Required for symbiotic nitrogen fixation. Plays a key role in the conversion of malate to acetyl-CoA for efficient tricarboxylic acid cycle function in nitrogen-fixating bacteria. (S)-malate + NAD(+) = CO2 + NADH + pyruvate Divalent metal cations. Prefers magnesium or manganese. Subject to substrate inhibition and shows allosteric regulation by acetyl-CoA. Homooctamer. In the N-terminal section; belongs to the malic enzymes family. In the C-terminal section; belongs to the phosphate acetyltransferase and butyryltransferase family.