Catalyzes oxidation of malate to oxaloacetate in the citric acid cycle. Donates electrons to quinones of the electron transfer chain (By similarity). (S)-malate + a quinone = a quinol + oxaloacetate The FAD is tightly bound. Carbohydrate metabolism; tricarboxylic acid cycle; oxaloacetate from (S)-malate (quinone route): step 1/1. Belongs to the MQO family.