Physical state of κ-carrageenan modulates the mode of action of κ-carrageenase from Pseudoalteromonas carrageenovora

Abstract

Pseudoalteromonas carrageenovora κ-carrageenase is a glycoside hydrolase involved in the bioconversion of carrageenans. Carrageenans are sulfated 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 characterize the enzymatic depolymerization of κ-carrageenan, soluble and solid substrates (in both gel and powder forms) were incubated with P. carrageenovora κ-carrageenase. The average molecular mass 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 mass of solid carrageenan decreased very slowly, and tetrasaccharide production was high. Interestingly, experimentally determined processivity was 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.