Physical state of κ-carrageenan modulates the mode of action of κ-carrageenase from Pseudoalteromonas carrageenovora
Résumé
Pseudoalteromonas carageenovora κ-carrageenase is a glycoside hydrolase involved in the bioconversion of carrageenans. Carrageenans are sulphated galactans that are densely packed in red algal cell walls. Previous crystallographic investigations revealed that the active site of κ-carrageenase has a tunnel-shaped topology, suggesting a processive mode of action for this enzyme. To biochemically characterise the enzymatic depolymerisation of κ-carrageenan, soluble and solid substrates (in both gel and powder forms) were incubated with P. carageenovora κ-carrageenase. The average molecular weight of soluble carrageenan decreased rapidly and all possible degradation products were observed, suggesting random degradation of κ-carrageenan. In contrast, as expected for a processive-type carrageenase, the average molecular weight of solid carrageenan decreased very slowly and tetrasaccharide production was high. Interestingly, experimentally determined processivity was very similar for gel and powder, suggesting that, in addition to an adapted catalytic site, the substrate must be in the solid state for κ-carrageenase processivity to operate, whatever the level of carrageenan ordering.
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