Three nonequilibrium reactions in glycolysis , catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase, prevent simple reversal of glycolysis for glucose syn thesis (Figure 1). They are circumvented as follows.

Fig1. Major pathways and regulation of gluconeogenesis and glycolysis in the liver. Entry points of glucogenic amino acids after transamination are indicated by arrows extended from circles. The key gluconeogenic enzymes are shown in double bordered boxes. The ATP required for gluconeogenesis is supplied by the oxidation of fatty acids. Propionate is important only in ruminants. Arrows with wavy shafts signify allosteric effects; dash-shafted arrows, covalent modification by reversible phosphorylation. High concentrations of alanine act as a “gluconeogenic signal” by inhibiting glycolysis at the pyruvate kinase step.

Pyruvate & Phosphoenolpyruvate

Reversal of the reaction catalyzed by pyruvate kinase in glycolysis involves two endothermic reactions. Mitochondrial pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate, an ATP-requiring reaction in which the vitamin biotin is the coenzyme. Biotin binds CO₂ from bicarbonate as carboxybiotin prior to the addition of the CO₂ to pyruvate . The resultant oxaloacetate is reduced to malate, exported from the mitochondrion into the cytosol and then oxidized back to oxaloacetate. A second enzyme, phosphoenolpyruvate carboxykinase, catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate using GTP as the phosphate donor. In liver and kidney, the reaction of succinate thiokinase in the citric acid cycle produces GTP (rather than ATP as in other tissues), and this GTP is used for the reaction of phosphoenol pyruvate carboxykinase. This provides a link between citric acid cycle activity and gluconeogenesis, to prevent excessive removal (ie, anaplerosis and cataplerosis have to be equal) of oxaloacetate for gluconeogenesis, which would impair citric acid cycle activity.

Fructose 1,6-Bisphosphate & Fructose-6-Phosphate

 The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, for the reversal of glycolysis, is catalyzed by fructose 1,6-bisphosphatase. Its presence determines whether a tissue is capable of synthesizing glucose (or glycogen) not only from pyruvate but also from triose phosphates (eg, glycerol). It is present in liver, kidney, and skeletal muscle, but is probably absent from heart and smooth muscle.

Glucose-6-Phosphate & Glucose

 The conversion of glucose-6-phosphate to glucose is catalyzed by glucose-6-phosphatase. It is present in liver and kidney (renal cortex), but absent from muscle, which, therefore, cannot export glucose derived from glycogen into the bloodstream.

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مواضيع ذات صلة

Blood Glucose is Derived From the Diet, Gluconeogenesis, & Glycogenolysis

Glycolysis & gluconeogenesis share the same pathway but in opposite directions, & are reciprocally regulated

Glucose-1-Phosphate & Glycogen

Gluconeogenesis & the Control of Blood Glucose: Biomedical Importance

Glycogen Metabolism is Regulated by A Balance in Activities Between Glycogen Synthase & Phosphorylase

cAMP Activates Glycogen Phosphorylase

Cyclic AMP Integrates The Regulation of Glycogenolysis & Glycogenesis

Glycogenolysis is not the Reverse of Glycogenesis, it is A Separate Pathway

Glycogenesis Occurs Mainly in Muscle & Liver

The Oxidation of Pyruvate to Acetyl-COA is The Irreversible Route from Glycolysis to The Citric Acid Cycle

Glycolysis is Regulated at Three Nonequilibrium Reactions

Tissues That Function Under Hypoxic Conditions or have Intrinsically High Rates of Glucose Oxidation Produce Lactate