chapter  10
36 Pages

Modication of Carboxyl Groups in Proteins

Aspartic acid and glutamic acid are dicarboxylic amino acids (Figure 10.1), which are among the 20 amino acids considered normal monomer units of proteins. Other amino acids that contain a hydroxyl function are serine, threonine, and tyrosine, which are not considered to be ionizable groups of proteins while the carboxyl groups of aspartic acid and glutamic acid are ionizable groups.1 γ-Carboxyglutamic acid (Figure 10.1) is a product of the posttranslational modication of glutamic acid. Carboxyl groups are unique in that there are two carbon-oxygen bonds with different bond lengths.2 Early work on the amino acid composition of proteins did not discriminate between the free acid and amide forms of aspartic acid and glutamic acid, and reported composition in terms of Asx and Glx3 as only aspartic acid and glutamic acid would be measured after the acid hydrolysis of proteins (asparagine and glutamine would be converted to the free acid forms). More recent work that is based on DNA sequence analysis permits the reporting of both free acid and amide forms.4 An average content in proteins of 6% has been reported for aspartic acid, while a value of 6.3% has been reported for glutamic acid.4 The selective hydrolysis of the carboxyl peptide bond of aspartic acid in dilute acid has been known for some time5-8 and continues to see active use for structural analysis of proteins.9-11 This reaction has been the subject of considerable study, and a mechanism involving a succinimide intermediate (Figure 10.1) was proposed by Piszkiewicz and coworkers12 noting the particular sensitivity of Asp-Pro peptide bond. The succinimide intermediate, which also results from asparagine, can be trapped by reduction with borohydride to yield homoserine and isohomoserine.13