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Structural and biochemical characterization of the laminarinase Zg LamC GH16 from Zobellia galactanivorans suggests preferred recognition of branched laminarin
Laminarin is a β-1,3-D-glucan displaying occasional β-1,6 branches. This storage polysaccharide of brown algae constitutes an abundant source of carbon for marine bacteria such as
Zobellia galactanivorans
. This marine member of the Bacteroidetes possesses five putative β-1,3-glucanases [four belonging to glycosyl hydrolase family 16 (GH16) and one to GH64] with various modular architectures. Here, the characterization of the β-glucanase
Zg
LamC is reported. The catalytic GH16 module (
Zg
LamC
GH16
) was produced in
Escherichia coli
and purified. This recombinant enzyme has a preferential specificity for laminarin but also a significant activity on mixed-linked glucan (MLG). The structure of an inactive mutant of
Zg
LamC
GH16
in complex with a thio-β-1,3-hexaglucan substrate unravelled a straight active-site cleft with three additional pockets flanking subsites −1, −2 and −3. These lateral pockets are occupied by a glycerol, an acetate ion and a chloride ion, respectively. The presence of these molecules in the vicinity of the O6 hydroxyl group of each glucose moiety suggests that
Zg
LamC
GH16
accommodates branched laminarins as substrates. Altogether,
Zg
LamC is a secreted laminarinase that is likely to be involved in the initial step of degradation of branched laminarin, while the previously characterized
Zg
LamA efficiently degrades unbranched laminarin and oligo-laminarins.