Enzyme hexokinase

ATP 2 ADR The structure was determined to 3.0 A resolution in the laboratory of Georg Schulz in Heidelberg, Germany, (c) The ATP-binding domain of the glycolytic enzyme hexokinase, which catalyzes the phosphorylation of glucose. 

A kinetic procedure employing the reverse reaction is coupled to the enzymes hexokinase and glucose-6-phosphate dehydrogenase, as used by Nielson and Ludvigson after the method of Oliver (J ). This procedure was later modified and optimized by Rosalki (38). 

The principle nnderlying this coupling is also explained by reference to the Gibbs free energy eqnation. An example, which iUnstrates the principle, is the phosphorylation of glncose in the reaction catalysed by the enzyme hexokinase ... 

There are two different glucose-phosphorylating enzymes, hexokinase and glucokinase, which catalyse the reaction ... 

The transporter binds glucose at a specific site, then changes its conformation, which results in transport of glucose across the membrane to be released on the other side. The enzyme hexokinase (or glucokinase) binds glucose at a specific site, then catalyses its phosphorylation and releases the product, glucose 6-phosphate. 

The physiological usefulness of this method for identifying the fuels that are used and their rates of utilisation in different cells is discussed in other chapters (Chapter 3). For example, measurement of the activity of the enzymes hexokinase and glutaminase in immune cells showed, for the first time, that glucose and glutamine are the major fuels utilised by these cells. This finding has had clinical significance (Chapter 17). 

B. Two key enzymes, hexokinase and glucokinase, catalyze the reaction of glucose with ATP to form glucose 6- ph osphate, which becomes trapped in the cell and subject to metabolism. 

Protection of an Enzyme against Denaturation by Heat When enzyme solutions are heated, there is a progressive loss of catalytic activity over time due to denaturation of the enzyme. A solution of the enzyme hexokinase incubated at 45 °C lost 50% of its activity in 12 min, but when incubated at 45 °C in the presence of a very large concentration of one of its substrates, it lost only 3% of its activity in 12 min. Suggest why thermal denaturation of hexokinase was retarded in the presence of one of its substrates. 

Figure 9-7 A shows the rate of exchange of isotopi-cally labeled glucose (glucose ) with glucose 6-phosphate as catalyzed by the enzyme hexokinase (Chapter 12). The exchange rate is plotted against the concentration of glucose 6-phosphate with the ratio [glucose] / [glucose 6-phosphate] constant at 1 /19, such that an equilibrium ratio for reactants and products is always...

Unlike many enzymes, hexokinase has a broad substrate specificity. A partial list of substrates for the sugar site is presented in Table II. Structures for some of these compounds are given in Fig. 10. It should be pointed out that kinetic constants for the various substrates were obtained under a variety of conditions with different forms and modifications of the enzyme. 

Fig. 9. Sequence of transformations catalysed by the supramolecular ATP-generating system [38, AcP, Mg2, ADP] (38 = [24]-N6C>2) and the enzymes hexokinase (HK), glucose-6-phosphate dehydrogenase (G-6-PDH) and 6-phospho-gluconate dehydrogenase (6-P-GDH) [5.32],...

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. 

Example 8.4. Phosphate transfer catalyzed by hexokinase. The enzyme hexokinase catalyzes transfer of phosphate to C6 sugars. Table 8.2 shows results reported for the reaction... 

This free glucose molecule is phosphorylated by the glycolytic enzyme hexokinase. Thus, the transferase and a-1,6-glucosidase convert the branched structure into a linear one, which paves the way for further cleavage by phosphorylase. It is noteworthy that, in eukaryotes, the transferase and the a-l,6-glucosidase activities are present in a single 160-kd polypeptide chain, providing yet another example of a bifunctional enzyme (Section 16.2.2). Furthermore, these enzymes may have additional features in common (Section 21.4.3). 

Phosphorylation and isomerization. Glucose, produced by the digestion of dietary carbohydrate., is first phosphorylated at the hydroxyl group on C6 by reaction with ATP in a process catalyzed by the enzyme hexokinase. The glucose 6-phosphate that results is isomerized by glucose 6-phosphate isomerase to fructose 6-phosphate. As the open-chain structures in Figure 29.4 show, this isomerization reaction takes place by keto-enol tautomerism (Section 22.1), since both glucose and fructose share a common enol ... 

Enzymes hexokinase in all tissues and also, in the liver, glucoki-nase. Both of these enzymes are subject to regulatory mechanisms. 

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