SUMMARY PARAGRAPH for XYL2
XYL2 encodes xylitol dehydrogenase, a zinc-containing member of the medium-chain dehydrogenase/reductase superfamily (polyol dehdrogenase family; 4, 5). Although XYL2 deletion mutants have cell wall defects (3), and XYL2 expression is induced in the presence of the sugar xylose (1), it is not entirely clear what role Xyl2p plays in S. cerevisiae. Unlike some fungi, such as Pichia stipitis, S. cerevisiae is unable to utilize or ferment xylose, the pentose sugar that is an abundant component of lignocellulose, one of the structural materials common in plants (1, 2). In fungi that do utilize xylose, xylitol dehydrogenase is part of the fungal xylose utilization pathway, a pathway of cytosolic enzymes that convert xylose to xylulose 5-phosphate (6), which is then used by the pentose phosphate pathway (1).
The construction of S. cerevisiae strains able to ferment xylose from woody plant materials is of major industrial interest, because these strains could be used for the efficient production of biofuels (1, 7, 8). The construction of xylitol-producing S. cerevisiae strains is also of industrial interest, primarily because xylitol tastes sweet but its metabolism is independent of insulin, making it safe for diabetics (9).
About the medium-chain dehydrogenase/reductase (MDR) family
Medium-chain dehydrogenase/reductases (MDRs), sometimes referred to as long-chain dehydrogenases (10), constitute an ancient and widespread enzyme superfamily with members found in Bacteria, Archaea, and Eukaryota (4, 5). Many MDR members are basic metabolic enzymes acting on alcohols or aldehydes, and thus these enzymes may have roles in detoxifying alcohols and related compounds, protecting against environmental stresses such as osmotic shock, reduced or elevated temperatures, or oxidative stress (4). The family also includes the mammalian zeta-crystallin lens protein, which may protect the lens against oxidative damage and enzymes which produce lignocellulose in plants (4).
MDR enzymes typically have subunits of about 350 aa residues and are two-domain proteins, with a catalytic domain and a second domain for binding to the nicotinamide cofactor, either NAD(H) or NADP(H) (4, 5). They contain 0, 1, or 2 zinc atoms (11). When zinc is present, it is involved in catalysis at the active site.
Based on phylogenetic and sequence analysis, the members of the MDR superfamily can be further divided into more closely related subgroups (4, 5). In families which are widespread from prokaryotes to eukaryotes, some members appear conserved across all species, while others appear to be due to lineage specific duplications. Some subgroups are only found in certain taxa. S. cerevisiae contains fifteen (4) or twenty-one (5) members of the MDR superfamily, listed below. The difference in number is due to six sequences that were included as members of the quinone oxidoreductase family by Riveros-Rosas et al. (5) but not by Nordling et al. (4).
Zinc-containing enzyme groups:
- PDH; "polyol" dehydrogenase family - BDH1, BDH2, SOR1, SOR2, XYL2
- ADH; class III alcohol dehydrogenase family - SFA1
- Y-ADH; "yeast" alcohol dehydrogenase family - ADH1, ADH2, ADH3, ADH5
- CADH; cinnamyl alcohol dehydrogenase family - ADH6, ADH7
Non-zinc-containing enzyme groups:
- NRBP; nuclear receptor binding protein (5) or MRF; mitochondrial respiratory function (4) family - ETR1
- QOR; quinone oxidoreductase family - ZTA1 (4, 5), AST1, AST2, YCR102C, YLR460C, YMR152W, YNL134C (5)
- LTD; leukotriene B4 dehydrogenases - YML131W
- ER; enoyl reductases (5) or ACR; acyl-CoA reductase (4) family - no members in S. cerevisiae
Last updated: 2008-08-19