Substrate specificity of platypus venom L-to-D-peptide isomerase.
The L-to-D-peptide isomerase from the venom of the platypus (Ornithorhyncus anatinus) is the first such enzyme to be reported for a mammal. In delineating its catalytic mechanism and broader roles in the animal, its substrate specificity was explored. We used N-terminal segments of defensin-like peptides DLP-2 and DLP-4 and natriuretic peptide OvCNP from the venom as substrates. The DLP analogues IMFsrs and ImFsrs (srs is a solubilizing chain; lowercase letters denote D-amino acid) were effective substrates for the isomerase; it appears to recognize the N-terminal tripeptide sequence Ile-Xaa-Phe-. A suite of 26 mutants of these hexapeptides was synthesized by replacing the second residue (Met) with another amino acid, viz. Ala, alpha-aminobutyric acid, Ile, Leu, Lys, norleucine, Phe, Tyr, and Val. It was shown that mutant peptides incorporating norleucine and Phe are substrates and exhibit L- or D-amino acid isomerization, but mutant peptides that contain residues with shorter, beta-branched or long side chains with polar terminal groups, viz. Ala, alpha-aminobutyric acid, Ile, Val, Leu, Lys, and Tyr, respectively, are not substrates. It was demonstrated that at least three N-terminal amino acid residues are absolutely essential for L-to-D-isomerization; furthermore, the third amino acid must be a Phe residue. None of the hexapeptides based on LLH, the first three residues of OvCNP, were substrates. A consistent 2-base mechanism is proposed for the isomerization; abstraction of a proton by 1 base is concomitant with delivery of a proton by the conjugate acid of a second base.
D000596:Amino Acids / Q000737:chemistry* / Q000378:metabolism; D000818:Animals; D036341:Intercellular Signaling Peptides and Proteins; D007535:Isomerases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010455:Peptides / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010987:Platypus; D011506:Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013237:Stereoisomerism; D013379:Substrate Specificity / Q000502:physiology; D014688:Venoms / Q000201:enzymology* / Q000235:genetics
D000596:Amino Acids; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D011506:Proteins; D014688:Venoms; C121145:defensin-like peptide 1, platypus; C114236:ovCNP-39 peptide; D007535:Isomerases