Question

From what reactions is NADH made in complex I in electron transport chain?

In Succinate --> Fumarate in Complex II what reaction is this, and in which pathway?

Answer 1

Nicotinamide adenine dinucleotide, NAD is a co-enzyme that exists in two forms a) an oxidized as NAD⁺ and b) a reduced form as NADH.

The redox reaction for conversion of NADH to NAD⁺ and vice versa isthe following-

                                    NADH     ?      NAD⁺ + H⁺ + 2e^-

In complex I, NADH is oxidized to NAD⁺ by losing two electrons. The pathway of electron transfer in complex I is-

In the first step, NADH is oxidized to NAD⁺ by reducing Flavin mononucleotide to FMNH₂ in one two-electron step and FMNH₂ is then oxidized in two one-electron steps, through a semiquinone intermediate in the second step. In this transfer of electrons, four protons are translocated from the mitochondrial matrix to the intermembrane space, thus producing a proton gradient.

In complex II of elecron transfer chain succinate is oxidized to fumerate through a series of reactions catalyzed by the enzyme succinate dehydrogenase, SDH or succinate-coenzyme Q reductase (SQR), In complex II, SQR catalyzes the oxidation of succinate to fumarate with the reduction of ubiquinone to ubiquinol. The redox reaction for the conversion of Succinate to Fumerate or vice versa is -

                                    Succinate   ?     Fumarate + 2H⁺ + 2e^–

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