Ans. is 'a' i.e., Fructose -1,6 phosphatase Rate limiting steps in various biochemical reactions taking place inside cells is of vital importance while formulating drugs to fight specific diseases affecting these reactions. So these rate-limiting steps are frequently asked in various entrance exams. Glycolysis - Phosphofructokinase Glycogen synthesis - Glycogen synthetase Glycogenolysis - Glycogen Phosphorylase Gluconeogenesis - Phosphoenolepyruvate; Fructose 1,6 biphosphatase TCA cycle- Isocitrate dehydrogenase Cholesterol synthesis- HMG CoA Reductase Bile acid synthesis - 7 alpha hydroxylase Fatty acid synthesis - acetyl CoA carboxylase Ketone body synthesis -HMG CoA synthetase Uric acid synthesis - Xanthine Oxidase Norepinephrine(catecholamine) synthesis- Tyrosine hydroxylase Tyrosine------->DOPA(Tyrosine hydroxylase) Acetylcholine synthesis- Uptake of choline Urea synthesis - Carbamoyl transferase Bio-chemistry facts - Rate limiting step in synthesis of bile salts: cholesterol hydroxylation= Enzyme: 7 alpha -hydroxylase - Rate limiting step in Glycolysis: Fructose 6-phosphate to fructose-1,6 biphosphate = Enzyme: phosphofructokinase (PFK) - Rate limiting step in Fatty acid synthesis: Carboxylation of Acetyl CoA: Enzyme: Acetyl CoA carboxylase (biotin dependent) - Rate limiting step in cholesterol synthesis: Mevalonate formation: Enzyme: HMG CoA reductase - Rate limiting step in HMP pathway: Glucose-6-phosphate to 6-phospho gluconolactone : Enzyme : Glucose 6-phosphate dehyrogenase - Rate limiting step in Gluconeogenesis: Fructose-1,6 bisphosphate to fructose-6-phosphate : Enzyme : Fructose -1,6 phosphatase (rate limiting step/enzyme) Reactions of gluconeogenesis o Gluconeogenesis involves glycolysis, the citric acid cycle plus some special reactions. Glycolysis and gluconeogenesis share the same partway 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 : (1) Pyruvate carboxylaseQ, (ii) Phosphoenolpyruvate carboxykinase, (iii) Fructose-1,6- bisphosphataseQ, (iv) Glucose-6-phosphatase. These four enzymes are the key enzy mes of gluconeogenesis (or gluconeogenesis enzymes). Among these four, pyruvate carboxylase is a mitochondrial enzyme and other three are cytoplasmic enzymes. o All three irreversible steps of glycolysis should be bypassed for gluconeogenesis to occur. These three bypass steps are circumvented by four special reactions. First bypass (conversion of pyruvate into phosphoenol pyruvate): - Conversion of pyruvate into phosphoenol pyruvate takes place through two reactions. Carboxylation of pyruvate : - First, pyruvate enters the mitochondria and is converted into oxaloacetate by pyruvate carboxylase. Pyruvate carboxylase is a mitochondrial enzyme, therefore this reactions occurs in mitochondria only. 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 converted to phosphoenol pyruvate by phosphoenolpyruvate (PEP) carboxykinase. Second bypass: - Conversion of fructose-1,6-bisphosphate into fructose-6-phosphate is catalyzed by fructose- 1,6-bisphosphatase (PGI 05). Its presence determines w hether a tissue is capable of synthesizing glucose (gluconeogenesis) or glycogen (glyconeogenesis (PGI 05).) not only from py ruvate, but also from triose phosphate. It is present in liver, kidney, and skeletal muscle, but is probably absent from heart and smooth muscle. C) Third bypass : - Conversion of glucose-6-phosphate to glucose is catalyzed by glucose-6-phosphatase.