BIOC 3560 Carb metabolism.docx

Inhibited by fructose 6 phosphate not glucose 6

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Inhibited by fructose 6-phosphate not glucose 6-phosphate o Inhibition by fructose 6-phosphae effected through glucokinase regulatory protein Hexokinase IV has relatively poor K0.5 Hexokinase I found in muscle does not increase its rate when blood glucose concentration is higher than optimal (~5 mM) Hexokinase IV has a much higher Km (~10mM) With hexokinase IV, the liver responds directly to increasing blood glucose concentration with increased turnover (converting substrate to product) Regulation of hexokinase IV When concentrations of fructose 6P is high, glucokinase regulatory protein sequesters hexokinase IV in the nucleus High concentrations of glucose weakens the enzyme/regulator interaction, encouraging cytosolic localization Phosphofructokinase-1 allostery Glucose 6-P has several possible fates in the cell Phosphorylation by PFK-1 commits fructose 6-P (in equilibrium with G6P) to glycolysis Allosteric regulation of PFK-1 is complex: o ATP binds to an allosteric site on PFK-1 and lowers affinity for fructose 6-P o ADP and AMP relieve inhibition by ATP o Citrate increases the inhibition by ATP o Fructose 2,6-biphosphate is a strong activator Regulation of PFK-1 by ATP High concentrations of ATP greatly reduces the affinity of PFK-1 for fructose 6-P When concentrations of ATP are low, higher F6P affinity allows PFK-1 to be more active Pyruvate kinase is inhibited by ATP Pyruvate kinase catalyzes the last step in glycolysis PK transfers Pi from phosphoenolpyruvate to ADP This yields pyruvate and a molecule of ATP
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High concentrations of ATP allosterically inhibits PK, decreasing its affinity for PEP PK inhibition Acetyl-CoA and long chain FAs also inhibit PK o Important fuels for citric acid cycle o When plentiful, so is ATP Other allosteric modulators of PK: o Alanine (-) o F 1,6-BP accumulation (+) Gluconeogenesis: the solution The body needs a steady supply of glucose to fuel certain key organs (eg. brain) However, our bodies can only store about a 1 day supply (glycogen) The body also needs to resynthesize glucose from the lactic acid produced from anaerobic exercise Gluconeogenesis is the synthesis of glucose from non-hexose precursors In gluconeogenesis, 7 of the 10 steps are the glycolytic reactions but run in reverse The 3 irreversible steps of glycolysis are bypassed Here, different enzymes catalyze one or more different steps to enable the reverse reaction o Step 10 of glycolysis is step 1 of gluconeogenesis: pyruvate is converted to oxaloacetate using pyruvate carboxylase (2 ATP 2ADP) and oxaloacetate is converted to phosphoenolpyruvate using PEP carboxylase (2 GTP 2GDP) o Step 3 of glycolysis is step 8 of gluconeogenesis: F 1,6-BP is converted to F6P by fructose 1,6-biphosphatase-1 (H2O Pi) o Step 1 of glycolysis is step 10 of gluconeogenesis: G6P is converted to glucose by glucose 6-phosphatase (H2O Pi) Precursors of gluconeogenesis Main precursors for gluconeogenesis are: o Lactic acid (via pyruvate) o Glycerol (from lipids) o Glucogenic amino acids In the mitochondria
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