Krebs Cycle

1. Kreb’s Cycle
(aka, tricarboxylic acid
(TCA)cycle, citric acid cycle)
“The wheel is turnin’ and the sugar’s a
burnin’”
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2. Overall goal
•Makes ATP
•Makes NADH
•Makes FADH
2
•Requires some carbohydrate to run
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3. Geography
•Glycolysis in the cytosol
•Krebs in mitochondrial matrix
•Mitochondrion
–Outer membrane very permeable
•Space between membranes called intermembrane space
(clever huh!)
–Inner membrane (cristae)
•Permeable to pyruvate,
•Impermeable to fatty acids, NAD, etc
–Matrix is inside inner membrane
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4. Conversion of pyruvate to Acetyl
CoA
CH
3
O
O
O
pyruvate
CO
2HSCoA
CH
3
SCoA
O
acetyl CoA
NADHNAD+
pyruvate dehydrogenase complex
•2 per glucose (all of Kreb’s)
•Oxidative decarboxylation
•Makes NADH
•-33.4kJ
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5. Fates of Acetyl CoA
CH
3
SCoA
O
acetyl CoA
Kreb's
CO
2
, ATP, NADH...energy
ketone bodies
no CHO present
TAG's
•In the presence of CHO an using energy
–Metabolized to CO
2
, NADH, FADH
2
,GTP and, ultimately, ATP
•If energy not being used (Lots of ATP present)
–Made into fat
•If energy being used, but no CHO present
–Starvation
–Forms ketone bodies (see fat metabolism slides)
–Danger!
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6. Kreb’s Cycle
CH
3
C
O
SCoA
acetyl CoA
CO
CH
2
C
O
C
O
O
O
oxaloacetate
CoASH
citrate synthase
C
O
O
CH
2
C
CH
2
C
OH C
O
O
O O
citrate
aconitase
C
OO
CH
CH
CH
2
C
C
O
O
OO
OH
isocitrate
NAD
NADH
CO
2
C
OO
C
CH
2
CH
2
C
O
O
O
isocitrate dehydrogenase
alpha ketoglutarate
NAD
NADH
CoASH
CO
2
C
CH
2
CH
2
C
OO
OSCoA
succinyl CoA
alpha ketoglutarate
dehydrogenase
GDP
GTP
CoASH
C
C
C
C
OO
O O
H
H
succinate
succinyl CoA
synthetase
FAD
FADH
2succinate
dehydrogenase
C
CH
2
CH
2
C
OO
O
O
fumarate
OH
2
C
CH
CH
2
C
O
O
O
O
OH
malate
fumarase
NAD
NADH
malate
dehydrogenase
Kreb's Cycle
OH
2
+
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7. Net From Kreb’s
•Oxidative process
–3 NADH
–FADH
2
–GTP
•X 2 per glucose
–6 NADH
–2 FADH
2
–2 GTP
•All ultimately turned into ATP (oxidative
phosphorylation…later)
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8. Citrate Synthase Reaction (First)
acetyl CoA
oxaloacetate
CoASH
citrate synthase
citrate
OH
2
CH
3
C
O
SCoA
CO
CH
2
C
O
C
O
O
O
C
O
O
CH
2
C
CH
2
C
OH C
O
O
O O
+
•Claisen condensation
•-32.2kJ
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9. Aconitase Reaction
citrate
aconitase
isocitrate
C
O
O
CH
2
C
CH
2
C
OH C
O
O
O O
C
OO
CH
CH
CH
2
C
C
O
O
OO
OH
•Forms isocitrate
•Goes through alkene intermediate (cis-aconitate)
–elimination then addition
•13.3kJ
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10. Isocitrate Dehydrogenase
isocitrate
NADNADHCO
2
isocitrate dehydrogenase
alpha ketoglutarate
C
OO
CH
CH
CH
2
C
C
O
O
OO
OH
C
OO
C
CH
2
CH
2
C
O
O
O
•All dehydrogenase reactions make NADH or FADH
2

