Which is not a step of gluconeogenesis?
Correct Answer: Conversion of phosphoenolpyruvate to pyruvate
Description: Conversion of phosphoenolpyruvate to pyruvate is a step of glycolysis (not of gluconeogenesis).Reaction in gluconeogenesis o Gluconeogenesis involves glycolysis, the citric acid cycle plus some special reactions. Glycolysis and gluconeogenesis share the same pathway but in opposite direction. Seven reactions of glycolysis are reversible and therefore are used with same enzyme in the synthesis of glucose by gluconeogenesis. However, three of the reactions of glycolysis are irreversible and must be circumvented by four special reactions which are unique to gluconeogenesis and catalyzed by : (I) Pyruvate carboxylase, (ii) Phosphoenolpyruvate carboxykinase,Fructose-I,6-bisphosphatase , (iv) Glucose-6-phosphatase. These four enzymes are the key enzymes of gluconeogenesis (or gluconeogenesis enzymes). Among these four, pyruvate carboxylase is a mitocondrial enzyme and other three are cytoplasmic enzymes.All three irreversible steps of glycolysis should be bypassed for gluconeogenesis to occur. These three bypass steps are circumvented by four special reactions.A) First bypass (conversion of pyruvate into phosphoenol pyruvate) :- Conversion of pyruvate into phosphoenol pyruvate takes place through two reactions :?i) Carboxylation of pyruvate : - First, pyruvate enters the mitochondria and is conveed into oxaloacetate by pyruvate carboxylase. Pyruvate carboxylase is a mitochondrial enzyme, therefore this reactions occurs in mitochondria only.ii) Conversion of oxaloacetate to phosphoenolpyruvate : - Oxaloacetate produced in the mitrochondria cannot cross the membrane. It is first reduced to malate, which then moves across the mitochondrial membrane into the cytosol. Malate is, then, reoxidized to oxaloacetate in the cytosol. Oxaloacetate is conveed to phosphoenol pyruvate by phosphoenolpyruvate (PEP) carboxykinase.B) Second bypass : - Conversion of fructose-1,6-bisphosphate into fructose-6-phosphate is catalyzed by fructose-1,6-bisphosphatase. Its presence determines whether a tissue is capable of synthesizing glucose (gluconeogenesis) or glycogen (glyconeogenesis) not only from pyruvate, but also from triose phosphate. It is present in liver, kidney, and skeletal muscle, but is probably absent from hea and smooth muscle.C) Third bypass : - Conversion of glucose-6-phosphate to glucose is catalyzed by glucose-6-phosphatase.
Category:
Biochemistry
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