Hexokinase structural changes

When hexokinase binds glucose, its structure changes in a way that brings together the elements of the active site (fig. 8.3). The enzyme literally closes like a set of jaws around the substrate Such a structural change is often referred to as an induced fit. 

Structural changes also contribute to the high specificity of some enzymatic reactions. In hexokinase, the structural change induced by glucose promotes the binding of the other substrate, ATP. ATP does not bind to the enzyme... 

Albumin ovalbumin carbonic anhydrase hemoglobin hexokinase MWCNTs MWCNT-COOH MWCNT-tyrosine MWCNT-isobutane amine CD Fluorescence The functionalized MWCNTs selectively induced protein secondary structure changes. Structural changes depend on the enzyme used and the functional group and the concentration of MWCNTs. [136]... 

FIGURE 6-22 Induced fit in hexokinase. (a) Hexokinase has a U-shaped structure (PDB ID 2YHX). (b) The ends pinch toward each other in a conformational change induced by binding of o-glucose (red) (derived from PDB ID 1HKG and PDB ID 1GLK). 

Although precise positioning of the reactants is a fundamental aspect of enzyme catalysis, most enzymes undergo some change in their structure when they bind substrates. A particularly dramatic example is hexokinase, which catalyzes the transfer of a phosphate group from adenosine triphosphate (ATP) to glucose. 

Structural studies of the oxy-Cope catalytic antibody system reinforce the idea that conformational dynamics of both protein and substrate are intimately intertwined with enzyme catalysis, and consideration of these dynamics is essential for complete understanding of biologically catalyzed reactions. Indeed, recent single molecule kinetic studies of enzyme-catalyzed reactions also suggest that different conformations of proteins are associated with different catalytic rates (Xie and Lu, 1999). In addition, a number of enzymes are known to undergo conformational changes on binding of substrate (Koshland, 1987) that lead to enhanced catalysis two examples are hexokinase (Anderson and Steitz, 1975 Dela-Fuente and Sols, 1970) and triosephosphate isomerase (Knowles, 1991). 

A. E. Aleshin, C. Kirby, X. Liu, G.P. Bourenkov, H.D. Bartunik, H.J. Fromm, and R.B. Honzatko. 2000. Crystal structures of mutant monomeric hexokinase I reveal multiple ADP binding sites and conformational changes relevant to allosteric regulation J. Mol. Biol. 296 1001-1015. (PubMed)... 

The first and clearest example of the use of nucleoside phosphorothioates to determine coordination geometry at an active site was the work of Jaffe and Cohn on hexokinase (22). They found that (7 p)-ATP)3S is a far better substrate with Mg(II) as the activating cation than with Cd(Il) however, (5p)-ATP/3S is a far better substrate with Cd(ll) than with Mg(ll) as the metal ion. That is, the favored complex depended on both the metal ion and the configuration at P of ATP/8S, and a change in metal ion from Mg(II) to Cd(II) led to a change in selectivity for the configuration at P. The structures of the preferred substrates were thereby deduced as those shown below. The other isomers, Mg(Sp)-ATP/3S and Cd(7 p)-ATP)8S, were less active or practically inactive. By the simplest interpretation of the data, both active complexes were the A-screw sense isomers shown here, in which Mg is coordinated to oxygen in one and Cd is coordinated to sulfur in the other. 

An example of induced fit is shown in Figure 24.4. The three-dimensional structure of the enzyme hexokinase is shown before and after binding glucose, its substrate. Notice the change in conformation that occurs upon binding the substrate. 

Induced fit. Enzymes are flexible. In this regard, they are different from solid catalysts, like the metal catalysts used in chemical hydrogenation. After an enzyme binds its sub-strate(s), it changes conformation and forces the substrates into a strained or distorted structure that resembles the transition state. For example, the enzyme hexokinase closes like a clamshell when it binds glucose. In this conformation, the substrates are forced into a reactive state. 

Fig. 13. Upper Summary of the Rq measurements for native hexokinase ( ) and in the presence of glucose (a) and gIucose-6-phosphate ( ). The concentration dependence of the Rq values was taken from the best Rq data set for the native enzyme. The values extrapolated to zero concentration are 2.473, 2.378, and 2.348 tun in that order, with errors of 0.02 nm. Lower A drawing is shown of the projected outline of the actual crystal structures of unligated hexokinase (left) and hexokinase-glucose (right). In the absence of glucose (G), the enzyme has an open structure with a deep slit, while in the presence of glucose the conformation changes to give a closed structure (93).

Sachsenheimer and Schulz report the structure of another crystalline form of adenylate kinase which appears to be related to the previously known form by a conformation change in a segment. The binding sites of ATP and AMP to this enzyme were also located. Similar work on the identification of binding sites and the assessment of conformational changes has been continued with ribo-nuclease, alcohol dehydrogenase, concanavalin A, " hexokinase, lysozyme," flavodoxin, and oxy-erythrocrurin." ... 

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