Other carbohydrate metabolism

Other carbohydrate metabolism - Chapter 23 Other Pathways...

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Unformatted text preview: Chapter 23 Other Pathways of Carbohydrate Metabolism Carbohydrate synthesis from simple precursors Sources of oxaloacetate Gluconeogenesis Pyruvate carboxylase Biotin and carboxybiotinyl-enzyme The reaction mechanism of pyruvate carboxylase PEP Carboxykinase The reaction mechanism of PEP carboxykinase The transport of PEP and oxaloacetate Regulation of gluconeogenesis • Substrate cycles • Fructose-2,6-bisphosphate (F26BP) – Activates phosphofructokinase – Inhibits fructose-1,6-bisphosphatase • [F26BP] is controlled by its enzymatic synthesis and degradation • Inhibition of pyruvate kinase by allosterism and phosphorylation Regulation of gluconeogenic enzymes F26BP F26BP Substrate cycles in glucose metabolism Regulation of PFK-1 and FBPase-1 Control of the formation and degradation of F26BP (F26BP) PFK-2 and FBPase-2 activities are located on different domains of the same protein (bifunctional protein) Effects of F26BP Hormonal regulation of [F26BP] [F26BP] Regulation of pyruvate kinase Cori cycle Glyoxylate cycle • Vertebrates cannot convert fatty acids or the acetate derived from them to carbohydrates • In plants, certain invertebrates, and some microorganism (including E. coli and yeast), the glyoxylate cycle convert the acetate to fourcarbon compounds, which are the precursors of the carbohydrate synthesis 2Acetyl-CoA + NAD+ + 2H2O → succinate + 2CoA + NADH + H+ Relationship between the glyoxylate and citric acid cycles Biosynthesis of oligosaccharides and glycoproteins • Nucleotide sugars power the formation of glycosidic bonds • O-linked oligosaccharides are posttranslationally synthesized in the Golgi apparatus • N-linked oligosaccharides are constructed on dolichol carrier Role of nucleotide sugars Synthesis of an O-linked oligosaccharide chain Dolichol pyrophosphate glycoside Dolichol-PP-oligosaccharide synthesis Pentose phosphate pathway (Hexose monophosphate shunt or phosphogluconate pathway) • An alternative pathway to glycolysis • Primarily anabolic rather than catabolic • Production of NADPH (a reducing power in biosynthetic pathways) by the oxidation of G6P • Synthesis of pentose sugars including ribose-5phosphate, an essential precursor in nucleotide biosynthesis NADPH and NADH are not metabolically interchangeable • NADH uses the free energy of metabolite oxidation to synthesize ATP (oxidative phosphorylation): [NAD+]/[NADH] = ~1000 • NADPH use the free energy of metabolite oxidation for reductive biosynthesis: [NADP+]/[NADPH] = ~ 0.01 NAD+ NADP+ Three stages of the pentose phosphate pathway 3 G6P + 6 NADP+ + 3 H2O ↔ 6 NADPH + 6 H+ + 3 CO2 + 2 F6P + GAP • Oxidative reactions – NADPH and ribulose-5-phosphates (Ru5P) 3 G6P + 6 NADP+ + 3 H2O ↔ 6 NADPH + 6 H+ + 3 CO2 + 3 Ru5P • Isomerization and epimerization reactions – Ribose-5phosphate (R5P) or xylulose-5-phosphate (Xu5P) 3 Ru5P ↔ R5P + 2 Xu5P • C-C bond cleavage and formation reactions R5P + 2 Xu5P ↔ 2F6P + GAP Stage 1: Oxidative reactions of NADPH production Stage 2: Isomerization and epimerization of ribulose-5-phosphate Stage 3: C-C bond cleavage and formation reactions Mechanism of transketolase Mechanism of transaldolase Relationship between glycolysis and the pentose phosphate pathway ...
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This note was uploaded on 10/16/2010 for the course CHEM 60280 taught by Professor Ryu during the Spring '09 term at TCU.

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