Protein Engineering, Vol 11, 1121-1128, Copyright © 1998 by Oxford University Press
S Fushinobu, K Ito, M Konno, T Wakagi and H Matsuzawa
Xylanase C from Aspergillus kawachii has an optimum pH of 2.0 and is stable
at pH 1.0. The crystal structure of xylanase C was determined at 2.0 A
resolution (R-factor = 19.4%). The overall structure was similar to those
of other family 11 xylanases. Asp37 and an acid-base catalyst, Glu170, are
located at a hydrogen-bonding distance (2.8 A), as in other xylanases with
low pH optima. Asp37 of xylanase C was replaced with asparagine and other
residues by site-directed mutagenesis. Analyses of the wild-type and mutant
enzymes showed that Asp37 is important for high enzyme activity at low pH.
In the case of the asparagine mutant, the optimum pH shifted to 5.0 and the
maximum specific activity decreased to about 15% of that of the wild-type
enzyme. On structural comparison with xylanases with higher pH optima,
another striking feature of the xylanase C structure was found; the enzyme
has numerous acidic residues concentrated on the surface (so-called
'Ser/Thr surface' in most family 11 xylanases). The relationship of the
stability against extreme pH conditions and high salt concentrations with
the spacially biased distribution of charged residues on the proteins is
discussed.
ARTICLES
Crystallographic and mutational analyses of an extremely acidophilic and acid-stable xylanase: biased distribution of acidic residues and importance of Asp37 for catalysis at low pH
Department of Biotechnology, University of Tokyo, Japan.
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