Involved in the caffeine degradation, which is the essential first step for assimilating the carbon and nitrogen in caffeine. Catalyzes the oxidation of NADH and transfers electrons to NdmA and NdmB, which catalyze the N-demethylation reactions. Binds 1 [2Fe-2S] cluster per subunit. It is probably a fusion between a ferredoxin and a reductase into a single ORF which encodes a functional reductase that is specifically coupled to NdmA and NdmB.