Functional interaction among catalytic residues in subtilisin BPN'

Variants of the serine protease, subtilisin BPN', in which the catalytic triad residues (Ser-221, His-64, and Asp-32) are replaced singly or in combination by alanine retain activities with the substrate N-succinyl-L-Ala-L-Ala-L-Pro-L-Phe-p-nitroanilide (sAAPF-pna) that are at least 10(3) to 10(4) above the non-enzymatic rate [Carter, P., Wells, J.A. Nature (London) 322:564-568, 1988]. A possible source of the residual activity was the hydrogen bond with the N delta 2 of Asn-155 that helps to stabilize the oxyanion generated in the tetrahedral transition state during amide bond hydrolysis by the wild-type enzyme. Replacing Asn-155 by Gly (N155G) lowers the turnover number (kcat) for sAAPF-pna by 150-fold with virtually no change in the Michaelis constant (KM). However, upon combining the N155G and S221A mutations to give N155G:S221A, kcat is actually 5-fold greater than for the S221A enzyme. Thus, the catalytic role of Asn-155 is dependent upon the presence of Ser-221. The residual activity of the N155G:S221A enzyme (approximately 10(4)-fold above the uncatalyzed rate) is not an artifact because it can be completely inhibited by the third domain of the turkey ovomucoid inhibitor (OMTKY3), which forms a strong 1:1 complex with the active site. The mutations N155G and S221A individually weaken the interaction between subtilisin and OMTKY3 by 1.8 and 2.0 kcal/mol, respectively, and in combination by 2.1 kcal/mol. This is consistent with disruption of stabilizing interactions around the reactive site carbonyl of the OMTKY3 inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)