Gluconeogenesis Lecture 4 PDF

Gluconeogenesis Medical Biochemistry and Molecular Biology Department Faculty of Medicine – Menofia University  Gluconeogenesis Definition: - Is the synthesis of glucose or glycogen from non carbohydrate sources. These sources include - Lactic acid - Pyr...

Gluconeogenesis Medical Biochemistry and Molecular Biology Department Faculty of Medicine – Menofia University  Gluconeogenesis Definition: - Is the synthesis of glucose or glycogen from non carbohydrate sources. These sources include - Lactic acid - Pyruvic acid - Glycerol - Amino acids - Propionic acid. Site:- – Organ site: 90% in the liver and 10% in the kidney. – Cellular site: Cytoplasm and mitochondria.  Importance of Gluconeogenesis 1) It provides blood with glucose during fasting more than 18 hours when glycogen stores are depleted. Glucose is required; 1. It is the only source of energy for nervous tissue, RBCs and skeletal muscles during exercise. 2. It is the precursor of milk sugar (lactose) in mammary gland. 2) Gluconeogenesis help to clear the blood from waste products as lactic acid produced in sk. muscles and RBCs.  Steps Gluconeogenesis is mainly the reversal of glycolysis except for the 3 irreversible kinases which are overcomed by the following enzymes. Enzyme of glycolysis Enzyme of gluconeogenesis 1- Pyruvate kinase - Pyruvate carboxylase (mitochondrial). - Phosphoenol pyruvate carboxykinase 2- Phosphofructokinase-1 - Fructose 1.6 biphosphatase 3- Hexokinase & glucokinase - Glucose-6-phosphatase  1.Conversion of pyruvate to phosphoenol pyruvate:- Pyruvate carboxylase enzyme is present in the mitochondria so pyruvate must be transported by special pyruvate transporter to enter the mitochondria. While phosphoenol Pyruvate carboxykinase is present in the cytoplasm. The conversion occurs as follow. Pyruvate carboxylase Malate dehydrogenase Pyruvate Oxaloacetate Malate CO2, ATP, Mn2+, Biotin Mitochondria Cytosol Phosphoenol pyruvate Malate carboxykinase dehydrogenase Phosphoenol pyruvate Oxaloacetate Malate H2O+CO2+GDP GTP NADH+ H+ NAD 2. Conversion of fructose 1,6 biphosphate into fructose-6- phosphate: Fructose 1,6 biphosphatase Fructose 1,6 biphosphate Fructose -6-phosphate H2 O Pi 3. Conversion of glucose-6-phosphate into glucose: Glucose-6-phosphatase Glucose-6-phosphate Glucose H2O Pi  Gluconeogenic substances 1. Gluconeogenic amino acids. 2. Pyruvate and lactate. 3. Propionate; – This occurs only in ruminants. – Propionic acid is converted into succinyl CoA as follow:- 4. Glycerol:- – Glycerol is derived from lipids of adipose tissue during fasting. – It is converted as follow. Glycerol-3-P CH2-OH Glycerol kinase CH2-OH Dehydrogenase CH2-OH CH-OH CH-OH C=O CH2-OH ATP ADP CH2-O-P NAD NADH+ H + CH2-O-P Glycerol Glycerol-3-P Dihydroxy acetone-P – Then dihydroxy acetone phosphate binds the common pathway.  Energy cost of gluconeogenesis For conversion of 2 molecules of pyruvate to glucose, 6 ATP molecules and 2 NADH+ H+ are consumed. 1. ATP 2 Pyruvate 2 oxaloacetate (2 ATP) 2 Oxaloacetate 2 phosphoenol pyruvate (2 GTP) 2 3 Phosphoglycerate 2 1.3 biphosphoglycerate (2 ATP) 1. NADH+H+ 2 1.3 Biphosphoglycerate 2 glyceraldhde-3-P (2 NADH+H+)  Regulation of gluconeogenesis Gluconeogenesis and glycolysis occur in coordinated manner i.e when one pathway is active the other should be inhibited. The key regulatory enzymes of gluconeogenesis include: 1. Pyruvate carboxylase. 2. Phosphoenol pyruvate carboxykinase. 3. Fructose 1,6 biphosphatase. 4. Glucose - 6 - phosphatase. These enzymes are regulated as follow: 1) Changes in the rate of enzyme synthesis:- – Glucocorticoids e.g cortisol stimulates gluconeogenesis By 1. Inducing synthesis of the key enzymes. 2. Provide gluconeogenic amino acids by increasing protein catabolism. – Insulin: It inhibits gluconeogenesis by repressing the synthesis of the key enzymes. 2) Covalent modification:- – Glucagon and epinephrine stimulate gluconeogenesis by phosphorylation of pyruvate kinase (inactive) this inhibit glycolysis and stimulate gluconeogenesis. 3) Allosteric regulation:- Acetyl CoA: - – Is an allosteric activator of the enzyme pyruvate carboxylase. – It is derived from FA oxidation thus; FA oxidation stimulates gluconeogenesis by providing. A. Acetyl CoA which activates pyruvate carboxylase. B. ATP needed by pyruvate carboxylase and phosphoenol pyruvate carboxykinase. Fructose 2,6 biphosphate:- mentioned in glycolysis.