine (pAfa of the phenol group 10.06) there will be negligible ionization at pH 7.0. 

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