FIGURE 3.

A diagram showing the steps in the C₄ NAD-malic enzyme-subtype pathway. In mesophyll (M) cells, PEP is carboxylated by PEPC using the CO₂ group from bicarbonate (HCO₃^-), which was produced from the hydration of atmospheric CO₂ by CA. The resulting OAA is transaminated to Asp by Asp aminotransferase (AST). In the mitochondria (blue oval) of BS cells, OAA is generated from Asp by a mitochondrial AST, and then reduced to malate by NAD-malate dehydrogenase (NAD-MDH). Malate is decarboxylated by NAD-malic enzyme (NAD-ME), and the released CO₂ diffuses into the BS chloroplast (green oval), where it is fixed by the PCR cycle. The pyruvate resulting from the decarboxylation reaction is converted to alanine (Ala) by Ala aminotransferase (ALT) in the BS cytosol. The Ala is transaminated back to pyruvate by ALT activity in the M cytosol. PPDK activity in M chloroplasts converts pyruvate to PEP.