Involved in the degradation of the pyridine ring of trigonelline (TG; N-methylnicotinate) into succinate and methylamine as carbon and nitrogen sources, respectively. TgnA catalyzes the reduction of flavin (FMN or FAD) by NADH and supplies the reduced flavin to the oxygenase component TgnB. a reduced flavin + NAD(+) = an oxidized flavin + 2 H(+) + NADH FADH2 + NAD(+) = FAD + 2 H(+) + NADH FMNH2 + NAD(+) = FMN + 2 H(+) + NADH Maximal reductase activity is achieved only upon trigonelline (TG) binding to the reductase component before interaction with NADH. It seems that TgnA undergoes an allosteric transition upon trigonelline (TG) binding accounting for the positive cooperativity toward NADH oxidation. kcat is 72 sec(-1) for NADH as substrate. Homodimer. The trigonelline monooxygenase is composed of a reductase component TgnA and an oxygenase component TgnB. Belongs to the non-flavoprotein flavin reductase family.