Ritva Verho, John Londesborough,
Merja Penttilä and Peter
Richard*
VTT Biotechnology
PO Box 1500
02044 VTT,
Phone: +358 9 456 7116
Fax: +358 9 455 2103
E-mail: Ritva.Verho@vtt.fi
An NADH-dependent L-xylulose reductase and the
corresponding gene were identified from the yeast Ambrosiozyma monospora. The enzyme is part of the
yeast pathway for L-arabinose catabolism. A fungal
pathway for L-arabinose utilization has been earlier
described for molds. In this pathway L-arabinose is
sequentially converted to L-arabinitol, L-xylulose, xylitol and D-xylulose and enters the pentose phosphate pathway as D-xylulose 5-phosphate. In molds the reductions are
NADPH-linked and the oxidations are NAD+-linked. In this
communication we show that in A. monospora the pathway is similar, i.e. it has the same
two reduction and two oxidation reactions, but the reduction by L-xylulose reductase is not performed
by a strictly NADPH-dependent enzyme as in molds but by a strictly
NADH-dependent enzyme. The ALX1 gene
encoding the NADH-dependent L-xylulose reductase is strongly expressed during growth on L-arabinose as shown by Northern analysis. The gene was functionally
overexpressed in Saccharomyces cerevisiae and the purified his-tagged
protein characterized. The reversible enzyme converts L-xylulose
to xylitol. It is specific for sugar alcohols where
in a Fischer projection the hydroxyl group of the C2 is in L and the hydroxyl
group of C3 in D-configuration. To our knowledge this is the first report of an
NADH-linked L-xylulose reductase.