Although the direct reduction of CoQ to CoQH2 is thermodynamically possible, this reaction is not a direct part of the electron transport chain via complexes I, III, and IV. Instead NADH reduces an FMN cofactor in complex I and the reduced FMNH2 kick-starts electron transport in the membrane.

Why is this electron handoff from NADH to FMN, as opposed to direct reduction of CoQ by NADH, a critical component of the electron transport chain?

This electron handoff from NADH to FMN, as opposed to direct reduction of CoQ by NADH, a critical component of the electron transport chain is important for Signaling Transduction and Metabolomics

Explanation:

The NADH-CoQ reductase reaction is catalyzed by Complex I. In this course of activity, following events takes place-  

a)FNM (NADH dehydrogenase flavoprotein) is reduced by the NADH  to FMNH2 through following reactions –  

NADH+H++E-FMN↔NAD++E-FMNH2

b)In the next phase coenzyme Q  receives electron from FMNH2  through the the iron–sulfur centers of the NADH-CoQ reductase

c)The iron atom undergoes oxidation–reduction cycles to conserve  mitochondrial protein  as lataxin and hence transport protons from the matrix to the intermembranal space thereby Signaling Transduction and Metabolomics

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