BIOL2171 Lecture Notes - Lecture 6: Succinyl-Coa, Acetyl-Coa, Pyruvate Dehydrogenase
Topic 6: TCA Cycle
1. Acetyl CoA (activated acetyl) joins with a four-carbon molecule called oxaloacetate.
o This releases the CoA group and forms a six-carbon carbon molecule called citrate
2. Citrate is converted to its isomer - isocitrate
o This is a 2 step process involving the removal and addition of a water molecule
3. Oxidation of isocitrate
o When isocitrate is oxidised, it releases a molecule of carbon dioxide and leave behind a
5-carbpon molecule, alpha-ketoglutarate
o NAD⁺ is redued to for NADH
o Enzyme catalysing the step is called isocitrate dehydrogenase and is important in
regulating the speed of the TCA cycle
4. Alpha-ketoglutarate is oxidized
o This process reduced NAD⁺ to NADH ad releases a oleule of aro dioide
o Remaining 4-carbon molecule picks up CoA forming the unstable compound succinyl
CoA
o The enzyme catalysing this step is alpha-ketoglutarate dehydrogenase and again is very
important in regulating the citric acid cycle
5. CoA on succinyl is replaced by a phosphate group
o This phosphate group is then transferred to ADP to make ATP
o In some cells GDP (guanine diphosphate) is used instead of ADP, and forms GTP
o The 4-carbon molecule produced in this step is called succinate
6. Succinate is oxidised
o This forms another 4-carbon molecule called fumarate
o 2 hydroge atos ad their eletros are trasferred to FAD to for FADH₂
o Enzyme that carries our this step is embedded in the inner membrane of the
itohodrio, so FADH₂ a trasfer eletros diretl ito the eletro trasport
chain
7. Water is added to the 4-carbon molecule fumarate, converting it into another 4-carbon
molecule called malate
8. Oxaloacetate (the starting 4-carbon compound) is regenerated via the oxidation of malate
o Aother oleule of NAD⁺ is redued to NADH i the proess
• Products from a single turn of the cycle:
o 2 carbons entre from acetyl CoA and 2 molecules of carbon dioxide is released
o 3 oleules of NADH ad 1 oleule of FADH₂ are geerated
o One molecule of ATP or GTP is produced
• Where's the ATP?
o Lots of ATP is produced indirectly, by way of the NADH ad FADH₂
o it generates. These electron carriers will connect with the last portion of cellular
respiration, depositing their electrons into the electron transport chain to drive
synthesis of ATP molecules through oxidative phosphorylation.
• Stuff is taken out must be put back in
• Pyruvate oxidation supplies the TCA cycle with acetyl-CoA
o Pyruvate produced by glycolysis in the cytoplasm
o Pyruvate oxidation takes place in the mitochondrial matrix
o Steps:
1. A carboxyl groups is snipped off pyruvate and release as a molecule of carbon
dioxide leaving behind a 2-carbon molecule
2. 2-carbon molecule is then oxidised and the electrons lost in the oxidation are
piked up NAD⁺ ad NADH
3. The oxidised 2-carbon molecule(the acetyl group) is then attached to CoA (an
organic molecule derived form vitamin B5) to form acetyl CoA
find more resources at oneclass.com
find more resources at oneclass.com
