tailieunhanh - USMLE ROAD MAP BIOCHEMISTRY – PART 5

Phản ứng này quan trọng là xúc tác bởi men carboxylase pyruvate. b. ATP phục vụ như là một nhà tài trợ năng lượng cho các phản ứng của pyruvate với CO2. c. Pyruvate men carboxylase đòi hỏi biotin liên kết cộng hóa trị ràng buộc như một coenzyme CO 2 được gắn tạm thời trong lúc di chuyển. | 84 USMLE Road Map Biochemistry a. This important reaction is catalyzed by pyruvate carboxylase. b. ATP serves as an energy donor for the reaction of pyruvate with CO2. c. Pyruvate carboxylase requires covalently bound biotin as a coenzyme to which CO2 is temporarily attached during the transfer. d. Oxaloacetate can then enter the tricarboxylic acid TCA cycle to produce energy through oxidative phosphorylation or it may be used for gluconeogenesis. 2. To initiate gluconeogenesis oxaloacetate is reduced to malate which is then transported to the cytosol in the reverse of the malate shuttle. 3. Oxaloacetate is re-formed in the cytosol by oxidation of malate. 4. Oxaloacetate is decarboxylated and simultaneously phosphorylated to PEP. a. This step requires the enzyme PEP carboxykinase. b. GTP hydrolysis provides the energy for this reaction and serves as the phosphate donor. E. The reactions of glycolysis converting fructose 1 6-bisphosphate to PEP are reversible so that when glucose levels in the cell are low equilibrium favors the conversion of PEP to fructose 1 6-bisphosphate Figure 6-8 . F. Conversion of fructose 1 6-bisphosphate to fructose-6-phosphate overcomes another of the irreversible steps of glycolysis and is catalyzed by fructose 1 6-bisphosphatase Figure 6-8 . 1. This is an important regulatory site for gluconeogenesis. 2. The reaction is allosterically inhibited by high concentrations of AMP an indicator of an energy-deficient state of the cell. Chapter 6 Carbohydrate Metabolism 85 Phosphoenolpyruvate 2-Phosphoglycerate 3-Phosphoglycerate ADP NAD 1 3-Bisphosphoglycerate Glyceraldehyde 3-phosphate Dihydroxyacetone phosphate Fructose 1 6-biphosphate P Fructose 6-phosphate - Fructose 1 6-bisphosphatase AMP Fructose 2 6-bisphosphate Glucose 6-phosphate Glucose 6-phosphatase Glucose Figure 6-8. Conversion of phosphoenolpyruvate to glucose during gluconeogenesis. Except for the indicated enzymes that are needed to overcome irreversible steps of glycolysis all .