*4.2. The Malate Valve*

The malate valve operates to adjust cellular ATP/NADPH supply by shuttling reducing power from the chloroplast to other organelles like the mitochondria via malate/oxaloacetate shuttles [32,57,58]. In the chloroplast, NADPH reduces oxaloacetate to malate via chloroplastic malate dehydrogenase (MDH). This malate is then exported from the chloroplast where it can be oxidized to form NADH in the cytosol, peroxisome or mitochondria via organelle-specific MDH enzymes. Reducing power shuttled to the mitochondria can fuel mitochondrial electron transport following transfer through the full complement of the electron transport complex proteins, generating additional *pmf* and ATP, or through only a portion of the electron transport complex proteins by dissipation of electrons via the alternative oxidase (AOX) or alternative mitochondrial electron carrier proteins. In all cases, the net effect is to increase ATP/NADPH supply either by decreasing NADPH or by simultaneously decreasing NADPH and increasing ATP. Chloroplastic NADP-MDH operates under tight light regulation via the Fd-thioredoxin (Fd-Trx) system, suggesting a role in photosynthetic energy balancing [59,60]. Importantly, the malate valve offers a way to "trade" NADPH for ATP by diverting reducing equivalence directly into mitochondrial electron transport.
