Lectures 22 & 23 The TCA Cycle-BW

22 23 why you need to condense actyl coa with 4

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Unformatted text preview: 23 Why you need to condense Actyl CoA with 4 carbons - can't carry out oxidzation and cleavege of Actyl CoA directly - have to carry out so you c an carry out cleavage. - during the regenratvie phase - you produce a NAD FADH2 and ATP - this reaction dont give you nothign in return. the fi rst reaction combine 4carbons of OAA TCA Cycle Chemistry TCA cycle seems like a complicated way to oxidize acetate units to CO2 But normal ways to cleave C-C bonds and oxidize don't work for acetyl-CoA. Therefore condense acetate with oxaloacetate and carry out a -cleavage. TCA cycle combines this with oxidation to form CO2, regenerate oxaloacetate and capture all the energy as NADH and ATP. 4/6 and 4/8/13 MCB 2000 Lectures 22 & 23 CoA lose to form citrate. Thioseter bond - liberate energy - need energy to insure that TCA cycle goes in the forward clockwise directionb because it c an go backwards. - pushes TCA c ycle in the forward direction. Citrate Synthase Involves a condensation reaction between OAA and acetyl CoA Ordered binding: OAA induces a major structural change to create a binding site for acetyl CoA Importance of hydrolysis of high energy thioester bond 4/6 and 4/8/13 MCB 2000 Lectures 22 & 23 Coupled Reactions I Citrate reaction favorable - the reaction that produce OAA (last reaction of TCA cycle) - not so favorable - how do you ever get the TCA cycle to go. c oupled reaction are coupled. produce low amount of OAA - imediate used in Citrate synthezie reaction - as you pull of OAA of produce. -advantage of forming more coupled reaction. TCA Cycle L- -8 kJ/mol What explains how the malate dehydrogenase catalyzed reaction can occur with such a high positive free energy change? Hydrolysis of high energy thioester bond ensures that the reaction proceeds irreversibly to the right. CS has large, negative G! 4/6 and 4/8/13 MCB 2000 Lectures 22 & 23 Regulation of Citrate Synthase Highly dependent on substrate concentration. most enzymes have substrate concentration that near or at the Km of the enzyme most concentration of substrate in the cell are need to Km - not the case for OAA. Oxaloacetate (substrate) concentration Feedback inhibition by product, citrate Cirtate sythase reaction - OAA is liminted as a substrate - dont ahve a lot of it. all the intermediate in the mitchondria the most rate liminting substrate. - in order to catalyze reaction - OAA must bind fi rst. look at c itrate synthase structure - no binding site of acetyl CoA in the abesence of OAA. - OAA must bind fi rst when it does it creates the binding site through conformation chain - for Actyl CoA - Actyl CoA can not bind in the absence of OAA. important in the effi cenity of the reaction. both OAA and actyl CoA bind indepeding of one an other - Actyl CoA (higher concentration hgier_ would be bound to enzyme and would not be avilable for a number of other reacitons. - only have productive binding when you can be use OAA is avilable to catalyze the raction in the TCA cycle - OA...
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