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 α-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 α-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 α-ketoglutarate in H. volcanii (Johnsen et al. 2009).