L-Arabinose degradation pathway in the haloarchaeon Haloferax volcanii involves a novel type of L-arabinose dehydrogenase

The pathway of l-arabinose degradation was studied in the haloarchaeon Haloferax volcanii. It is shown that l-arabinose is oxidatively degraded to alpha-ketoglutarate. During growth on l-arabinose, l-arabinose dehydrogenase (l-AraDH) was induced. The enzyme was purified as a 130 kDa homotetrameric protein catalyzing the oxidation of l-arabinose with both NADP(+) and NAD(+). The gene encoding l-AraDH was identified as HVO_B0032 and recombinant l-AraDH showed similar properties as the native enzyme. The l-AraDH deletion mutant did not grow on l-arabinose, but grew unaffected on glucose and d-xylose, indicating a specific involvement in l-arabinose degradation. Phylogenetic analyses attribute the first archaeal l-AraDH to the extended short-chain dehydrogenase/reductase (SDRe) family, where it is part of a novel cluster and thus differs from known archaeal and bacterial pentose dehydrogenases. Further, cell extracts of H. volcanii catalyzed the NADP(+)-dependent conversion of l-arabinoate to alpha-ketoglutarate. The genes involved in that conversion were identified by analyses of transcripts and deletion mutants as HVO_B0038A, HVO_B0027 and HVO_B0039 recently reported to be involved in d-xylonate conversion to alpha-ketoglutarate in H. volcanii (Johnsen et al. 2009).