Transition state analogy of phosphonic acid peptide inhibitors of pepsin

A series of 11 phosphonate peptide analogs, RO(2)C-Xaa-Yaa-Phe-(PO2--O)-Phe O-(3-(4-pyridyl)propyl ester), were synthesized and evaluated as inhibitors of the aspartic peptidase pepsin. For the inhibitors with Gly or Ala at the P-2 position, the K-i values correlate with the K-m/k(cat) values of the corresponding substrates, demonstrating that these analogs mimic the transition state in the way the P-2-P-4 residues bind. Deviations from the correlation for the other inhibitor/substrate pairs imply a different binding orientation as a result of N-substitution at P-2 (Pro), contamination with the more potent diastereomer (D-Ala), or a change in rate-limiting step for turnover (lie).