Enzymes glucose kinase

The biological transformations that involve ATP are both numerous and funda mental They include for example many phosphorylation reactions m which ATP trans fers one of its phosphate units to the —OH of another molecule These phosphoryla tions are catalyzed by enzymes called kinases An example is the first step m the metabolism of glucose... 

Feed-forward control is more likely to be focused on a reaction occurring at or near the end of a pathway. Compounds produced early in the pathway act to enhance the activity of the control enzyme and so prevent a back log of accumulated intermediates just before the control point. An example of feed-forward control is the action of glucose-6-phosphate, fructose-1,6-bisphosphate (F-l,6bisP) and phosphoenol pyruvate (PEP), all of which activate the enzyme pyruvate kinase in glycolysis in the liver. 

Serious diseases often result from the inability of the T organism to metabolize certain substances that are ingested as nutrients. In such cases the substance accumulates and often causes irreversible harm to the liver or other vital organs. Galactosemia is a disease that results from the inability to convert galactose into glucose. Reference to figure 16.2 shows that three enzymes, a kinase, a transferase, and an epimerase are required for this conversion. In the... 

As an example of enzyme action, look in Figure 24.11 at the enzyme hexose kinase, which catalyzes the reaction of adenosine triphosphate (ATP) with glucose to yield glucose-6-phosphate and adenosine diphosphate (ADP). The enzyme first binds a molecule of ATP cofactor at a position near the active site, and glucose then bonds to the active site with its C6 hydroxyl group held rigidly in position next to the ATP molecule. Reaction ensues, and the two products are released from the enzyme. 

Figure 2.5 Gluconeogenesis is the reversal of glycolysis, attained through the use of four unique enzymes glucose-6-phosphatase (A), fructose-1,6-bisphosphatase (5), PEP carboxykinase (6) and pyruvate carboxylase (7). Although phosphoglycerate kinase is shared with glycolysis, in gluconeogenesis this reaction requires the input of ATP.

Because of the very high price of ATP, reaction (5.7) must be coupled with a regenerating system, the transfer of phosphate to ADP starting from the enol phosphate of pyruvic acid (an easily accessible and inexpensive phosphate), catalysed by the enzyme pyruvate kinase (reaction (5.8). In the same flask are mixed glucose, phosphoenolpyruvate, hexokinase, pyruvate kinase, and a catalytic quantity of ATP (about 1% mol) and the system produces D-glucose 6-phosphate until the phosphoenolpyruvate runs out. The kinases are easily accessible and, if they are immobilized on an insoluble support (see Section 10.4.1), they are reusable a certain number of times. In this way glucose 6-phosphate can be easily prepared on a 250 g scale (Poliak et al. 1977). 

The adipocyte, or fat cell, is designed for continuous synthesis and breakdown of triacylglycerols, with breakdown controlled largely via the activation of hormone-sensitive lipase. Because adipocytes lack the enzyme glycerol kinase, some glucose catabolism must occur for triacylglycerol synthesis to take place—specifically, the formation of dihydroxyacetone phosphate, for reduction to glycerol-3-phosphate (see here). 

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