Catalyzes the reduction of a single double bond in the isoprenoid tail of menaquinone (MK-9) in M.tuberculosis, likely the beta-isoprene unit, forming the predominant form of menaquinone found in mycobacteria, MK-9(II-H2). Is required for M.tuberculosis survival in host macrophages. AH2 + menaquinone-9 = A + beta-dihydromenaquinone-9 Quinol/quinone metabolism; menaquinone biosynthesis. Constitutively expressed at a low level, not induced by salicylate. Cells lacking this gene show complete abolition of the synthesis of MK-9(II-H2) accompanied by accumulation of MK-9. They have similar growth rates than wild-type in both aerated and hypoxic culture conditions. However, despite having similar levels of adhesion and infection at the earliest time point, the survival of mutant bacilli in the macrophages is dramatically reduced. Belongs to the geranylgeranyl reductase family.