Glucokinase and hexokinase

Figure 3.20 A plot of the percentage of maximal activity of glucokinase and hexokinase against the glucose concentration. Hexokinase is present in most if not all cells. Glucokinase is only present in hepatocytes and p-cells of the Islets of Langerhans in the pancreas (known as the endocrine pancreas). The enzyme is physiologically very important in both tissues e.g. a low activity in the p-cells can be a cause of one type of diabetes mellitus.

There are several different glucose transporters. There are two glucose phosphorylating enzymes glucokinase and hexokinase (Chapter 6). 

FIGURE 15-16 Comparison of the kinetic properties of hexokinase IV (glucokinase) and hexokinase I. Note the sigmoidicity for hexokinase IV and the much lower Km for hexokinase I. When blood glucose rises above 5 itim, hexokinase IV activity increases, but hexokinase I is already operating near /max at 5 itim glucose and cannot respond to an increase in glucose concentration. Hexokinase I, II, and... 

Glucokinase and hexokinase are relatively inactive, so free glucose is not reconverted to glucose 6-phosphate. 

Fig. 11-9 Enzyme kinetics of glucokinase and hexokinase. This is a schematic only and shows glucokinase having a maximal velocity that is 10 times greater than that of hexokinase, while the K of hexokinase for glucose (0.1 mM) is 1/lOOth of that for glucokinase (10 mM).

Vinuela, E., Salas, M., Sols, A. Preliminary communication Glucokinase and hexokinase in liver in relation to glycogen synthesis. J. biol. Chem. 238, 1175-1177 (1963)... 

The main differences between glucokinase and hexokinase are fisted below ... 

Fructose can be metabolized by two routes. In adipose tissue and muscle, hexokinase can phosphorylate fructose to fructose 6-phosphate that then enters glycolysis. In liver, most of the enzyme present is glucokinase not hexokinase and this does not phosphorylate fructose. In this tissue, fructose is metabolized instead by the fructose 1-phosphate pathway. 

Four enzymes are unique to the process of gluconeogenesis and are required to circumvent the unidirectional steps of glycolysis (Fig. 13.1). Two of these are pyruvate carboxylase, which converts pyruvate to oxaloacetate, and PEP carboxykinase, which converts oxaloacetate to phosphoenolpyruvate. These enzymes are required to effectively reverse the action of pyruvate kinase. The other two are fructose-1,6-bisphosphatase and glucose-6-phosphatase, which effectively reverse the actions of PFK and hexokinase or glucokinase. 

There are three reactions that are counter-productive relative to gluconeogenesis. These are pyruvate kinase, PFK I, and glucokinase (or hexokinase). 

This enzyme also catalyses reactions with /3-D-glucose and other sugars. It catalyses the reaction whereby glucose-6-phosphate enters the pentose phosphate shunt. It may be used as an indicator reaction for glucokinase, or hexokinase. It is assayed by u.v. spectroscopy. 

Glycolysis is regulated by (i) glucose transporters (GLUTs), (ii) glucokinase or hexokinase, (in) phosphofructokinase 1, (iv) pyruvate kinase, and (v) pyruvate dehydrogenase. 

The way in which fungi sense the presence of D-glucose and signal carbon catabolite repression to CREl is only poorly understood. In A. nidulans and T. reesei a single glucokinase and a single hexokinase are present and only in the absence of both of them carbon catabolite derepression is observed (Flipphi et al 2003). Stranss et aL (1999) have shown data that suggested that A. nidulans requires a... 

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