Chapter 12 (part 1)
Citric Acid Cycle
Gylcolysis
Electron Transport and
Oxidative phosphorylation
TCA Cycle
The TCA Cycle
(aka Citric Acid Cycle, Krebs Cycle)
• Pyruvate (actually acetate) from
glycolysis is degraded to CO2
• Some ATP is produced
• More NADH is made
• NADH goes on to make more ATP in
electron transport and oxidative
phosphorylation
Entry into the TCA Cycle
• Pyruvate is translocated from the cytosol to the
mitochondria
• Pyruvate is oxidatively decarboxylated to form
acetyl-CoA
• Pyruvate dehydrogenase uses TPP, CoASH, lipoic
acid, FAD and NAD+
• Acetyl-CoA then enters TCA cycle thru citrate
synthase
Pyruvate Dehydrogenase
Complex
Composed of three enzymes:
• pyruvate dehydrogenase (E1) (cofactor =
TPP)
• Dihydrolipoamide acetyltransferase (E2)
(cofactor = Lipoamide, CoA)
• Dihydrolipoamide dehydrogenase (E3)
(cofactor = FAD, NAD+)
Pyruvate Dehydrogenase
Citrate Synthase
• Only step in TCA cycle that involves the
formation of a C-C bond
Aconitase
• Isomerization of Citrate to Isocitrate
• Citrate is a poor substrate for oxidation
• So aconitase isomerizes citrate to yield
isocitrate which has a secondary -OH, which
can be oxidized
• Aconitase uses an iron-sulfur cluster to position
citrate (binds –OH and carboxyl of central
carbon)
Isocitrate Dehydrogenase
• Oxidative decarboxylation of isocitrate to yield
 -ketoglutarate
• Classic NAD+ chemistry (hydride removal)
followed by a decarboxylation
• Isocitrate dehydrogenase is a link to the
electron transport pathway because it makes
NADH
• Rxn is metabolically irreversible
 -Ketoglutarate Dehydrogenase
• A second oxidative decarboxylation
• This enzyme is nearly identical to pyruvate
dehydrogenase - structurally and
mechanistically
• Five coenzymes used - TPP, CoASH, Lipoic acid,
NAD+, FAD
Succinyl-CoA Synthetase
• A substrate-level phosphorylation
• A nucleoside triphosphate is made (ATP
in plants/bacteria and GTP in animals)
• Its synthesis is driven by hydrolysis of a
CoA ester
Succinate Dehydrogenase
• An oxidation involving FAD
• Mechanism involves hydride removal by FAD and
a deprotonation
• This enzyme is actually part of the electron
transport pathway in the inner mitochondrial
membrane
• The electrons transferred from succinate to
FAD (to form FADH2) are passed directly to
ubiquinone (UQ) in the electron transport
pathway
• Enzyme inhibited by malonate
Fumarase
• Hydration across the double bond
• trans-addition of the elements of
water across the double bond
• Stereospecific reaction
Malate Dehydrogenase
• An NAD+-dependent oxidation
• The carbon that gets oxidized is the one
that received the -OH in the previous
reaction
• This reaction is energetically expensive
• Go' = +30 kJ/mol
Reduced Coenzymes Fuel ATP
Production
• Acetyl-CoA + 3 NAD+ + Q + GDP + Pi +2 H20 
HS-CoA + 3NADH + QH2 + GTP + 2 CO2 + 2 H+
•
•
•
•
•
Isocitrate Dehydrogenase
-ketoglutarate dehydrogenase
Succinyl-CoA synthetase
Sunccinate dehydrogenase
Malate Dehydrogenase
1 NADH=2.5 ATP
1 NADH=2.5 ATP
1 GTP=1 ATP
1 QH2=1.5 ATP
1 NADH=2.5 ATP
• Total of 10 ATPs gained from oxidation of 1 AcetylCoA
Regulation
of TCA
Cycle
Protein/amino acid
Catabolites feed
Into the TCA Cycle
Fats breakdown
and feed
into the TCA Cycle
TCA Cycle provides
intermediates for
many biosynthetic
processes
The Anaplerotic Reactions
The "filling up" reactions
• PEP carboxylase - converts PEP to oxaloacetate
• Pyruvate carboxylase - converts pyruvate to
oxaloacetate
• Malic enzyme converts pyruvate into malate
Following the carbons
through the TCA cycle
The Glyoxylate Cycle
• A variant of TCA for plants and bacteria
• Acetate-based growth - net synthesis of
carbohydrates and other intermediates from
acetate - is not possible with TCA
• Glyoxylate cycle offers a solution for plants and
some bacteria and algae
• The CO2-evolving steps are bypassed and an
extra acetate is utilized
• Isocitrate lyase and malate synthase are the
short-circuiting enzymes
Glyoxylate Cycle
• Rxns occur in specialized organelles
(glycoxysomes)
• Plants store carbon in seeds as oil
• The glyoxylate cycle allows plants to use
acetyl-CoA derived from B-oxidation of
fatty acids for carbohydrate synthesis
• Animals can not do this! Acetyl-CoA is
totally oxidized to CO2
• Malate used in gluconeogenesis