Lysine ionizing groups

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... 

For amino acids with more than two ionizable groups, e.g. lysine, the same formula is used but the two pK values used are those of the two groups that lose and gain a charge from the neutral form of the amino acid. 

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. 

When homopolymers consist of ionizable groups, for example, poly(L-lysine), the helix—coil transition is inducible by a change of pH, with the pH of... 

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. 

We also see that lysine carries the maximum total number of charges at the first equivalence point (all ionizable groups charged). 

The next two ionizable groups are both lysines. The average of their pKas will be 10.3. 

Certain amino acids also possess side chains with ionizable groups. For example, the side chain of lysine possesses an ionizable amino group. Because of their structures, alanine, lysine, and the other amino acids can act as effective... 

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). 

Write the form of lysine most prevalent at pH 1.0, and then show its reaction with each of the following (consultTable 18.2 for pKg values of the ionizable groups in lysine) (See Example 18.2) ... 

Edlbacher (126,127) studied the action of dimethyl sulfate on edestin and other proteins and concluded that mainly lysine NH, groups reacted. Those proteins which contained no lysine failed to react. He also isolated methyl -amino lysine from methylated casein (128). Haurowitz (129) has recently treated crystalline horse hemoglobin and egg albumin with dimethylsulfate and obtained as an upper limit 6.5% and 3.9%, respectively, of OCHg groups. This is 142 and 54 methoxyl groups per mole protein and represents a large proportion of the total ionizable groups for these two proteins (18). 

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