Research Notes: Kreb's Cycle

Kreb's cycle showing entry points of pyruvate, fatty acids and amino acids. Graphic from http://www.ruf.rice.edu/~bioslabs/studies/mitochondria/mitokrebs.html

Amino acids and odd-chain fatty acids can be metabolized into Krebs substrates (intermediates) and enter at several points and intermediates can be 'siphoned off' for use in biosynthetic pathways. They must be replaced in order to maintain energy balance, however the fate of a Krebs intermediate is not necessarily to cycle through the enzymes until it is completely oxidized to carbon dioxide and water.

Succinate to fumarate

When succinate is brought into or generated in the mitochondria matrix in sufficient quantity, succinate molecules bind to succinate dehydrogenase (SDH), also known as complex II of the electron transport chain. The oxidation of succinate to fumarate is the only Krebs reaction that takes place on the inner membrane itself. The energy carrier flavin adenine dinucleotide (FAD) is also a part of the succinate dehydrogenase complex and because both are part of the same complex, the only step needed to initiate succinate oxidation is the binding of succinate to SDH. Even in severely compomised mitochondria, succinate supported respiration can usually be accomplished, as long as fragments of the inner membrane remain.