Catalyzes the reduction of free flavins (FMN or FAD) by NADH. Subsequently, the reduced flavins diffuse to the HsaA oxygenase subunit. a reduced flavin + NAD(+) = an oxidized flavin + 2 H(+) + NADH Lipid metabolism; steroid biosynthesis. HsaAB monooxygenase consists of an oxygenase component HsaA and a reductase component HsaB. Induced by KstR. Cholesterol metabolism contributes to the survival of M.tuberculosis in the host by helping the bacterial multiplication during earlier stages of infection and to the dissemination of the pathogen in the host. Belongs to the non-flavoprotein flavin reductase family.