After two turns of the Krebs cycle (recall that two pyruvate and thus two acetyl groups are generated for each glucose that we started with) all six carbons of the original glucose have been oxidized completely to CO2. These oxidations mean that the electrons must go somewhere. So far the ultimate repository for the electrons has NOT been involved! Rather the electrons have been transferred to NAD+ and to a flavin ( which is a cofactor for and therefore a permanent part of succinate dehydrogenase) Summary of complete reaction sequence for the complete oxidatino of one glucose so far
|2 ATP||2 NADH||none|
2 acetyl-CoA + 2 CO2
|+2||succinyl-CoA ligase |
|+2||isocitrate dehydrogenase||+2||succinate dehydrogenase|
|2 ATP (as GTP)||6 NADH||2 reduced flavin|
|4 ATP||10 NADH||2 reduced flavin|
This of course is not sustainable for very long. There is a limited amount of NAD+ in a cell to reduce and the reduced flavin... since it is a permanent part of the enzyme it can only do its reaction once and then it cannot repeat. There must be a place for isocitrate dehydrogenase and all those NADH to dump their electrons so that cell metabolism can continue.