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Galactomannans are hemicelluloses composed of a β-1,4-linked mannose backbone with α-1,6-galactose substitutions.
They are part of our diet as seed storage polysacchardies and food thickeners and are utilised
by several human gut bacteria (1). One such bacteria, Bacteroides ovatus, contains a gene cluster encoding
two glycoside hydrolase family 26 β-mannanases, BoMan26A and BoMan26B (2). BoMan26B generates a
range of product lengths upon mannan hydrolysis, prefers longer substrates and is less restricted by galactose
side-groups than BoMan26A, which mainly generates mannobiose (3,4). The results suggest that BoMan26B
performs the initial attack on galactomannan, generating oligosaccharides that are further hydrolysed by Bo-
Man26A. Crystal structures of these two enzymes reveal the structural basis for their biochemical differences.
BoMan26B, with galactosyl-mannotetraose bound in subsites –5 to –2, has an open and long active site cleft
with W112 in subsite –5 concluded to be involved in mannosyl interaction (4). Moreover, K149 in the –4 subsite
interacted with the galactosyl side-group of the ligand, which may indicate a preference in for substituted
manno-oligosaccharides (4). BoMan26A instead revealed a narrow active site cleft that is restricted in one end
by a loop, explaining its preference for generating shorter products (6).
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Gilbert, D. N. Bolam and J. I. Gordon PLoS Biol (2011). 9: e1001221 - Bågenholm, V., S. K. Reddy, H. Bouraoui, J. Morrill, E. Kulcinskaja, C. M., Bahr, O. Aurelius, T. Rogers,
Y.Xiao, D. T. Logan, E. C. Martens, N. M. Koropatkin and H. Stålbrand J Biol Chem (2017). 292: 229-243 - Bågenholm, V., M. Wiemann, S. K. Reddy, A. Bhattacharya, A. Rosengren, D. T. Logan and H. Stålbrand J
Biol Chem (2019). 294: 9100-9117
