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Chapter 9
Chem 341
Suroviec Fall 2016
I. Citric Cycle Overview
•
•
8 reactions
Oxidizes acetyl group of Acetyl CoA
to 2 CO2
1.
2.
I. General Features
Circular Pathway oxidizes
acetyl groups from many
sources
Net reaction
3.
In eukaryotes all enzymes of CAC are located in
mitochondria
4.
1 CO2 produced in 1 round of the cycle
5.
Oxidation of acetyl groups to 2 CO2 requires transfer
of 4 pair of electrons
II. Synthesis of Acetyl CoA
A.
Pyruvate Dehydrogenase: Multienzyme Complex
•
Group of non covalently associated enzymes that catalyze
2+ sequential steps in metabolic pathway
• Formed from pyruvate through
oxidative decarboxylation
– Pyruvate dehydrogenase (E1)
– Dihydrolipoyl transacetylase (E2)
– Dihydrolipoyl dehydrogenase (E3)
a) 24 E2 proteins associated as trimers at the corners of cube
b) 24 E1 proteins form dimers that associate with E2 core along the 12
edges.
The 12 E3 proteins form dimers that attach to the 6 faces of E2 cube
c) Combining a) and b) forms a 60 subunit complex
Section 9.2: Citric Acid Cycle
Decarboxylation
Action of lipoic acid
Figure 9.10 Reactions Catalyzed by the
Pyruvate Dehydrogenase Complex
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
B. Pyruvate Dehydrogenase
Overall Reaction
1.
Pyruvate dehydrogenase
(E1)
A TPP requiring enzyme
TPP acts as electron sink in the
reaction
2. Lipoamide
• Hydroxyethyl group
transferred to E2
• Lipoamide uses lysine
• Cyclic disulfide reversibly
reduced
3. E2 transesterification
• Yields acetyl CoA and
dihydrolipoamide-E2
4. Regenerate E2
• E3 becomes reduced
• Regenerates E2
• Disulfide interchange
reaction
5. Reoxidize E3
•
Reoxidize E3
Section 9.2: Citric Acid Cycle
Figure 9.15 Citrate Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.15 Citrate
Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
III. Enzymes of CAC
1.
Citrate synthase
1.
Aconitase
3.
Isocitrate dehydrogenase
3.
a-ketoglutarate dehydrogenase:
5.
Succinyl CoA synthetase
5.
Succinate dehydrogenase
7.
Fumerase
7.
Malate dehydrogenase
III. Enzymes of the CAC
A.
•
•
•
•
Citrate Synthase
Catalyzes the condensation of acetyl-CoA and
oxaloacetate
Free enzyme is a dimer
Active site closes when oxaloacetate binds
Conformational changes seals oxaloacetate in binding
site and shuts out the solvent
B. Aconitase
•
Catalyzes reversible isomerization of citrate to
isocitrate
C. NAD+ Dependant Isocitrate Dehydrogenase
•
Catalyzes oxidative decarboxylation of isocitrate to a-ketogluterate
D. a-ketoglutarate dehydrogenase
•
•
•
•
•
Catalyzes oxidative decarboxylation
of a-ketogluterate
Is a multienzyme complex
E1: a-ketoglutarate dehydrogenase
E2: dihydrolipoyl transsuccinylase
E3: dihydrolipoyl dehydrogenase
E. Succinyl-CoA Synthetase
•
•
Cleaves “high-energy” succinyl-CoA to synthesis of GTP
Reaction almost energy neutral.
F. Succinate Dehydrogenase
•
•
Catalyzes stereospecific dehydrogenation of succinate to fumerate
Inhibited by malonate
G. Fumerase
Catalyzes the hydration of double bond of fumarate to
form malate
•
H. Malate dehydrogenase
•
Regeneration of oxaloacetate
IV. Regulation of CAC
•
•
•
Availibity of substrates
Need for CAC intermediates
Demand for ATP
A. Regulation of pyruvate decarboxylation
1.
2.
Product inhibition by NADH and acetyl-CoA
•
NADH, acetyl-CoA compete with NAD+ and CoA for
binding sites
•
Drive with E2 and E3
Covalent modification by phosphorylation/dephosphorylation
of E1
B. Rate-Controlling Enzymes
• Flux of metabolites through the CAC is proportional to the rate
of cellular oxygen consumption
• 3 main mechanisms
– Substrate availability
– Product inhibition
– Competitive feedback
inhibition
B. Rate-Controlling Enzymes
•
•
Regulators are acetyl-CoA,
oxaloacetate, NADH
Flux varies with substrate
concentration
V. Pathways that use CAC intermediates
1.
Glucose biosynthesis
2.
Fatty acid synthesis
3.
Amino Acid synthesis