The Link Reaction
Pyruvate enters the matrix of a mitochondrion and is:
- decarboxylated (CO2 removed)
- dehydrogenated (hydrogen removed)
- combined with coenzyme A to give acetylcoenzyme A (ACoA)
Pyruvate (3C) + coenzyme A -----carbon dioxide-----NAD>NADH----> acetyl (2C) coenzyme A
Coenzyme A consists of:
- Ribose (making a nucleoside together with adenine)
- Pantothenic acid (a B vitamin)
Coenzyme A transfers an acetyl group (with two carbon atoms) from pyruvate into the Krebs Cycle and plays a central role in respiration. It is present in small quantities in a cell and is recycled.
- An acetyl group with two carbon atoms from ACoA is combined with a 4 carbon compound (oxaloacetate) to give a 6 carbon compound (citrate or citric acid). Coenzyme A is re-formed.
- Citrate is then converted back to oxaloacetate in a series of small steps involving decarboxylation and dehydrogenation.
- The carbon dioxide removed is given off as a waste product.
- The hydrogens removed are accepted by NAD or the related flavin adenine dinucleotide (FAD). One FAD and three NAD molecules are reduced during each turn of the cycle. The main role of the Krebs cycle is to generate a pool of reduced hydrogen carriers to pass on to the next stage.
- The regenerated oxaloacetate can combine with another ACoA
- One molecule of ATP is made directly by substrate-level phosphorylation for each ACoA entering the cycle. So two molecules of ATP are made per glucose molecule entering glycolysis.