title: Substrate-Induced Facilitated Dissociation of the Competitive Inhibitor from the Active Site of O-Acetyl Serine Sulfhydrylase Reveals a Competitive-Allostery Mechanism
creator: Singh, Appu Kumar
creator: Ekka, Mary Krishna
creator: Kaushik, Abhishek
creator: Pandya, Vaibhav
creator: Singh, Ravi P.
creator: Banerjee, Shrijita
creator: Mittal, Monica
creator: Singh, Vijay
creator: Kumaran, S.
subject: QR Microbiology
description: By classical competitive antagonism, a substrate and competitive inhibitor must bind mutually exclusively to the active site. The competitive inhibition of O-acetyl serine sulfhydrylase (OASS) by the C-terminus of serine acetyltransferase (SAT) presents a paradox, because the C-terminus of SAT binds to the active site of OASS with an affinity that is 4-6 log-fold (104-106) greater than that of the substrate. Therefore, we employed multiple approaches to understand how the substrate gains access to the OASS active site under physiological conditions. Single-molecule and ensemble approaches showed that the active site-bound high-affinity competitive inhibitor is actively dissociated by the substrate, which is not consistent with classical views of competitive antagonism. We employed fast-flow kinetic approaches to demonstrate that substrate-mediated dissociation of full length SAT-OASS (cysteine regulatory complex) follows a noncanonical "facilitated dissociation" mechanism. To understand the mechanism by which the substrate induces inhibitor dissociation, we resolved the crystal structures of enzyme·inhibitor·substrate ternary complexes. Crystal structures reveal a competitive allosteric binding mechanism in which the substrate intrudes into the inhibitor-bound active site and disengages the inhibitor before occupying the site vacated by the inhibitor. In summary, here we reveal a new type of competitive allosteric binding mechanism by which one of the competitive antagonists facilitates the dissociation of the other. Together, our results indicate that "competitive allostery" is the general feature of noncanonical "facilitated/accelerated dissociation" mechanisms. Further understanding of the mechanistic framework of "competitive allosteric" mechanism may allow us to design a new family of "competitive allosteric drugs/small molecules" that will have improved selectivity and specificity as compared to their competitive and allosteric counterparts.
publisher: ACS
date: 2017
type: Article
type: PeerReviewed
relation: http://dx.doi.org/10.1021/acs.biochem.7b00500
identifier:   Singh, Appu Kumar and Ekka, Mary Krishna and Kaushik, Abhishek and Pandya, Vaibhav and Singh, Ravi P. and Banerjee, Shrijita and Mittal, Monica and Singh, Vijay and Kumaran, S.  (2017) Substrate-Induced Facilitated Dissociation of the Competitive Inhibitor from the Active Site of O-Acetyl Serine Sulfhydrylase Reveals a Competitive-Allostery Mechanism.  Biochemistry, 56 (37).  pp. 5011-5025.  ISSN 0006-2960     
relation: http://crdd.osdd.net/open/2057/