•Oxidative decarboxylation
•-20.9kJ
•Energy from increased entropy in gas formation
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11. α-ketoglutarate dehydrogenase
alpha ketoglutarate
NAD
NADH
CoASH
CO
2
succinyl CoA
alpha ketoglutarate
dehydrogenase
C
OO
C
CH
2
CH
2
C
O
O
O
C
CH
2
CH
2
C
OO
OSCoA
•Same as pyruvate dehydrogenase reaction
•Formation of thioester
–endergonic
–driven by loss of CO
2

•increases entropy
•exergonic
•-33.5kJ
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12. Succinyl CoA synthetase
succinyl CoA
GDP
GTPCoASH
succinate
succinyl CoA
synthetase
C
CH
2
CH
2
C
OO
OSCoA
C
CH
2
CH
2
C
OO
O
O
•Hydrolysis of thioester
–Releases CoASH
–Exergonic
•Coupled to synthesis of GTP
–Endergonic
–GTP very similar to ATP and interconverted later
•-2.9kJ
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13. Succinate dehydrogenase
succinate
FAD
FADH
2
succinyl CoA
dehydrogenase
fumarate
C
CH
2
CH
2
C
OO
O
O
C
C
C
C
OO
O O
H
H
•Dehydrogenation
•Uses FAD
–NAD used to oxidize oxygen-containing groups
•Aldehydes
•alcohols
–FAD used to oxidize C-C bonds
–0kJ
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14. Fumarase
fumarate
OH
2
malate
fumarase
C
C
C
C
OO
O O
H
H
C
CH
CH
2
C
O
O
O
OH
O
•Addition of water to a double bond
•-3.8kJ
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15. Malate Dehydrogenase
oxaloacetate
malate
NAD NADH
malate
dehydrogenase
C
CH
CH
2
C
O
O
O
OH
O
CO
CH
2
C
O
C
O
O
O
•Oxidation of secondary alcohol to ketone
•Makes NADH
•Regenerates oxaloacetate for another round
•29.7 kJ
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16. Net From Kreb’s
•Oxidative process
–3 NADH
–FADH
2
–GTP
•X 2 per glucose
–6 NADH
–2 FADH
2
–2 GTP
•All ultimately turned into ATP (oxidative
phosphorylation…later)
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17. Total Energy per glucose
•Cytosol
–Glycolysis
•2 NADH
•2 ATP
•Mitochondrion
–Pyruvate dehydrogenase
•2 NADH
•Krebs
–6 NADH
–2 FADH
2
–2 GTP
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18. Total Energy/glucose
•In mitochondrion:
–Each NADH makes 2.5 ATP
–Each FADH
2 makes 1.5 ATP
–GTP makes ATP
•So…
–From in mitochondrion
•8 NADH X 2.5 ATP/NADH = 20 ATP
•2 FADH
2
X 1.5 ATP/FADH
2
= 3 ATP
•2 GTP X 1 ATP / GTP = 2 ATP
•TOTAL in mitochondrion 25 ATP
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19. Total Energy/ glucose
•Cytosol
–2 ATP
–2 NADH
•NADH can’t get into mitochondrion
•In eukaryotes two pathways,
–transferred to FADH
2
»get 1.5 ATP/ FADH
2
–Or transferred to NADH
»Get 2.5 ATP/ NADH
–(Not a problem in prokaryotes (why?))
–2 NADH X 1.5 ATP = 3 ATP
–Or 2 NADH X 2.5 ATP = 5 ATP
»+ =2 ATP
»Total 3+ 2 or 5 + 2 so either 5 or 7
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20. ATP/glucose
•Eukaryotes
–Mitochondrial: 25 ATP
–Cytosolic: 5 or 7 ATP
–Total 30 or 32 ATP/glucose
–30 ATP X 7.3kcal X 4.18 kJ = 915 kJ
ATP kcal
If 32 ATP = 976 kJ
•Prokaryotes
–32 ATP X 7.3kcal X 4.18 kJ = 976 kJ
ATP kcal
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