Enzymatic peptide synthesis by the recombinant proline-specific endopeptidase from Flavobacterium meningosepticum and its mutationally altered Cys-556 variant.

Proline-specific endopeptidase (PSE) (EC 3.4.21.26) was investigated for its potential as a catalyst in peptide synthesis. Using an activated peptide ester or a peptide amide as the acyl component, the enzyme catalyzed kinetically controlled aminolysis and transpeptidation respectively, with various amino acid amides as acyl acceptors. To a certain extent the nucleophile preference reflected the amino acid preference in the S1'-position of the enzyme in peptide hydrolysis: the highest fractions of aminolysis were obtained using amino acid amides with hydrophobic side-chains (e.g. Leu-NH2, Phe-NH2). PSE also catalyzed the thermodynamically controlled condensation of short peptides with a free carboxyterminus and various amino acid amides. This enabled us to examine the acceptance of different acyl components in the substrate-binding site of the enzyme with regard to their amino acid composition: In the S1 position proline was clearly favored, but alanine was also accepted, whereas the S2 subsite accepted various amino acids rather unspecifically. Since PSE was shown to be extremely sensitive against water-miscible organic solvents, an alternative approach was used to increase yields in enzymatic peptide synthesis: a derivative of PSE in which the catalytic Ser-556 is converted to a Cys was constructed by protein engineering. This mutant (PSEcys) exhibited a dramatically increased peptide ligase activity in aqueous solution.