BIOC 212 Lecture Notes - Lecture 49: Citric Acid Cycle, Coenzyme A, Lactate Dehydrogenase
Metabolism XI
Citric Acid Cycle
• Citric acid cycle turns acetyl-CoA into more FADH2 & NADH in the matrix
o Ultimate end point for all acetyl-CoA that is generated by catabolic
metabolism
o Series of enzymes that cycle substrate between each other
• New acetyl-CoAs coming from glycolysis, beta oxidation, or breakdown of aa
enter this pathway
o Conjugated to oxaloacetate (4 carbon carbohydrate)
o Not really a sugar since has extra oxygens in it
• The acetyl group (2 carbons) is added to oxaloacetate (4 carbons) to make
citrate (6 carbons)
o Addition of acetyl group generates citrate molecules
o Citrate is the conjugated base of citric acid, which is where name citric acid
cycle comes from
• Once citrate molecules enter this cycle, go through a series of steps that are very
similar to beta oxidation pathway
o Series of NAD+ and FAD molecules will oxidize C-C bonds
o Sequentially, one at a time, will release both of the carbons that have been
added to citrate as CO2
• Citrate is progressively oxidized one carbon at a time back to oxaloacetate
o Oxidize the carbon-carbon bonds one at a time
o Get 4C intermediate that is converted back to oxaloacetate --ready to accept
more acetyl-CoA generated from pyruvate in mt
• Key molecules are oxaloacetate, and citrate
• Another enzyme in the pathway, succinate dehydrogenase
o Involved in electron transport chain itself
• Products of this cycle are very similar to beta oxidation
o Primarily reduction equivalents that are generated in the matrix and need to
be detoxified by using oxygen as its ultimate electron acceptor
• The acetyl group (2 carbons) is added to oxaloacetate (4 carbons) to make
citrate (6 carbons)
• Citrate is progressively oxidized one carbon at a time back to oxaloacetate
• This drives the reduction of NAD+ and FAD to NADH and FADH2
o Also generates one molar equivalent of GTP from GDP
• Other product that get is GTP
• GTP in the mitochondria is used as an energetic equivalent to ATP