Download Citric Acid Cycle

Citric Acid Cycle
Krebs Cycle
Tricarboxylic Acid Cycle
Citric Acid Cycle
• Reduces Acetyl CoA to CO2, with
production of NADH,FADH2 and GTP
• Acetyl CoA comes from:
– Lipids
– Pyruvate from Glycolysis
– Amino Acid breakdown
• Acetoacetate or Acetyl-CoA
• pyruvate
• Produces oxaloacetate from various
compounds for glucose synthesis
Oxidative Decarboxylation of
Pyruvate
• The first step of Citric Acid Cycle
oxidation of carbohydrates is
decarboxylation of pyruvate.
• The Acetyl group is attached to
CoA.
• The process oxidation results in
reduction of NAD+ to NADH
• The process happens on a large
complex in 3 stages.
Pyruvate Dehydrogenase Complex
Enzyme
Abbreviation
Number
of
chains
Prosthetic
group
Pyruvate
dehydrogenase
component
E1
24
TPP
Dihydrolipoyl
transacetylase
E2
24
Lipoamide
Dihydrolipoyl
dehydrogenase
E3
12
FAD
Reaction catalyzed
Oxidative decarboxylation
of pyruvate
Transfer of the acetyl
group to CoA
Regeneration of the
oxidized form of lipoamide
Pyruvate
Dehydrogenase
Complex
Mechanism of Pyruvate
Decarboxylation
• TPP is first deprotonated.
• It can then attack pyruvate
to displace CO2.
Transfer of Acetyl to CoA
• Transfer to
Lipoamide
• Transfer to
CoA
• Oxidation of
Lipoamide
Flipping of Lipamide between
subunits
Acetyl CoA breakdown
• Acetyl CoA is broken down to produce
– 1 GTP or ATP
– 3 NADH
– 1 FADH2
Citrate Synthase
• Citrate Synthase adds acetyl group to
oxaloacetate.
Citrate Synthase conformation
change ensure correct reaction
Citryl CoA Synthesis Mechanism
• Citrate synthase goes through a second
conformation change after this reaction to allow
citryl-CoA hydrolysis.
Aconitase converts Citrate to
Isocitrate
• The reaction is
stereospecific to
one side of citrate.
• The enzyme uses
an iron-sulfur
complex for
binding citrate.
Isocitrate is then oxidized &
decarboxylated
• Oxalosuccinate is an unstable intermediate,
which is decarboxylated on the enzyme.
• This is an important step in regulating the cycle.
a-Ketoglutarate dehydrogenase
catalyzes oxidative decarboxylation
• The reaction is by the
same mechanism as
pyruvate
dehydrogenase.
• The same kind of large
complex with E1, E2 and
E3 is used.
– E3 (dihyrolipoyl
dehydrogenase is
identical in the two
complexes.
Succinyl CoA synthase runs in
reverse to generate GTP
• Some organisms use ATP instead of GTP.
• These NTPs are converted by dinucleotide
kinase
Oxidation of Succinate to Oxaloacetate
• Oxaloacetate is regenerated by oxidation
of succinate to the keto acid in 3 steps.
– Succinate Dehydrogenase oxidizes succinate
to fumarate with reduction of an enzymebound FAD to FADH2.
• Succinate dehydrogenase in membrane bound &
directly transfers e- to e- transport chain.
– Fumarate is hydrated to malate.
• Hydration is stereospecific.
– Malate is oxidized to oxaloacetate with
production of NADH.
Citric Acid
Cycle
Summary
Regulation of Citric Acid Cycle
• Decarboxylation of pyruvate
is committed step, since in
animals acetyl-CoA cannot
be converted back to
pyruvate & glucose.
• This step is highly regulated.
• Other regulated steps include
the two decarboxylation
steps.
Pyruvate Dehydrogenase
Regulation
Citric Acid Cycle Generation of
Biosynthetic Precursors
• Anaplerotic reactions can replenish the reaction intermediates.
• i.e. generation of oxaloacetate by pyruvate carboxylation.
• No net conversion of Acetyl CoA to intermediates or pyruvate.
The Glyoxylate Cycle
• Plants and many
bacteria can
convert acetyl CoA
to TCA cycle
intermediates by
the Glyoxylate
cycle.
• Allows these
organisms to grow
on acetate.
Conclusions
• The Citric Acid Cycle allows organisms to extract
electrons from pyruvate and other Acetyl-CoA precursors
for transport to the mitochondria electron transport chain.
• One NADH is made converting pyruvate to Acetyl-CoA.
• Three NADH, one FADH2 & 1 GTP/ATP is made in the
citric acid cycle.
• The citric acid cycle can be used to make precursors for
other molecules.
• Citric acid cycle intermediates can be made from
pyruvate and amino acids to increase flow through or
replace those used in biosynthesis.
– Also from acetyl CoA in plants & some bacteria, but not animals.
• The citric acid cycle is regulated by NADH, and
ultimately by oxygen availability.