What is the first reaction of the citrate cycle?
Reactants: Oxaloacetate + Acetyl-CoA
Enzymes: Citrate Synthase
Products: Citrate + CoA-SH + H+
Notes: IRREVERSIBLE + oxaloacetate can be considered catalytic because it is regenerated by the cycle
What is the first part of the second reaction of the citrate cycle?
Dehydration of citrate to an intermediate...
Product: Cis-Aconitate Intermediate
What two things can inhibit aconitase and what results from this?
- buildup of its product isocitrate
results in free citrate which can be removed to the cytosol for fatty acid synthesis
- results in free citrate which can be removed to the cytosol for fatty acid synthesis
fluoracetate - in plants or metabolized from chemo meds
- inhibits aconitase via fluoracetyl-CoA > fluorocitrate formation + causes citrate accumulation
What is the 2nd part of the second reaction of the citrate cycle?
Rehydration of an intermediate to isocitrate...
Reactants: Cis-aconitate intermediate
What is the 1st part of the 3rd reaction of the citric acid cycle (acting on the isomer formed in reaction 2)?
Reactant: Isocitrate + NAD+
Enzyme: Isocitrate Dehydrogenase
Product: Oxalosuccinate + NADH + H+
Notes: IRREVERSIBLE + product remains enzyme-bound for 2nd part of rxn
What is the 2nd part of the third reaction of the citrate cycle?
Enzyme: Isocitrate Dehydrogenase
Notes: IRREVERSIBLE + requires Mg2+ or Mn2+
What is the fourth reaction of the citrate cycle, occurring after decarboxylation via isocitrate dehydrogenase?
Notes (especially about the enzyme)?
Another oxidative decarboxylation...
Reactants: α-Ketoglutarate dehydrogenase + NAD+ + HS-CoA
Enzyme: α-Ketoglutarate dehydrogenase complex
Product: Succinyl-CoA + NADH
Notes: IRREVERSIBLE + α-KGDH Complex is very similar to PDH complex (same E3)
Considering that alpha-ketoglutarate dehydrogenase complex is so similar to PDH complex...
what are its prosthetic groups + cofactors?
- Thiamin Pyrophosphate
- Lipoic Acid
Cofactors: NAD + CoA (via CoA-SH)
What is the 5th reaction of the citrate cycle, coming after the reaction catalyzed by an enzyme complex?
Substrate level phosphorylation...
Reactants: Succinyl-CoA + Pi + GDP
Enzyme: Succinyl-CoA Synthetase
Products: Succinate + CoA-SH + GTP
Notes: GTP can pass the phosphoryl group to ADP via NDP Kinase
What is special about Succinyl-CoA Synthetase in different tissues with regard to its effect on nucleoside diphosphates?
Two isoenzymes exist:
Gluconeogenic tissues (liver/kidney): GDP/ADP-specific versions
- GDP version makes GTP for decarboxylation of oxaloacetate to phosphoenolpyruvate
- links citric acid cycle + gluconeogenesis
Non-gluconeogenic tissues: only ADP-specific version
What is the sixth reaction of the citrate cycle, following a substrate level phosphorylation?
Dehydrogenation of succinate...
Reactants: Succinate + FAD
Enzyme: Succinate Dehydrogenase
Products: Fumarate + FADH2
What is special about the location and function of succinate dehydrogenase?
- it is a membrane-bound flavoprotein bound to the inner surface of the inner mitochondrial membranse
- it is part of complex II of the respiratory chain
- it reduces CoQ via electrons transported by its Iron-Sulfur Proteins
What is the 7th reaction of the citrate cycle, right after the step catalyzed by a membrane bound enzyme?
Hydration of fumarate...
Note: the hydration is stereospecific, always forming the L-enantiomer
What inhibits succinate dehydrogenase?
Via what kind of inhibition?
- via competitive inhibition
What is the 8th and final step of the citrate cycle, reforming the intial compound that contributes to the first step?
Oxidization of malate...
Reactants: Malate + NAD+
Enzyme: Malate Dehydrogenase
Products: Oxaloacetate + NADH
What 3 citric acid cycle steps produce NADH?
Remember: all dehydrogenase steps except succinate dehydrogenase.
- Isocitrate Dehydrogenase (oxidation of isocitrate to oxalosuccinate)
- Alpha-ketoglutarate Dehydrogenase (oxidative decarboxylation of AKG to succinyl-CoA)
- Malate Dehydrogenase (oxidation of L-malate to oxaloacetate)
Which step of the citrate cycle produces FADH2?
- also technically E3 of AKGDH Complex I guess, but that remains bound to E3 as its prosthetic group
Which step of the citrate cycle is a substrate-level phosphorylation?
What is special about the free energy change of the final step of the citrate cycle?
And because of this, what moves it forward?
non-spontaneous under standard conditions
- driven by high [malate]/[oxaloacetate] and [NAD+]/[NADH] ratios
Which two steps of the citrate cycle produce CO2?
The two oxidative decarboxylations leading up to ADP/GDP phosphorylation
- Isocitrate Dehydrogenase (oxalosuccinate to AKG)
- AKG-Dehydrogenase Complex (AKG to Succinyl-CoA)
What is the net reaction of the citrate cycle?
What inhibits alpha-ketoglutarate dehydrogenase complex?
- also high [ammonia]
What are the 3 irreversible steps of the citrate cycle?
- Citrate Synthase
- Isocitrate Dehydrogenase
- AKG-Dehydrogenase Complex
What activates the citrate cycle?
Think ratios of products and reactants.