Citric Acid Cycle Generates Reductive Equivalents Utilized in Respiration

1
Citric Acid Cycle Generates Reductive Equivalents
Utilized in Respiration
2
Citric Acid Cycle is a Source for Biosynthetic
Precursors
3
Citric Acid Cycle Intermediates Replenished as
Metabolites are Drawn Off for Biosynthesis
Anapleurotic reactions lead to a replenishment of
pathway components (e.g. pyruvate carboxylase).
4
Citric Acid Cycle A Two Stage Process
Stage One C4 C2 condensation C6
tricarboxylic acid Oxidative decarboxylation -2
CO2 Succinyl CoA Net Result 2NADH
2CO2 Succinyl CoA
5
C4 C2 Condensation Reaction
Condensation catalyzed by enzyme dimer Citrate
Synthase
What undesirable side reactions might occur? How
can unfavorable side reactions be minimized?
6
Citrate Synthase Conformational Forms
Oxaloacetate binding leads to acetyl CoA binding
site formation (partially closed). Citryl CoA
formation closes active site for
hydrolysis. Production formation opens active
site.
7
Citrate to Isocitrate Conversion via Aconitase
Hydroxyl shift for subsequent oxidative
decarboxylation
How does fluoroacetate function in aconitase
catalysis? (Hint fluoroaceyl CoA fluorocitrate
generated first.)
8
Fluoroacetate - Fluoroacetyl-CoA Conversion via
Acetate Thiokinase
What benefit might Australian plants from the
Gastroblonium genus gain in storing the aconitase
suicide-inhibitor fluoroacetate?
9
Isocitrate Dehydrogenase Oxidation and
Decarboxylation
Oxalosuccinate not released from the enzyme
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a-Ketoglutarate Dehydrogenase Oxidation and
Decarboxylation
How similar is a-ketoglutarate dehydrogenase with
pyruvate dehydrogenase in terms of substrate
components? Reaction mechanism?
11
Citric Acid Cycle Part Two Oxaloacetate
Regeneration
Succinyl Coenzyme A Synthetase couples hydrolysis
of Coenzyme A (-33 kJ/mol) with GTP formation
(-30 kJ/mol)
The energetics works but what about the reaction
mechanism?
12
Succinyl CoA-Sythetase Generates GTP
What is the Nu-, E, and leaving group with GTP
generation?
13

• Oxaloacetate Enzyme Regeneration from Succinate
• Succinate Dehydrogenase
• Fumerase
• Malate Dehydrogenase

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A Color-Coded Citric Acid Cycle
Isocitrate is prochiral Succinate is a symmetric
molecule Generated products include 2CO2
3NADH 1FADH2 GTP CoA
15
Test Your Knowledge
When feeding the shown substrate, none of the
radiolabel was lost in generating a-ketoglutarate
while succinate was devoid of radiolabel. Why
were early investigators surprised with these
results?
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Citric Acid Energetics
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Citric Acid Cycle Regulation Points
ATP and NADH are indicators of a high energy
state ADP an indicator of a low energy
state Regulatory sites include Isocitrate
dehydrogenase a-Ketoglutarate dehydrogenase Pyru
vate dehydrogenase
18
Test Your Knowledge
Malonate is a competitive inhibitor of succinate
dehydrogenase.
How will the concentrations of citric acid cycle
intermediates change after the addition of
malonate? Why is malonate not a substrate for
succinate dehydrogenase?
19
Chapter 19 Problems 1-6 and 10-15.