Citric Acid Cycle and Glyoxylate Cycle Flashcards
Basic Metabolic Fate of pyruvate
Glucose –> Pyuvate –> Lactate
- 2NAD+ –> 2NADH.H+
- ATP produced
- O2 can regenerate NAD
Basic Metabolic Fate of pyruvate IN YEAST
Pyruvate –> Ethanal –> Ethanol
- NADH.H+ –> NAD+
- Enzyme Pyruvate Decarboxylase
Link Reaction
Pyruvate + CoASH + NAD+ –> Acetyl CoA + CO2 + NADH.H+
Enzymes in Prokaryotes + Eukaryotes
- E2 is catalytic and structural core of complex
- E.coli has 24 copies of E2
- E1 and E3 bound to it
- 48 E1 : 24 E2 : 24 E3
- Eukaryote has 60 copies in a pentagonal dodecahedron 5 x 12 = 60
PDHC Link Reaction ENZYMES
Stage 1 -
E1 - Pyruvate Decarboxylase
E2 - Lipolylacetyltransferase
Cofactor [TPP-H] Thiamin Pyro-Phosphate
PDHC Link Reaction ENZYMES
Stage 2 -
E2 -
[S-S] Lipoic Acid
\lip/
PDHC Link Reaction ENZYMES
Stage 3 -
E3 - Dihydro lipamide dehydrogenase
Complex formed which has 1) Flavin Adenine Dinucleotide 2) Histidine
What is E2 in Link Reaction consisting of?
- Long swinging Lysine arm
- Lipoic Acid
- Lipoic acid communicates with E1,2,3
Carbons of the Citric Acid Cycle
C3 —> C2 + C4 —> C6 —> C4 (C2 RELEASED INBETWEEN C6 AND C4, C4 GOES BACK ROUND TO C6)
General equation for Citric Acid Cycle
Pyruvate + 3H2O –> 3CO2 + 10H
Enzymes in order of the CAC
1) Citrate Synthase
2) Aconitase
3) Isocitrate Dehydrogenase
4) 2-Oxoglutarate Dehydrogenase Complex
5) Succinate Thiokinase
6) Succinate Dehydrogenase
7) Fumerase
8) Malate Dehydrogenase
Enzymes in order of the Glyoxylate Cycle
9) Isocitrate Lyase
10) Malate Synthase
Step 1 - Citrate Synthase
- attraction of a proton off acetyl CoA
- asp-COO- attacks CH3 group of acetyl CoA to get proton
- Eq lies to left (+^G) between O=C,(CH2COOH)–COOH
and Citryl-SCoA - Hydrolysis of C-SCoA bond -^G
Mechanism of Isocitrate Dehydrogenase
- enzyme determines that COOH in central C is decarboxylated
- Isocitrate —> 2 intermediates —> 2-oxoglutarate
- H removed
Enzymes forming 2-Oxoglutarate dehydrogenase complex
E1 - 2-oxoyglutarate decarboxylase
E2 - Lipoyl-succinyl transferase (24 of these)
E3 - Dihydrolipoamide dehydrogenase
^G = -RT ln Keq
^G = ^H - T^S
- ^S is highly positive ^G is highly negative in decarboxylation reaction
Succinyl-SCoA has high free energy meaning
High -^G to make atp
Functions of the CAC 1) Final Oxidation
1) Final Oxidation - Carbs-->Sugars-->Pyruvate Fats-->fattyacids-->Acetyl-SCoA (C2) Fats/Tryglycerides --> Glycerol Proteins--->Amino acids - Transamination
Functions of the CAC 2) Energy
NADH + H+ --ETC--> 3ATP X 4 --> 12 ATP FADH2 ---> 2ATP X 1 ---> 2 ATP SUCCTK ---> 1 ATP TOTAL - 15 PER PYRUVATE 30 PER GLUCOSE 2PGM IN GLYCOLYSIS
Functions of the CAC 3) Biosynthesis
- Oxaloacetate —> Carbohydrates (gluconeogenesis)
- Acetyl CoA —> Fatty Acids
- Pyruvate —> Alanine (via reverse transalanation)
Citric acid cycle regulation of energy
- kinase carry out reactions with high energy compounds
- PDHC ——–> PDHC-P
- substrates include, pyruvate and CoA
- products include, acetyl SCoA, NADH
- Ca2+ inhibits kinase/activates phosphatase
- insulin stimulates phosphatase
- 4 kinases
- 3 phophates added to E1 component
- E1 is enzyme that’s phosphorylated
- insulin phosphorylates phosphate
Regulation of Citrate Synthase
- ATP- inhibits citrate synthase
- ATP is an analog of CoA (competitive active site)
- inhibition between ATP and CoA
Regulation of Isocitrate dehydrogenase
- Ca2+, ADP, AMP all activators
- ATP inhibitor
- ADP mammalian AMP yeast
Regulation of Oxoglutarate dehydrogenase complex
- Ca2+ activates it
- 3 enzymes PDHC, ISO DH, OGDHC, all activated by Ca2+
- Ca2+ floods into cells and mitochondria
- muscular action occurs
- hormones e.g. adrenaline also stimulate muscular action
Anaplerotic reactions
- filling up
- intermediates taken out of cycle for biosynthesis
- OAA is not regenerated
- OAA —> Citrate
Complications affecting overall yield…
- ATP being used up so there’s not 30 made
- pyruvate carboxylase requires acetyl SCoA as if acetyl SCoA builds up theres not enough OAA to take it up so it stimulates pyruvate decarboxylase and inhibits kinase
Coupling reactions
- lysine - biotin (coupled)
Need this because: - enzyme - biotin + ATP + HCO3- Co2 - biotin - enzyme + ADP + Pi
- CO2 - biotin -enzyme +pyruvate biotin-enz + OAA
Glyoxylate Cycle
- only one thing normally happens to acetyl coa in this context, it goes to make CO2
- can’t grow on acetate/acetyl coa if you rely on citric acid cycle as all your carbons go to CO2
- eat fatty food —> store or use, can’t convert it to glucose using CAC
- plants therefore bypass Citrate —> OAA where CO2 is release called GC
What is done in Glyoxylate Cycle?
- make extra C4
- C4 + 2C2 —> C4 + C4
- OAA + A.COA —> SUCCINATE + MALATE
- succinate goes back to OAA which is used again
- malate goes to make OAA which is used for biosynthesis
Pyruvate carboxylase requires what to work?
requires an enzyme attached biotin moiety for catalysis
The conversion of pyruvate to acetyl-SCoA is catalysed by…
the pyruvate dehydrogenase complex
Eukaryotic PDHC is covalently regulated by:
phosphorylation
eukaryotic 2-oxoglutarate dehydrogenase complex is stimulated by:
Ca2+
In plants, in which organelle are the enzymes of the glyoxylate cycle situated?
glyoxysome
What is the cofactor involved in the mechanism of pyruvate carboxylase?
biotin
