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Carbohydrate Biosynthesis

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Carbohydrate Biosynthesis

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    87. Gluconeogenesis, the synthesis of glucose from 3-carbon compounds (mainly pyruvate) is highly conserved in all organisms. Gluconeogenesis shares most of the reactions occurring in glycolysis, but bypassing the three irreversible reactions (using different enzymes). Gluconeogenesis consumes more energy than glycolysis releases. Most of the amino acids, but not fatty acids can be used for net production of glucose in vertebrates. The gluconeogenesis and glycolysis are reciprocally regulated by molecules like acetyl CoA, AMP, fructose 1,6BP.

    88. To limit futile cycling, the two pathways are under reciprocal allosteric control, mainly achieved by opposite effects of F2,6BP on PFK-1 and FBPase-1. Glucagon or epinephrine decreases F2,6BP. The hormones do this by raising [cAMP] and bringing about phosphorylation of the bifunctional enzyme, PFK-2/FBPase-2. Phosphorylation inactivates PFK-2 and activates FBPase-2, leading to breakdown of F2,6BP. Insulin increases [F2,6BP] by activating a phosphoprotein phosphatase that dephosphorylates (activates) PFK-2.

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