Peptides adjacent protein properties

From these observations it follows that (1) some of the chemical properties of a phosphoamino acid may change considerably on incorporation into a peptide or protein, (2) that the adjacent molecular configuration may be responsible for the stability of the phosphate group, and (3) that the acidity of the medium determines whether migration of a phosphoric acid residue from the —0— to the —— position occurs. As outlined in a previous section in this article A-phosphorylserine and A-phosphorylthreonine, respectively, w-ould represent possible configurations of the A-terminal amino acid of a peptide chain in a native protein. 

Many secreted proteins, as well as smaller peptide hormones, are acted upon in the endoplasmic reticulum by tryptases and other serine proteases. They often cut between pairs of basic residues such as KK, KR, or RR.214-216 A substilisin-like protease cleaves adjacent to methionine.217 Other classes of proteases (e.g., zinc-dependent carboxypeptidases) also participate in this processing. Serine carboxypeptidases are involved in processing human prohormones.218 Among the serine carboxypeptidases of known structure is one from wheat219 and carboxypeptidase Y, a vacuolar enzyme from yeast.220 Like the pancreatic metallocarboxypeptidases discussed in Section 4, these enzymes remove one amino acid at a time, a property that has made carboxypeptidases valuable reagents for determination of amino acid sequences. Carboxypeptidases may also be used for modification of proteins by removal of one or a few amino acids from the ends. 

Such observations show that the physicochemical properties of the phosphorus-coirtaining amino acids change on their incorporation into a peptide or into a protein and hence are sensitive to the adjacent molecular configuration. Such considerations led the author to the development of enzymatic methods for investigating the nature of phosphorus linkages... 

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