Plant Molecular Biology 47: 73–91, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
Structure-function relationships of β-
-glucan endo- and exohydrolases
from higher plants
Maria Hrmova and Geoffrey B. Fincher
Department of Plant Science, University of Adelaide, Waite Campus, Glen Osmond, SA 5064, Australia (
for correspondence; e-mail geoff.ﬁncher@adelaide.edu.au)
Key words: catalytic mechanism, cell wall hydrolysis, monocotyledons, protein modelling, substrate binding,
-Glucansrepresent an important component of cell walls in the Poaceae family of higher plants.
A number of glycoside endo- and exohydrolases is required for the depolymerization of (1→3),(1→4)-β-
glucans in germinated grain or for the partial hydrolysis of the polysaccharide in elongating vegetative tissues.
The enzymes include (1→3),(1→4)-β-
-glucan endohydrolases (EC 18.104.22.168), which are classiﬁed as family 17
glycoside hydrolases, (1→4)-β-
-glucan glucohydrolases (family 1) and β-
-glucan exohydrolases (family 3).
Kinetic analyses of hydrolytic reactions enable the deﬁnition of action patterns, the thermodynamics of substrate
binding, and the construction of subsite maps. Mechanism-based inhibitors and substrate analogues have been
used to study the spatial orientation of the substrate in the active sites of the enzymes, at the atomic level. The
inhibitors and substrate analogues also allow us to deﬁne the catalytic mechanisms of the enzymes and to identify
catalytic amino acid residues. Three-dimensional structures of (1→3),(1→4)-β-
-glucan glucohydrolases and β-
-glucan exohydrolases are available or can be reliably modelled from the
crystal structures of related enzymes. Substrate analogues have been diffused into crystals for solving of the three-
dimensional structures of enzyme-substrate complexes. This information provides valuable insights into potential
biological roles of the enzymes in the degradation of the barley (1→3),(1→4)-β-
-glucans during endosperm
mobilization and in cell elongation.
Abbreviations: 3D, three-dimensional; HCA, hydrophobic cluster analysis; 4NPG, 4-nitrophenyl β-
Primary cell walls of the Poaceae family of higher
plants comprise cellulosic microﬁbrils embedded in a
matrix that consists predominantly of glucuronoara-
binoxylans and (1→3),(1→4)-β-
amounts of xyloglucans, pectic polysaccharides, glu-
comannans and structural proteins may also be present
(Bacic et al., 1988; Carpita and Gibeaut, 1993).
The relative proportions of these wall components
vary between species and between particular tissues
in a single species. Furthermore, primary walls are
dynamic structures in which the amounts and ﬁne
structural characteristics of constituent polysaccha-
rides change at different stages of development, and in
response to abiotic and biotic stresses. Clearly, a large
number of synthetic and hydrolytic enzymes will be
involved in these processes (Henrissat et al., 2001).
Here we will focus on enzymes that hydrolyse, or
potentially hydrolyse, the (1→3),(1→4)-β-
during normal cell wall metabolism in the Poaceae.
-glucans are especially
abundant in walls of the starchy endosperm, where
they represent an important source of stored glu-
cose for the developing seedling (Morrall and Briggs,
1978). In barley they constitute about 75% of cell