Crystal structures of an atypical aldehyde dehydrogenase having bidirectional oxidizing and reducing activities

Abstract Aldehyde dehydrogenases (ALDHs) are NAD(P) + -dependent oxidoreductases that catalyze the oxidation of a variety of aldehydes to their acid forms. In this study, we determined the crystal structures of ALDH from Bacillus cereus ( Bc ALDH), alone, and in complex with NAD + and NADP + . This enzyme can oxidize all- trans -retinal to all- trans -retinoic acid using either NAD + or NADP + with equal efficiency, and atypically, as a minor activity, can reduce all- trans -retinal to all- trans -retinol using NADPH. Bc ALDH accommodated the additional 2′-phosphate of NADP + by expanding the cofactor-binding pocket and upshifting the AMP moiety in NADP + . The nicotinamide moiety in NAD + and NADP + had direct interactions with the conserved catalytic residues (Cys300 and Glu266) and caused concerted conformational changes. We superimposed the structure of retinoic acid bound to human ALDH1A3 onto the Bc ALDH structure and speculated a model of the substrate all- trans -retinal bound to Bc ALDH. We also proposed a plausible mechanism for the minor reducing activity of Bc ALDH. These Bc ALDH structures will be useful in understanding cofactor specificity and the catalytic mechanism of an atypical bacterial Bc ALDH and should help the development of a new biocatalyst to produce retinoic acid and related high-end products.