ABSTRACTA novel bacterial aldehyde dehydrogenase (ALDH) that converts retinal to retinoic acid was first identified inBacillus cereus. The amino acid sequence of ALDH fromB. cereus(BcALDH) was more closely related to mammalian ALDHs than to bacterial ALDHs. This enzyme converted not only small aldehydes to carboxylic acids but also the large aldehyde all-trans-retinal to all-trans-retinoic acid with NAD(P)+. We newly found thatBcALDH and human ALDH (ALDH1A1) could reduce all-trans-retinal to all-trans-retinol with NADPH. The catalytic residues inBcALDH were Glu266 and Cys300, and the cofactor-binding residues were Glu194 and Glu457. The E266A and C300A variants showed no oxidation activity. The E194S and E457V variants showed 15- and 7.5-fold higher catalytic efficiency (kcat/Km) for the reduction of all-trans-retinal than the wild-type enzyme, respectively. The wild-type, E194S variant, and E457V variant enzymes with NAD+converted 400 μM all-trans-retinal to 210 μM all-trans-retinoic acid at the same amount for 240 min, while with NADPH, they converted 400 μM all-trans-retinal to 20, 90, and 40 μM all-trans-retinol, respectively. These results indicate thatBcALDH and its variants are efficient biocatalysts not only in the conversion of retinal to retinoic acid but also in its conversion to retinol with a cofactor switch and that retinol production can be increased by the variant enzymes. Therefore,BcALDH is a novel bacterial enzyme for the alternative production of retinoic acid and retinol.IMPORTANCEAlthough mammalian ALDHs have catalyzed the conversion of retinal to retinoic acid with NAD(P)+as a cofactor, a bacterial ALDH involved in the conversion is first characterized. The biotransformation of all-trans-retinal to all-trans-retinoic acid byBcALDH and human ALDH was altered to the biotransformation to all-trans-retinol by a cofactor switch using NADPH. Moreover, the production of all-trans-retinal to all-trans-retinol was changed by mutations at positions 194 and 457 inBcALDH. The alternative biotransformation of retinoids was first performed in the present study. These results will contribute to the biotechnological production of retinoids, including retinoic acid and retinol.
Crystal structure of human aldehyde dehydrogenase 1A3 complexed with NAD+ and retinoic acid
