Hexokinase product inhibition

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... 

Many examples of product inhibition are to found. Some dehydrogenases are inhibited by NADH (a co-product of the reaction), e.g. PDH and isocitrate dehydrogenase (ICD), which are involved with the glycolysis and the TCA cycle are two such examples. Hexokinase isoenzymes in muscle (but not liver) and citrate synthase are inhibited by their products, glucose-6-phosphate and citrate respectively offering a very immediate fine tuning of reaction rate to match cellular requirements and possibly allowing their substrates to be used in alternative pathways. 

Specific activation or inhibition Transport of glucose can be increased or decreased by specihc compounds insulin increases the transport whereas phloridzin, a plant glycoside, inhibits glucose transport by muscle. Insulin increases glucokinase activity in liver, whereas a plant sugar, mannoheptulose, inhibits glucokinase activity. Hexokinase is inhibited by its product, glucose 6-phosphate. 

Rate experiments that are typically carried out in the presence of different concentrations of an alternative product (or product analog) while using the normal substrates . This approach can be particularly useful when the normal product cannot be used because it is unstable, insoluble, or ineffective (the latter indicated by a very high Ki value). Moreover, the normal product may be consumed as an essential substrate in a coupled assay system for the primary enzyme. Fromm and Zewe used the alternative product inhibition approach in their study of hexokinase. Wratten and Cleland later applied this procedure to exclude the Theorell-Chance mechanism for liver alcohol dehydrogenase. See Abortive Complexes... 

The inflowing glucose is phosphorylated by the enzyme hexokinase with a Km in the order of 0.1-0.2 mM [17]. It is therefore able to keep Gi low, so the net influx in practice is independent of Gj. This is, however, a truth with modifications. The activity of hexokinase is inhibited by its product G6P, so if G6P is not removed fast enough, the activity of the hexokinase goes down. 

Hexokinase does not yield parallel reciprocal plots, so the Ping Pong mechanism can be discarded. However, initial velocity studies alone will noi discriminate between the rapid equilibrium random and steady-state ordered mechanisms. Both yield ihe same velocity equation and families of intersecting reciprocal plots. Other diagnostic procedures must be used (e.g., product inhibition, dead-end inhibition, equilibrium substrate binding, and isotope exchange studies). These procedures are described in detail in the author s Enzyme Kinetics behavior and Analysis of Rapid Equilibrium and Steady-State Enzyme Systems, Wiley-Interscience (1975),... 

Hexokinase, in all cells, can phosphorylate other 6-carbon monosaccharides such as fructose and galactose, whereas glucokinase is specific for glucose and is only found in the liver. Hexokinase is inhibited by its product, glucose... 

To detect the presence of abortive ternary complexes, the kinetidst may raise the concentration of dissimilar substrate-product pairs (such as glucose-ADP and ATP-glucose-6-P in the hexokinase reaction, or acetaldehyde-NAD and ethanol-NADH in alcohol dehydrogenase reaction). This wiU lead to the inhibition of all exchanges irrespective of the kinetic mechanism, and provide useful information about the abortive complex formation (Wong Hanes, 1964 Wedler Boyer, 1972 Punch Allison, 1980, 2000). Nevertheless, product inhibition is stiU unrivaled as the means for detecting abortive complex formation. 

Renz A. and Stitt M. 1993. Substrate specificity and product inhibition of different forms of fructo-kinases and hexokinases in developing potato tubers. Planta 190 166-175. 

A good example of allosteric inhibition is given by hexokinase (HK) isoenzymes of muscle. The product of the HK reaction, glucose-6-P allosterically inhibits the enzyme, so matching the phosphorylation of glucose to its overall metabolism, helps to regulate... 

Hexokinase, the activity of which is inhibited by its product, glucose 6-phosphate, which is relieved by phosphate (i.e. phosphate can activate the enzyme when it is inhibited by glncose 6-phosphate). 

The glucose-6-P remains in the coordination sphere of the cobalt. Only one-half of the mixture of the two isomers was a substrate the part remaining was inert, although it did inhibit. The inert material and the product were separated and a degradation product of the inert material was crystallized. The absolute stereochemistry was determined by x-ray crystallography (44). The active isomer of Co(NH3)4ATP used by hexokinase was found to correspond to the A isomer of /i,y-bidentate MgATP (43). 

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