Phosphorylation of Glucose

Glucose is phosphorylated by hexokinase (in extrahepatic tissues) or glucokinase (in the liver). 

The Mg is complexed with ATP and is present as MgATP. This reaction, essentially an irreversible 

Four isoenzymes of hexokinases (types I-IV) constitute a family of enzymes that probably arose from a common ancestral gene by gene duplication and fusion events. Hexokinases I-III (M.W. 100,000) have a Km of about 0.1 mM and are allosterically inhibited by glucose-6-phosphate. Due to their low K, hexokinases I-III are saturated with the substrate glucose at 

Enzyme Coenzymes and Cofactors Allosteric Modulators Positive Negative Equilibrium Constant atpH7.0(K ) AG° kcal/mol (kj/mole) 

Pyruvate kinase Mg2+, K+ Fructose 1,6 bisphosphate ATP, alanine, acetyl-CoA 200,000 -7.5 (-31.4) (nonequilibrium)  

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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. 

Hexokinase catalyzes the phosphorylation of glucose from ATP, yielding glncose-6-P and ADR Using the values of Table 3.3, calculate the standard-state free energy change and equilibrium constant for the hexokinase reaction. 

FIGURE 10.26 Glucose transport in E. coli is mediated by the PEP-dependent phosphotransferase system. Enzyme I is phosphorylated in the first step by PEP. Successive phosphoryl transfers to HPr and Enzyme III in Steps 2 and 3 are followed by transport and phosphorylation of glucose. Enzyme II is the sugar transport channel. 

Phosphorylation of glucose and conversion to 2 molecules of glyceraldehyde-3-phosphate ... 

Reaction 1 Phosphorylation of Glucose hy Hexokinase or Glucokinase—The First Priming Reaction... 

FIGURE 19.4 Phosphorylation of glucose to glucose-6-phosphate by ATP creates a charged molecule that cannot easily cross the plasma membrane. 

Glycolysis glucose to pyruvate (cytosol) Phosphorylation of glucose -1 -1... 

Yet another difference is that laboratory reactions are often done using relatively small, simple reagents such as Br2, HC1, NaBH4, OO3, and so forth, while biological reactions usually involve relatively complex "reagents" called coenzymes. In the hexokinase-catalyzed phosphorylation of glucose just shown,... 

When (1) and (2) are coupled in a reaction catalyzed by hexokinase, phosphorylation of glucose readily proceeds in a highly exergonic reaction that under physiologic conditions is irreversible. Many activation reactions follow this pattern. 

Another enzyme used for the measurement of glucose is hexokinase (EC 2.7.1.1) which catalyses the phosphorylation of glucose to produce glucose-6-phosphate with adenosine triphosphate as the phosphate donor and magnesium ions as an activator. The rate of formation of glucose-6-phosphate can be linked to the reduction of NADP by the enzyme glucose-6-phosphate dehydrogenase (EC 1.1.1.49). This indicator reaction can be monitored spectrophotometrically at 340 nm or fluorimetrically ... 

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... 

The first step in glycolysis is the phosphorylation of glucose to give the ester glucose 6-phosphate. The glucose starting material may well have come from hydrolysis of starch obtained in the diet, or by utilization of glycogen reserves. 

The biological activity of magnesium is manifold. It serves as an indicator for many enzyme reactions, predominantly phosphate metabolism (e.g. as a co-factor in the phosphorylation of glucose). It also plays a part in the regulation of the neuromuscular excitation process. 

Calculate Keq for the overall reaction. For the ATP-dependent phosphorylation of glucose, what concentration of glucose is needed to achieve a 250 /xm intracellular concentration of glucose 6-phosphate when the concentrations of ATP and ADP are 3.38 mM and 1.32 mM, respectively Does this coupling process provide a feasible route, at least in principle, for the phosphorylation of glucose in the cell Explain. 

Glycogen is synthesized from molecules of a-D-glucose. The process occurs in the cytosol, and requires energy supplied by ATP (for the phosphorylation of glucose) and uridine triphosphate (UTP). 

The net reaction at this point produces more ATP than is consumed in the phosphorylation of glucose and fructose (see Exercise 20-20). 

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