Gluconeogenesis (de novo synthesis of glucose)
Gluconeogenesis Gluconeogenesis is the biosynthesis of new glucose. The main purpose of gluconeogenesis is to maintain the constant blood Glc concentration. Gluconeogenesis occurs mainly (90%) in the liver and in the cortex of kidney (10%). The major noncarbohydrate precursors: lactate amino acids glycerol gy
Rections of gluconeogenesis Seven of the reactions of glycolysis are reversible and are used in the synthesis of Glc from lactate or pyruvate. 3 of the reactions, catalyzed by hexokinase, phosphofructokinase and pyruvate kinase are irreversible and must be circumvented by alternate reactions: - Carboxylation of pyruvate. - Decarboxylation and phosphorylation of cytosolic OA. - Dephosphorylation of F1.6 bisp. - Dephosphorylation of G-6-P.
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1. Carboxylation of pyruvate (Pyruvate to Phosphoenolpyruvate (PEP)) Conversion of pyruvate to PEP requires the action of two mitochondrial enzymes. The first is an ATP-requiring reaction catalyzed by pyruvate carboxylase, (PC). Pyruvate carboxylase contains biotin that is covalently bound to apoenzyme. PEP carboxykinase (PEPCK) requires GTP in the decarboxylation of OA to yield PEP.
Human cells contain almost equal amounts of mitochondrial and cytosolic PEPCK.
2. Fructose-1,6-bisphosphate to Fructose-6- phosphate (Dephosphorylation of F1,6bisP) The reaction is catalyzed by fructose-1,6- bisphosphatase (F1,6BPase). F1,6BPase reaction is a major point of control of gluconeogenesis.
3. Dephosphorylation of Glucose-6-phosphate to Glucose G6P is converted to glucose through the action of glucose-6-phosphatase (G6Pase).
The net equation of gluconeogenesis 2Pyruvate + 4ATP + 2GTP + 2NADH + H + + 6H 2 O Glucose + 2NAD + + 4ADP + 2GDP + 6Pi + 6H +
Substrates for Gluconeogenesis Lactate: Lactate is released into blood by cells that lack mitochondria, such as red blood cells and exercising skeletal muscle. Lactate diffuses out of active skeletal muscle into the blood and is carried to the liver. In the liver lactate is oxidized to pyruvate. Pyruvate is converted into Glc by the gluconeogenic pathway in liver. Glc enters the blood and is taken up by skeletal muscle. In skeletal muscles Glc is converted to lactate by the glycolysis pathway. This relationship between glycolysis in muscles and gluconeogenesis in liver is called the Cori cycle.
Glycerol:
Amino Acids: All 20 of the amino acids, except leucine and lysine, can be degraded to TCA cycle intermediates.
Regulation of Gluconeogenesis Regulation by energy levels within the cell: F-1.6-bisphosphatase is inhibited by elevated levels of AMP. Conversely, high levels of ATP and low of AMP stimulate gluconeogenesis. g Allosteric regulation: Pyruvate carboxylase is an allosteric enzyme activated by acetyl-coa. Fructose 1,6-bisphosphatase is an allosteric enzyme inhibited by fructose 2,6-bisphosphate. p Regulation through the amount of enzymes: PEP carboxykinase, fructose 1,6- bisphosphatase and glucose 6- phosphatase are inducible enzymes whose amounts are increased in respose to hormones (e.g. glucagon and glucocorticoids hormones of adrenal cortex).
Pentose phosphate pathway (PPP)
Functions of PPP Provides NADPH as a reductant Provides ribose 5-phosphate needed for synthesis of nucleotides (e.g. AMP) and nucleic acids. Converts glucose into other sugars.
Location In tissues, the pathway is most active in the liver, mamary glands, adipose tissue and the adrenal cortex. Within the cell the enzymes of pathway are located in Within the cell, the enzymes of pathway are located in the cytoplasm.
Stages of the PPP Hexokinase Glucose 6-P dehydrogenase 6-Phosphogluconate dehydrogenase OXIDATIVE BRANCH BRANCH ATIVE B N-OXIDA NON
The stoichiometry of the pentose phosphate pathway: 3 G6P + 6 NADP + 2 F6P + 3 CO 2 + G3P + 6 NADPH + 6 H +
Regulation The cellular concentration of NADP+ is the major factor in regulating flux through the pathway. Its availability regulates the rate-limiting G6PD reaction.
To read at home: Metabolism of fructose and galactose