University:
Amherst College
Course:
BIOL 330 | Biochemical Principles of Life at the Molecular Level
Academic year:

2011
Views:

120

Pages:

14
Author:

Zara Mcneil

Carbohydrate Metabolism Nov 3, 2011 • Intro to Metabolism –ATP, the energy currency of the cell –sugar structure •Glycolysis Phase I – gly 1-5 •Glycolysis Phase II – gly 6-10 •Control in Glycolysis Control of Glycolysis • Energy Coupling (review phosphoryllation potential slide 5) – Gly 3? driven by ATP hydrolysis • uphill part is phosphorylation of sugar Major control point – Gly 7? driven by BPG hydrolysis • uphill part is ATP synthesis Not likely control since it is the last step in glycolysis – Gly 10? driven by PEP hydrolysis 2 • uphill part is ATP synthesis Free Energy 0 -2 -4 -6 Phosphoglceratekinase gly 7 -8 -10 Pyruvate kinase gly10 Phosphofructokinase gly 3 -12 Start Hexokinase PGI PFK Aldolase TIM G-3-P DH PGK PGM Enolase PK Free Energy of Hydrolysis 20 mM in heart..Phosophocreatine 10 mM in heart… ATP PFK--Committed Step: Allosteric Control Dimer of PFK shown Substrate binding sites in center Allosteric sites for ATP R state, high affinity for F6P T state, low affinity for F6P PFKT ↔ PFKR Only T state conformation binds ATP at inhibitor site, high ATP, shift to T, low affinity for f6P Substrate Cycling phosphatase kinase glycolysis gluconeogenesis Different enzymes catalyze the forward and backward reactions Reciprocal Regulation: Fructose 2,6 bisphosphate stimulates kinase and inhibits phosphatase F2,6BP production F2,6 BP is made from F6P by PFK-2, a different enzyme that ALSO has a phosphatase activity associated with it. In the liver, the PFK-2 system (11 and 12 in image at left) is under the control of glucagon, a major hormone that signals when glucose is low and glycogen needs to be made (glycolysis inhibited). Glycogen causes (eventually) the phosporyllation of the PFK-2 system and shuts down the production of F2,6BP which shuts down glycolysis. Control of Glycolysis (1) • Velocity = Vmax [St] = kcat [Et] [St] • Km + [St] Km + [St] • Typical enzyme concentrations, pM-mM • How can Enzyme levels be controlled? – Sequestered storage, triggered release – Zymogens (inactive precursors) • quick inefficient – Transcriptional activation (small molecule metabolites or hormones bind to the genes) • slow, efficient – mRNA processing activation; (small molecules bind to untranslated nascent mRNA and affect translation) riboswitches • quick efficient Control of Glycolysis (2) • Velocity = Vmax [St] = kcat [Et] [St] • Km + [St] Km + [St] • Typical substrate concentrations, 10mM-10 mM • How can substrate levels be controlled? – Sequestered storage (glycogen stores in muscle/liver), hormone triggered release (glucagon) – Conversion of related molecule (lactate to pyruvate) – Hunger signal to organism (hormone) Control of Glycolysis (3) • Velocity = Vmax [St] = kcat [Et] [St] • Km + [St] Km + [St] • Typical kcat, 102-106 s-1 • How can kcat be increased/decreased? – Allosteric effectors (example PFK-gly3) – Reversible covalent modification • Phosphoryllation, adenylation, methylation, acetyllation, others (example pyruvate dehydrogenase) Control of Glycolysis (3b) • Velocity = Vmax [St] = kcat [Et] [St] • Km + [St] Km + [St] • Typical Km, 10-1000 uM • How can Km be increased/decreased? – Self Inhibition Fates of Glucose: Fermentation LDH Mechanism: NADH redox B H O O H3 C C NH2 NH2 HS H COO HR O N R H3 C E'o = +157 m V overall favorable as written. H C H O HS H N+ R COOR •redox potential of NADH varies in different enzymes: • transfer of the proHR or proHs hydride to substrate depends on enzyme class • binding site selects conformation of the nicotinamide ring and only one stereoselected H is transferred (for reduction) or added (for oxidation). •His 195 donates a proton to ketone, accepts a proton from alcohol •Both His 195 and Arg 171 interact electrostatically to orient carboxyllic acid of pyruvate in enzyme active site Fermentation: Alcohol * An example of decarboxylation followed by reduction to ethanol. * On Tuesday, we will see how this system has been coopted in pyruvate dehydrogenase to perform OXIDATIVE decarboxylation Pyruvate Decarboxylase Mechanism Thiamine pyrophosphate, coenzyme Voet, Voet, and Pratt Fundamentals of Biochemistry

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