Glutamate ionizing groups

With multiple ionizable groups, such as in amino acids and proteins, each group titrates separately according to its pKa. The titration curves shown in Fig. 23-5 are for the amino acids glycine, histidine, and glutamate. 

In addition to the a-amino and a-carboxyl groups those amino acids with an extra ionizable group will also have a pvalue. Glutamic acid is an example of an amino acid with an extra acidic group (COOH) on the y-carbon, and lysine is an example of an amino acid with an extra amino group on the e-carbon atom. As a result they each have three ionizable groups and three pKa... 

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. 

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 -amino group of the reactive lysine residue has an abnormally low pKa of 7.7-8.0 at 30° as measured by the rate of inactivation of the enzyme as a function of pH with pyridoxal (84,280), pyridoxal 5 -phosphate (280,281), or cyanate (282). In the last instance, the reactive lysine was demonstrated to be the same as that found with pyridoxal phosphate. It should be noted that the rate of oxidation of glutamate has been shown to be dependent upon an ionizable group in the enzyme complex having a pKa of 7.7-7.S (283). 

Given p/Cg values for ionizable groups from Table 18.2, sketch curves for the titration of (a) glutamic acid with NaOH and (b) histidine with NaOH. 

The situation is more complicated for amino adds that carry extra ionizable groupings, e.g. the basic and acidic amino acids. For example, glutamic add which has a second -COOH group in... 

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. 

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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