Perfection in enzyme catalysis: the energetics of triosephosphate isomerase

Abstract

During the last lo9 years, the structures of biological macromolecules have been refined and improved so as to optimize the functioning of the organisms that use them. Enzymes are more specific and more effective than the simple molecular units of which they are made, and compared to simple catalysts (e.g., acetic acid or imidazole), the free-energy barriers for an enzymecatalyzed transformation are much lower, allowing the organism to synthesize less catalyst to produce a given flux of substrate.Can we say how far this process has gone? For some enzymes, there is going to be a conflict between changes that maximize catalytic effectiveness and changes that optimize control, and for these enzymes we may expect that some catalytic power has been sacrificed to the higher good of metabolic control.' On the other hand, very many enzyme systems appear not to be involved in metabolic control (at least not at the level of governing the flux of substrate through a metabolic pathway), and for these we can ask to what extent the selective pressures of evolution have produced the perfect catalyst.In this Account, we shall suggest one measure of perfection and further show how a combination of mechanistic approaches using isotope methods allows the essentially complete description of the energetics of the reaction catalyzed by triosephosphate isomerase from muscle. This enzyme appears to have arrived at the end of its evolutionary development as a catalyst.In muscle, adenosine triphosphate (ATP) is produced (at least partially) by the anaerobic conversion of muscle Jeremy Knowies is an Englishman for whom we have already had occasion to publish a biographical sketch: Acc. Chem. Res., 5, 155 (1972). During the intervening 5 years he migrated from Oxford to Harvard, where he is now Professor of Chemistry. John Albery is a University Lecturer in Physical Chemistry at the University of Oxford and a Fellow of University College, Oxford. He read Chemistry at Balliol College, Oxford, and R. P. Bell supervised his D. Phil. research on rapid proton-transfer reactions. During 1965 he worked with Stanley Bruckenstein at the University of Minnesota. He has published monographs on ring-disk electrodes and electrode kinetics. His research interests include photogalvanic cells, eiecbanatytical techniques, interfacial transfer reactions, and kinetic isotope effects.He also writes regularly for the theater, and one of his musicals, "On the Boil", IS set in a laboratory not very far from Oxford. glycogen to lactate. Glycogen is converted in a number of steps to fructose 1,6-bisphosphate, and these conversions consume 1 mol of ATP per mol of glucosyl unit (Figure 1). Fructose 1,6-bisphosphate is then cleaved by aldolase into two three-carbon units, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), and only GAP is further catabolized to lactate. Each mole of GAP that is converted to lactate yields 2 mol of ATP.From Figure 1 it is clear that, without triosephosphate isomerase (TIM), which allows the utilization of DHAP, musc...

Substrate Effect on Catalytic Loop and Global Dynamics of Triosephosphate Isomerase

“…This interesting situation indicates that electrostatic interactions between substrate and loop 6 are not strong enough to keep the catalytic loop closed over the active site for extended periods. In fact, this could provide a means for effective product release (DHAP), which is not the rate-limiting step in the conversion of GAP to DHAP [1,14]. Also in conformity with our current findings, there are two crystal structures reported, in which the loop 6 is in open conformation despite the presence of a ligand [1,15,16].…”

Characterization of the Transition State of Functional Enzyme Dynamics

“…1,2 Motions on this time scale are clearly of central importance to enzymatic activity, and recent experiments have illuminated the intimate connection between these conformational changes and catalytic turnover. [3][4][5][6] However, the mechanistic details of these motions are not fully known.…”

An aspartate and a water molecule mediate efficient acid-base catalysis in a tailored antibody pocket

“…Antibody 34E4, generated against hapten 4 (7), is among the most active catalysts for this transformation because it effectively exploits a combination of hydrogen bonding, stacking, and van der Waals interactions to align the substrate with Glu H50 , the carboxylate base that was induced in response to haptenic charge (8). Although 34E4 achieves large rate accelerations (7,9), it is substantially less efficient than catalysts like triose phosphate isomerase (TIM) and ketosteroid isomerase (KSI), which promote proton transfers near the diffusion limit (10,11). Conformational isomerism of the free antibody (12) and reliance on a single catalytic residue (8, 9) apparently limit its efficacy.…”