a-L-rhamnosidases (E.C. 3.2.1.40) are found widely distributed
in nature and have been reported in animals,
plants, yeast, fungi and bacteria, where they are responsible
for the cleavage of a-L-rhamnose from a wide range of
compounds.1 a-L-rhamnose is found in plants and bacteria
as components of polysaccharides, such as pectins,2 and
the O antigen polysaccharides, responsible for determining
the antigenicity of pathogenic bacteria3; it is also found in
rhamnolipids4 and it is attached to small molecule natural
products, such as rutin. There is industrial interest in arhamnosidases
for use in the debittering of citrus juices
and for the release of flavonoids from rhamnosylated precursors;
in wine production they play a role in the hydrolysis
of glycosylated terpene aroma compounds.
KoRha structure
The crystal structure of KoRha with rhamnose bound was
determined to 2.7A ° resolution. The final model consisted of
two KoRha subunits related by a non-crystallographic twofold
axis (giving a corresponding solvent content of 73%) in
the asymmetric unit, with each monomer containing a
bound rhamnose. Dynamic light scattering had suggested
that KoRha was a homodimer in solution and the structure
of KoRha confirmed this, giving a dimer interface of 1389.9
A °
2 (as calculated using the PISA server (http://www.ebi.ac.
uk/pdbe/pisa/).
Each monomer of KoRha is composed of two
domains. Domain A, the catalytic domain, is mainly ahelical,
consisting of residues 11–30 and 180–523, and
contains the bound rhamnose. Domain B, the dimerization
domain, is a b-sandwich domain consisting of residues
31–179.
