Substrate, enzyme complex with

Michaelis—Menten mechanism A model of enzyme catalysis in which the enzyme and its substrate reach a rapid pre-equilibrium with the bound substrate-enzyme complex. 

The high catalytic activity of enzymes has a number of sources. Every enzyme has a particular active site configured so as to secure intimate contact with the substrate molecule (a strictly defined mutual orientation in space, a coordination of the electronic states, etc.). This results in the formation of highly reactive substrate-enzyme complexes. The influence of tfie individual enzymes also rests on the fact that they act as electron shuttles between adjacent redox systems. In biological systems one often sees multienzyme systems for chains of consecutive steps. These systems are usually built into the membranes, which secures geometric proximity of any two neighboring active sites and transfer of the product of one step to the enzyme catalyzing the next step. 

Larsen, T.M., Wedeking, J.E., Rayment, I. and Reed, G.H. (1996) A carboxylate oxygen of the substrate bridges the magnesium ions at the active site of enolase structure of the yeast enzyme complexed with the equilibrium mixture of 2-phosphoglycerate and phosphoenolpyruvate at 1.8 A resolution, Biochemistry, 30, 4349-4358. 

Figure 10-9 Representation of the course of enzyme-induced hydration of fumaric acid (trans-butenedioic acid) to give L-malic acid (L-2-hydroxy-butanedioic acid). If the enzyme complexes with either—C02H (carboxyl) group of fumaric acid, and then adds OH from its right hand and H from its left, the proper stereoisomer (l) is produced by antarafacial addition to the double bond. At least three particular points of contact must occur between enzyme and substrate to provide the observed stereospecificity of the addition. Thus, if the enzyme functions equally well with the alkenic hydrogen or the carboxyl toward its mouth (as shown in the drawing) the reaction still will give antarafacial addition, but o,L-malic acid will be the product.

Detailed structures for the active sites of enzymes are difficult to obtain and have been worked out only for a few enzymes that have been studied extensively by both chemical and x-ray methods. Very revealing information has been obtained by x-ray diffraction studies of complexes between the enzyme and nonsubstrates, which are molecules similar to actual substrates and complex with the enzyme at the active site, but do not react further. These substances often inhibit reaction of the normal substrate by associating strongly with the enzyme at the active site and not moving onward to products. The x-ray studies of enzymes complexed with nonsubstrates show that the active site generally is a cleft or cavity in the folded structure of the enzyme that is largely hydrophobic in character. The enzyme-substrate complex can... 

ESR-silent, suggesting low-spin Fe11. This has been confirmed by the use of Mossbauer spectroscopy. Kinetic studies suggest that a substrate-enzyme complex is formed before interaction with dioxygen. Binding of substrate has no effect upon the Mossbauer spectrum, so it is not certain that it binds to the Fe11 centre. 

This means that this dehydrogenase can form binary complexes with steroid substrates, binary complexes with pyridine nucleotide, and ternary complexes with both substrates. This behavior contrasts with that usually observed for NAD-linked dehydrogenases in which the ketone or aldehyde substrate can bind only to the NADH-enzyme binary complex but not to free enzyme (29). 

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