Active-Site Glu165 Activation in Triosephosphate Isomerase and Its Deprotonation Kinetics

• Type of Material: Article
• Authors: Hua Deng, Albert Einstein College of Medicine; Brian Dyer, Emory University; Robert Callender, Albert Einstein College of Medicine
• Language: English
• Date: 2019-05-16
• Publisher: American Chemical Society
• Publication Version: Author Accepted Manuscript (After Peer Review)
• Copyright Statement: © 2019 American Chemical Society.
• Final Published Version (URL): http://dx.doi.org/10.1021/acs.jpcb.9b02981
• Title of Journal or Parent Work: Journal of Physical Chemistry B
• ISSN: 1520-6106
• Volume: 123
• Issue: 19
• Start Page: 4230
• End Page: 4241
• Grant/Funding Information: This research was supported by grants GM068036 (R.C.) and GM53640 (R.B.D.) from the National Institutes of Health.
• Keywords: LOOP MOTION; STRUCTURAL BASIS; Chemistry; Science & Technology; CATALYTIC-SITE; DIHYDROXYACETONE PHOSPHATE; HYDROGEN-BOND; DIHYDROFOLATE-REDUCTASE; LACTATE-DEHYDROGENASE; MICHAELIS COMPLEX; Physical Sciences; SECONDARY S96P MUTATION; PROTON-TRANSFER; Chemistry, Physical
• Research Categories: Chemistry, Biochemistry

Abstract

Triosephosphate isomerase (TIM) catalyzes the interconversion between dihydroxyacetone phosphate (DHAP) and d-glyceraldehyde 3-phosphate (GAP) via an enediol(ate) intermediate. The active-site residue Glu165 serves as the catalytic base during catalysis. It abstracts a proton from C1 carbon of DHAP to form the reaction intermediate and donates a proton to C2 carbon of the intermediate to form product GAP. Our difference Fourier transform infrared spectroscopy studies on the yeast TIM (YeTIM)/phosphate complex revealed a C=O stretch band at 1706 cm -1 from the protonated Glu165 carboxyl group at pH 7.5, indicating that the pK a of the catalytic base is increased by >3.0 pH units upon phosphate binding, and that the Glu165 carboxyl environment in the complex is still hydrophilic in spite of the increased pK a . Hence, the results show that the binding of the phosphodianion group is part of the activation mechanism which involves the pK a elevation of the catalytic base Glu165. The deprotonation kinetics of Glu165 in the μs to ms time range were determined via infrared (IR) T-jump studies on the YeTIM/phosphate and ("heavy enzyme") [U- 13 C,- 15 N]YeTIM/phosphate complexes. The slower deprotonation kinetics in the ms time scale is due to phosphate dissociation modulated by the loop motion, which slows down by enzyme mass increase to show a normal heavy enzyme kinetic isotope effect (KIE) ∼1.2 (i.e., slower rate in the heavy enzyme). The faster deprotonation kinetics in the tens of μs time scale is assigned to temperature-induced pK a decrease, while phosphate is still bound, and it shows an inverse heavy enzyme KIE ∼0.89 (faster rate in the heavy enzyme). The IR static and T-jump spectroscopy provides atomic-level resolution of the catalytic mechanism because of its ability to directly observe the bond breaking/forming process.