Subdomain II of α-Isopropylmalate Synthase Is Essential for Activity INFERRING A MECHANISM OF FEEDBACK INHIBITION

Background: Isopropylmalate synthases (IPMSs) with and without a regulatory domain were found. Results: IPMS subdomain II is essential for activities and likely involved in acetyl-CoA binding-mediated conformation transition. Conclusion: The N-terminal domain and the two subdomains comprise the complete and independently functional catalytic module of IPMS. Significance: The IPMS catalytic module was defined and characterized, which inferred a probable feedback inhibition mechanism. The committed step of leucine biosynthesis, converting acetyl-CoA and -ketoisovalerate into -isopropylmalate, is catalyzed by -isopropylmalate synthase (IPMS), an allosteric enzyme subjected to feedback inhibition by the end product l-leucine. We characterized the short form IPMS from Leptospira biflexa (LbIPMS2), which exhibits a catalytic activity comparable with that of the long form IPMS (LbIPMS1) and has a similar N-terminal domain followed by subdomain I and subdomain II but lacks the whole C-terminal regulatory domain. We found that partial deletion of the regulatory domain of LbIPMS1 resulted in a loss of about 50% of the catalytic activity; however, when the regulatory domain was deleted up to Arg-385, producing a protein that is almost equivalent to the intact LbIPMS2, about 90% of the activity was maintained. Moreover, in LbIPMS2 or LbIPMS1, further deletion of several residues from the C terminus of subdomain II significantly impaired or completely abolished the catalytic activity, respectively. These results define a complete and independently functional catalytic module of IPMS consisting of both the N-terminal domain and the two subdomains. Structural comparison of LbIPMS2 and the Mycobacterium tuberculosis IPMS revealed two different conformations of subdomain II that likely represent two substrate-binding states related to cooperative catalysis. The biochemical and structural analyses together with the previously published hydrogen-deuterium exchange data led us to propose a conformation transition mechanism for feedback inhibition mediated by subdomains I and II that might associated with alteration of the binding affinity toward acetyl-CoA.