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
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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- Fall '12
- Cellular Respiration, TCA