The crystal structure of the GH78 family a-rhamnosidase from Klebsiella oxytoca (KoRha) has been determined at 2.7 A °
resolution with rhamnose bound in the active site of the catalytic domain. Curiously, the putative catalytic acid, Asp 222, is
preceded by an unusual non-proline cis-peptide bond which helps to project the carboxyl group into the active centre. This
KoRha homodimeric structure is significantly smaller than those of the other previously determined GH78 structures.
Nevertheless, the enzyme displays a-rhamnosidase activity when assayed in vitro, suggesting that the additional structural
domains found in the related enzymes are dispensible for function.
Function
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.5 In the
context of expanding and diversifying the suite of enzymes
available to us for carbohydrate biotransformations,6–13 we
were drawn to consider a-rhamnosidases.
In the CAZy database,14 rhamnosidases are currently
classified into GH28 pectin hydrolases, GH78 containing
exclusively rhamnosidases, and GH106 containing a single enzyme from Sphingomonas
Disease
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Structural highlights
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