Aerobic Glucose Metabolism

End result of the the Krebs cycle

Where the electrons have gone... So Far

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

Pathway	ATP	Enzyme	NADH	Enzyme	Reduced Flavin	Enzyme

Glycolysis yields 2 pyruvate	-1	hexokinase	+2	glyceraldehyde-3P dehydrogenase	none
	-1	phosphofructokinase
	+2	glycerate kinase
	+2	pyruvate kinase

Glycolysis totals	2 ATP		2 NADH		none

Pyruvate dehydrogenase yields 2 acetyl-CoA + 2 CO₂	none		+2		none

Krebs cycle yields 4 CO₂	+2	succinyl-CoA ligase (as GTP)	+2	isocitrate dehydrogenase	+2	succinate dehydrogenase
			+2	α-ketoglutarate dehydrogenase
			+2	malate dehydrogenase

Kreb's cycle totals	2 ATP (as GTP)		6 NADH		2 reduced flavin

Overall Totals	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.