hard · MCAT bio-biochem
A researcher studies a mitochondrial matrix dehydrogenase that oxidizes its substrate using NAD^+ as cofactor. The purified enzyme has a K_M of 20 μ M for substrate and is normally saturated. In a cell, a point mutation in an unrelated transporter collapses the NADH/NAD^+ ratio from its normal value of about 0.1 down to 10, while total substrate concentration is held constant well above K_M.
Treating NAD^+ as a co-substrate whose availability now limits turnover, what is the MOST likely effect on the reaction, and why?
- Forward flux falls sharply, because the rise in NADH both depletes free NAD^+ co-substrate and makes the reaction's mass-action ratio approach equilibrium, lowering the net driving force.
- Forward flux is unchanged, because the organic substrate remains far above its K_M, and substrate saturation alone fixes the catalytic turnover rate of the matrix dehydrogenase.
- Forward flux rises, because the abundant NADH product feeds back to allosterically activate the dehydrogenase, strongly accelerating oxidation of the saturating substrate pool overall.
- Forward flux falls only because the elevated NADH competitively raises the apparent K_M for the organic substrate, an effect that is fully and easily reversible simply by adding more substrate.
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