Hexokinase conformational changes

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). 

Schematic representation of the change in conformation of the hexokinase enzyme on binding substrate. E and E are the inactive and active conformations of the enzyme, respectively. G is the sugar substrate. Regions of protein or substrate surface excluded from contact with solvent are indicated by a crinkled line. Figure 8.3 presents a more detailed view of the hexokinase molecule. (Source From W. S. Bennett and T. A. Steitz, Glucose-induced conformational changes in yeast hexokinase, Proc. Natl. Acad. Sci. USA 75 4848, 1978.)...

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)... 

Bennett, W. S., and Steitz, T. A. Glucose-induced conformational change in yeast hexokinase. Proc. Natl. Acad. Sci. USA 75, 4848-4854 (1978). 

McDonald, R. C., Steitz, T. A. and Engelman, D. M. (1979) Yeast Hexokinase in Solution Enhibits a Large Conformational Change upon Binding Glucose or Glucose 6-Phosphate, Biochemistry 18, 338-342. 

Less dramatic conformational changes are involved in cleft closure or closure of lids over active sites in many enzymes, including triosephosphate isomerase, hexokinase, phosphonatase, and adenylosuccinate lyase." Such motions sequester active sites, preventing escape of reactive intermediates or inadvertent reaction with the solvent, and often help to orient substrates and/or catalytic residues. 

Induced fit can therefore only be used to explain the rates of reactions of very poor substrates compared with very good ones, e.g. the rate of phosphoryl transfer to water compared with that to glucose catalysed by hexokinase. Although water can almost certainly bind to the active site it must have insufficient binding energy to induce the necessary conformational change in the enzyme. 

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