Abstract
Plant Aldehyde Dehydrogenase10 (ALDH10) enzymes catalyze the oxidation of ω-primary or ω-quaternary aminoaldehydes, but, intriguingly, only some of them, such as the spinach (Spinacia oleracea) betaine aldehyde dehydrogenase (SoBADH), efficiently oxidize betaine aldehyde (BAL) forming the osmoprotectant glycine betaine (GB), which confers tolerance to osmotic stress. The crystal structure of SoBADH reported here shows tyrosine (Tyr)-160, tryptophan (Trp)-167, Trp-285, and Trp-456 in an arrangement suitable for cation-π interactions with the trimethylammonium group of BAL. Mutation of these residues to alanine (Ala) resulted in significant K(m)(BAL) increases and V(max)/K(m)(BAL) decreases, particularly in the Y160A mutant. Tyr-160 and Trp-456, strictly conserved in plant ALDH10s, form a pocket where the bulky trimethylammonium group binds. This space is reduced in ALDH10s with low BADH activity, because an isoleucine (Ile) pushes the Trp against the Tyr. Those with high BADH activity instead have Ala (Ala-441 in SoBADH) or cysteine, which allow enough room for binding of BAL. Accordingly, the mutation A441I decreased the V(max)/K(m)(BAL) of SoBADH approximately 200 times, while the mutation A441C had no effect. The kinetics with other ω-aminoaldehydes were not affected in the A441I or A441C mutant, demonstrating that the existence of an Ile in the second sphere of interaction of the aldehyde is critical for discriminating against BAL in some plant ALDH10s. A survey of the known sequences indicates that plants have two ALDH10 isoenzymes: those known to be GB accumulators have a high-BAL-affinity isoenzyme with Ala or cysteine in this critical position, while non GB accumulators have low-BAL-affinity isoenzymes containing Ile. Therefore, BADH activity appears to restrict GB synthesis in non-GB-accumulator plants.
Highlights
Plant Aldehyde Dehydrogenase10 (ALDH10) enzymes catalyze the oxidation of v-primary or v-quaternary aminoaldehydes, but, intriguingly, only some of them, such as the spinach (Spinacia oleracea) betaine aldehyde dehydrogenase (SoBADH), efficiently oxidize betaine aldehyde (BAL) forming the osmoprotectant glycine betaine (GB), which confers tolerance to osmotic stress
To our knowledge for the first time, we present the crystal structure of a chloroplastic ALDH10 with high BADH activity, the one from spinach (Spinacia oleracea; SoBADH), and report the results of sitedirected mutagenesis of the residues involved in the binding of BAL as well as of the critical residue involved in discriminating BAL from other aminoaldehydes in those plant ALDH10s with poor BADH activity
In a first study with SoBADH, Glu-103, which is strictly conserved in the known plant ALDH10 enzymes, was thought to be this residue; when mutated to Gln, there were no changes in the kinetics with BAL as substrate and only a small negative effect on those with APAL and ABAL (Incharoensakdi et al, 2000)
Summary
Plant Aldehyde Dehydrogenase (ALDH10) enzymes catalyze the oxidation of v-primary or v-quaternary aminoaldehydes, but, intriguingly, only some of them, such as the spinach (Spinacia oleracea) betaine aldehyde dehydrogenase (SoBADH), efficiently oxidize betaine aldehyde (BAL) forming the osmoprotectant glycine betaine (GB), which confers tolerance to osmotic stress. Given the structural similarity between the trimethylammonium and dimethylsulfonium groups, some plant BADHs can use as substrate 3-dimethylsulfoniopropionaldehyde to produce the osmoprotectant 3-dimethylsulfoniopropionate (Trossat et al, 1997; Vojtechovaet al., 1997; Fig. 1) Because of their relatively broad specificity, the plant ALDH10 enzymes may be involved in the synthesis of these two osmoprotectants and in the synthesis of others, such as b-Ala betaine (Rathinasabapathi et al, 2000) and 4-aminobutyric acid (Boucheand Fromm, 2004), as well as in polyamine catabolism and the synthesis of carnitine (Fig. 1). To our knowledge for the first time, we present the crystal structure of a chloroplastic ALDH10 with high BADH activity, the one from spinach (Spinacia oleracea; SoBADH), and report the results of sitedirected mutagenesis of the residues involved in the binding of BAL as well as of the critical residue involved in discriminating BAL from other aminoaldehydes in those plant ALDH10s with poor BADH activity
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