文档介绍：Chapter 20 Carbohydrate Biosynthesis
1. Gluconeogenesis: The universal pathway for synthesis of glucose.
2. Biosynthesis of glycogen, starch, and sucrose.
3. CO2 fixation in plants (the Calvin Cycle).
4. Regulation of carbohydrate metabolism in plants.
1. Carbohydrates are synthesized from simple precursors via gluconeogenesis
A few three-pounds (including lactate, pyruvate, glycerol, and 3-phosphoglycerate) serve as the major precursors for carbohydrate (glucose) biosynthesis, or gluconeogenesis.
The reactions of gluconeogenesis are essentially the same in anisms.
The conversion of pyruvate to glucose is the central pathway in gluconeogenesis.
2. The opposing pathways of glycolysis and gluconeogenesis have 3 reactions different and 7 reactions mon
The reversible reactions between pyruvate and glucose are shared by gluconeogenesis and glycolysis, but the irreversible reactions are different (“bypassed” in gluconeogenesis).
Opposing pathways of
glycolysis and gluconeogenesis:
with 3 different and mon
reactions
3. Pyruvate is converted to phosphoenoylpyruvate (PEP) via two alternative paths
In both paths, pyruvate is converted to oxaloacetate (with the catalysis of pyruvate carboxylase) in mitochondria.
In one path, oxaloacetate is converted directly to PEP in the matrix of mitochondria in a reaction catalyzed by the mitochondrial PEP carboxykinase isozyme, PEP is then transported to the cytosol for further conversion.
In another path, oxaloacetate is first converted to malate in the matrix, which is then transported to the cytosol, where it is converted to oxaloacetate, and then PEP in a reaction catalyzed by cytosolic PEP carboxykiase isozyme.
Both paths involve a carboxylation-decarboxylation sequence, acting as a unique way to activate pyruvate.
Two high-energy phosphate equivalents must be expended to convert one pyruvate to one PEP.
From pyruvate
to PEP: two
alternative paths
4. Conversion of fructose 1,6-bisphosphate to fructose 6-phosphate is the