Transglutaminase is an enzyme which catalyzes the Ca++-activated incorporation of amines into proteins. More detailed studies of the action of this enzyme showed that the incorporated amines displaced ammonia, and that only L-glutamine and not asparagine residues in the protein or peptide are substrates for this enzyme. This enzyme will not act on free glutamine, hence is not to be confused with glutaminase or transamidases which will act on free glutamine as a substrate. A variety of substituted glutamine peptides have been found to have substrate activity for this enzyme. Neidle and Acs, and more recently Folk and Cole, have studied the effects of varying the nature of the amino acids surrounding the active glutamine residue. While the nature of the amino acid present will alter the quantitative response of the given peptide as a substrate, a large number of protected glutamine dipeptides seem to possess substrate activity. There are other factors which must be involved in determining the substrate specificity of a glutamine peptide as there are some proteins which, in the native state, do not act as substrates for transglutaminase but do so on denaturation. Thus, in Zn-free insulin, only the two glutamine residues of the A chain seem to react readily, while after oxidation of the chains, the glutamine residue in the B chain is quite a good substrate for transglutaminase. With hemoglobin, no substrate activity appears until the protein is denatured, and, in addition, a detailed study by Pincus has shown that certain glutamine residues in the protein do not act as substrates for transglutaminase, even after denaturation. (These are the residues which are foB owed by lysine and arginine.)
Clarke, Donald Dudley PhD and Nicklas, Ruth, "Automation of the assay for transglutaminase / Donald D. Clarke and Ruth Nicklas" (1966). Chemistry Faculty Publications. Paper 48.