Biochemistry Topics
The first step of glycolysis phosphorylates glucose in order to trap it inside the cell. In most tissues, this is done by catalysis of hexokinase, which transfers a phosphoryl group from ATP to the 6' position of glucose.
Liver cells contain an additional isoenzyme, glucokinase, which has a high Km to cope with the high glucose concentration taken up from the portal vein by GLUT2.
Like other enzymatic reactions using ATP, both hexokinase and glucokinase require Mg2+ as a cofactor to facilitate phosphoryl transfer by chelating the negative charges. Glucokinase has a Km ~ 10 mM for glucose, while that of hexokinase is < 0.1 nM. Since the Km of hexokinase is lower tan the normal blood glucose level, it usually operates at maximal velocity, while the activity of glucokinase is proportional to the glucose blood concentration.
Hexokinase is strongly inhibited by its product, by glucose 6-phosphate, and slowly controlled by induction and repression, requiring several hours to notice a change. Glucokinase is not inhibited by glucose 6-phosphate, and it seems to be induced by insulin because the diabetic liver is deficient in it. Both enzymes are regulated by the ratio of substrate to product concentrations.
Galactose can also be converted to glucose 6-phosphate in order to enter glycolysis. It is first converted to galactokinase, which is the epimerazed at C-4 to glucose 1-phosphate by a mutase. The epimerization reaction involves an important activated derivative of glucose, UDP-glucose.
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Fructose Phosphates
The second step of glycolysis is the conversion of glucose 6-phosphate into fructose 6-phosphate in a reversible reaction catalyzed by glucose 6-phosphate isomerase. The reverse reaction is the next to last step in gluconeogenesis.
In the third step of glycolysis, fructose is further phosphorylated in its 1' position by phosphofructokinase 1.
This seems to be the committed step of glycolysis until now glucose 6-phosphate could have gone to other pathways, like glycogen synthesis or the pentose phosphate pathway. In addition, dietary fructose can enter glycolysis at this reaction after being phosphorylated in muscle by hexokinase to fructose 6-phosphate.
In the fourth step of glycolysis, aldolase cleaves fructose 1,6-bisphosphate into two triose phosphates. The reverse reaction forms fructose 1,6-bisphosphate during gluconeogenesis.
Dietary fructose can also enter glycolysis in a similar series of reactions in the liver, were fructokinase phosphorylates fructose at the 1' position. Fructose 1-phosphate is the cleaved by fructose 1-phosphate aldolase, yielding glyceraldehyde and dihydroxyacetone phosphate. A triose kinase is required to phosphorylate the glyceraldehyde.
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