Brain hexokinase

Brain hexokinase is inhibited by its product glucose-6-phosphate and to a lesser extent by adenosine diphosphate 539... 

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... 

C7. Crane, R. K., and Sols, A., The non-competitive inhibition of brain hexokinase by glucose-6-phosphate and related compounds. J. Biol. Chem. 210, 597-606 (1954). 

Haldane relationships can also be useful in characterizing isozymes or the same enzyme isolated from a different source. Reactions catalyzed by isozymes must have identical equilibrium constants, but the magnitudes of their kinetic parameters are usually different (e.g., the case of yeast and mammalian brain hexokinase ). Note that the Haldane relationship for the ordered Bi Bi mechanism is = Hmax,f p i iq/(f max.r ia b)- This same... 

For example, Bachelard used [Mgtotai]/[ATPtotai ] = 1 in his rate studies, and he obtained a slightly sigmoidal plot of initial velocity versus substrate ATP concentration. This culminated in the erroneous proposal that brain hexokinase was allosterically activated by magnesium ions and by magnesium ion-adenosine triphosphate complex. Purich and Fromm demonstrated that failure to achieve adequate experimental control over the free magnesium ion concentration can wreak havoc on the examination of enzyme kinetic behavior. Indeed, these investigators were able to account fully for the effects obtained in the previous hexokinase study. ... 

Figure 1. Plot of v/V ax versus the millimolar concentration of total substrate for a model enzyme displaying Michaelis-Menten kinetics with respect to its substrate MA (i.e., metal ion M complexed to otherwise inactive ligand A). The concentrations of free A and MA were calculated assuming a stability constant of 10,000 M k The Michaelis constant for MA and the inhibition constant for free A acting as a competitive inhibitor were both assumed to be 0.5 mM. The ratio v/Vmax was calculated from the Michaelis-Menten equation, taking into account the action of a competitive inhibitor (when present). The upper curve represents the case where the substrate is both A and MA. The middle curve deals with the case where MA is the substrate and where A is not inhibitory. The bottom curve describes the case where MA is the substrate and where A is inhibitory. In this example, [Mfotai = [Afotai at each concentration of A plotted on the abscissa. Note that the bottom two curves are reminiscent of allosteric enzymes, but this false cooperativity arises from changes in the fraction of total "substrate A" that has metal ion bound. For a real example of how brain hexokinase cooperatively was debunked, consult D. L. Purich H. J. Fromm (1972) Biochem. J. 130, 63.

Initial rate kinetics of bovine brain hexokinase (A) with glucose and ATP as substrates, and (B) with fructose and ATP. Note that the apparent parallel-line kinetics observed with glucose conform to a rate equation lacking a < 12 term. 

The results indicate that brain hexokinase does indeed operate by way of a sequential kinetic mechanism, and subsequent kinetic studies with reversible inhibitors support this conclusion. These comments reinforce the wisdom of remaining wary of mechanistic inferences drawn solely on the basis of initial rate studies alone. See also Initial Rate Enzyme Assays... 

Affinity reagent classical V-bromoacetyl-D- glucosamine brain hexokinase 62... 

Lewis BE, Schramm VL (2003) Binding equilibrium isotope effects for glucose at the catalytic domain of human brain hexokinase. J. Am. Chem. Soc. 125 4785-4798... 

Aleshin AE, Zeng C, Bartunik HD, Fromm HJ, Honzatko RB. Regulation of hexokinase I crystal structure of recombinant human brain hexokinase complexed with glucose and phosphate. J Mol Biol 1998 282 345-57. 

Liu F, Dong Q, Myers AM, Fromm HJ. 1991. Expression of human brain hexokinase in Escherichia coli Purification and characterization of the expressed enzyme. Biochem Biophys Res Commun 177 305-311. 

Two reports of a manganese-specific brain hexokinase [68,69] are of special interest since as this enzyme is normally considered magnesium-activated. Mn(H) is also reported to activate a phosphoinositide kinase, part... 

Brain Hexokinase, the Prototype Am-biquitous Enzyme John E. Wilson... 

Tissues vary, however, in the relative amounts of the various isoenzymes present, and isoenzymes have been named after the tissues in which they predominate. Thus, type I is called brain hexokinase, type II muscle, and type IV liver. Type III seems to be... 

Glucose easily penetrates the blood-brain barrier of nephrectomized and eviscerated animals, but its distribution in the brain is not affected by insulin. Fructose and ribose do not pass the brain-blood barrier at all. Yet fructose is a good substrate for brain hexokinase and is metabolized in brain homogenates. From the observations made in vitro, fructose seems to be a good substrate for brain metabolism in vivo. But such an interpretation is false since direct in vivo experiments have shown that fructose cannot enter the brain. 

The stimulation of brain hexokinase activity by hypophysectomy and adrenalectomy does not neces-... 

Sols A, Crane RK. Substrate specificity of brain hexokinase. J Biol Chem 210 581 -595, 1954 Kipnis DM, Cori CF. The penetration and phosphorylation of 2-deoxyglucose in the rat diaphragm. J Biol Chem 234 171-177, 1959. 

Womak, F. C. and Colvrick, S. P. (1979) Proton-dependent inhibition of yeast and brain hexokinase by aluminum in ATP preparations. Proc. Natl. Acad. Sci. USA 76, 5080. 

These results have been challenged by Mirsky and Stadie and their co-workers, who have not confirmed these observations. However, the recent work of Chaikoff and the experiments of Weil-Malherbe, who finds that the plasma of untreated diabetic patients inhibits brain hexokinase and that the degree of inhibition is correlated with the severity of the disease, strongly support the original contentions of the Cori group. 

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