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I'm looking at the gluconeogenic pathway:

Pyruvate is shuttled into the mitochondria from the cytosol, converted to oxaloacetate by pyruvate carboxylase, converted to malate which is then shuttled out of the mitochondria, and then converted back to oxaloacetate in the cytosol. Why is the mitochondrial step necessary? Why can't pyruvate just be made into oxaloacetate in the cytosol?

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Got it. The ratio of [NADH]/[NAD+] is about 100000 lower in the cytosol than it is in the mitochondria. The conversion of malate to OAA reduces a molecule of NAD+ to NADH, and that NADH can be used for the later step in gluconeogenesis: the conversion of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate requires a molecule of NADH.

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  • $\begingroup$ It's a way to transport reduction equivalents across membranes, you are absolutely right. $\endgroup$ – Chris Feb 21 '15 at 20:49

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