The Department of Biochemistry's weekly BCH 252 seminar series is presented this week by

Rittik Ghosh, BCMB Graduate Student, Len Mueller Group, UC Riverside 

Seminar Title: "Mutation of βGln114 to Ala Alters the Stabilities of Allosteric States in Tryptophan Synthase Catalysis"

Abstract: The tryptophan synthase (TS) bienzyme complexes found in bacteria, yeasts and molds are pyridoxal 5’-phosphate (PLP) requiring enzymes that synthesize L-Trp. In the TS catalytic cycle, switching between open and closed states of the α- and β- subunits via allosteric interactions is key to the efficient conversion of 3-indole-D-glycerol-3’-phosphate and L-Ser to L-Trp. In this process, the roles played by β-site residues proximal to the PLP cofactor have not yet been fully established. βGln114 is one such residue. To explore the roles played by βQ114 we conducted a detailed investigation of the βQ114A mutation on the structure and function of tryptophan synthase. Initial steady-state kinetic and static UV/Vis spectroscopic analyses showed the Q to A mutation impairs catalytic activity and alters the stabilities of intermediates in the β-reaction. Therefore, we conducted X-ray structural and solid-state NMR spectroscopic studies to compare the wild-type and βQ114A mutant enzymes. These comparisons establish the protein structural changes are limited to the Gln to Ala replacement, the loss of hydrogen bonds between the side chains of βGln114, βAsn145 and βArg148, and the inclusion of waters into the cavity created by substitution of the smaller Ala side chain. Since the conformations of open and closed allosteric states are not changed by the mutation, we hypothesize that the altered properties arise from the lost hydrogen bonds which alter the relative stabilities of the open (βT state) and closed (βR state) conformations of the β-subunit and consequently alter the distribution of intermediates along the β-subunit catalytic path.

Seminar Host: Dr. Jikui Song; jikui.song@ucr.edu