Aldehyde dehydrogenases (ALDHs) represent a superfamily of NAD(P)+-dependent enzymes that catalyze oxidation of aldehydes to carboxylic acids. So far, 13 ALDH families have been described in plants. ALDHs play a crucial role in detoxifying aldehydes produced by various metabolic pathways and during adaptation upon various stress conditions. Plant ALDH2 family (EC 1.2.1.-) comprises mitochondrial and cytosolic isoforms and share ~ 60 % amino-acid identity with human ALDH2 (hALDH2). hALDH2 plays a role in ethanol metabolism by catalyzing the oxidation of ethanol-derived acetaldehyde to acetate. The first maize (Zea mays) ADLH2 gene was identified as a male fertility restorer RF2A followed by RF2B. Both genes code for homotetrameric mitochondrial ALDHs with an acetaldehyde activity. In contrast, the role and function of cytosolic ALDH2 members in maize is not known. ALDH7 (EC 126.96.36.199) is also known as Δ1-piperideine-6-carboxylate (P6C) dehydrogenase or antiquitin. Well-studied human ALDH7 (hALDH7) shares ~ 60 % sequence identity with plant orthologues. The enzyme primarily involved in the metabolism of lysine catalyzes the conversion of α-aminoadipic semialdehyde (AASAL) to α-aminoadipate. Only little is known about substrate preferences of plant isoforms. Here we present a detailed biochemical characterization of plant ALDH2 and ALDH7 families by analyzing maize and pea ALDH7 (ZmALDH7 and PsALDH7) and four maize cytosolic ALDH2 isoforms RF2C, RF2D, RF2E and RF2F. Kinetic analysis demonstrates that AASAL is the preferred substrate for plant ALDH7. Aromatic aldehydes including benzaldehyde, anisaldehyde, cinnamaldehyde, coniferaldehyde and sinapaldehyde are the best substrates for cytosolic ALDH2. All ALDH2 isoforms display activity with 3-methyl-2-butenal (isopentenal), which is formed by the oxidation of isoprenoid cytokinins by cytokinin oxidase/dehydrogenase. To better understand substrate specificity, we solved the crystal structures of ZmALDH7, RF2C and RF2F with NAD+ at 2.95, 2.25 and 2.40 Å resolution, respectively and compared them with the human enzymes.
This work was supported by grant 15-22322S from the Czech Science Foundation and grant IGA_PrF_2014020 from Palacký University.