Hexokinase binding energy

Hexokinase provides an excellent example of induced fit as a means of using substrate binding energy. 

The induction of the correct geometry in the active site of an enzyme is paid for by a good substrate, with binding energy. An alternative explanation to that of induced fit is that some small molecules (e.g., HzO in the hexokinase example) bind nonproductively, i.e., their small size allows them to assume many orientations with respect to the other substrate (ATP in the case of hexokinase) that do not lead to reaction. Large substrates are restricted in motion and are held in a catalytically correct orientation millions of times more often during molecular vibrations than is, say, water. 

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. 

Brain hexokinase is inhibited by its product glucose-6-phosphate and to a lesser extent by adenosine diphosphate. The isoenzyme of hexokinase found in brain may be soluble in the cytosol or be attached firmly to mitochondria [2 and references therein]. An equilibrium exists between the soluble and the bound enzyme. The binding changes the kinetic properties of hexokinase and its inhibition by Glc-6-P resulting in a more active enzyme. The extent of binding is inversely related to the ATP ADP ratio, i.e. conditions in which energy utilization... 

Bustamante E, Morris HP, Pedersen PL. Energy metabolism of tumor cells. Requirement for a form of hexokinase with a propensity for mitochondrial binding. J Biol Chem 1981 256 8699-8704. 

Recently we have found a type of tightly bound ATP which exists even in the presence of saturating concentrations of hexokinase (Aflalo, Shavit, 1982) but is exchangeable with exogenous ATP. This bound ATP appears to arise from a free species of ATP sequestered near the active site during photophosphorylation. The slowed down diffusion of the newly made ATP to the outer medium space allows its binding to proximal non-catalytic sites. Thus, mass transfer of nucleotides between the active site and the bulk medium may limit the steady state rate of ATP synthesis and should be considered in proposals on the mechanism of energy transduction. 

---