Action Pattern and Specificity of an Exo-β-(1 → 3)-d-glucanase from Basidiomycetes Species QM 806

Abstract

Abstract The action pattern and specificity of a highly purified exo-β-(1 → 3)-d-glucanase from Basidiomycetes species QM 806 have been investigated. The glucanase removes single glucose residues from a β-(1 → 3)-linked glucan chain commencing at the nonreducing terminal. It can bypass β-(1 → 6) linkages, yielding gentiobiose quantitatively from this structural feature, and it can cleave β-(1 → 3) linkages adjacent to other types of linkage in the main chain. The enzyme, although typically exo in its mode of attack, can initiate an endo cleavage in that it attacks endo-β-(1 → 3) bonds adjacent to β-(1 → 6) linkages. The enzyme does not catalyze transglycosylation reactions. The action pattern of the enzyme is the same on all structures attacked, regardless of their molecular size, but the rate at which these are attacked varies widely. Although the enzyme shows the greatest preference for substrates containing consecutive β-(1 → 3)-linked glucopyranosyl residues, it will degrade a variety of substrate analogues. It was found that the specificity of the enzyme extends over a number of residues of the substrate and is not directed toward any one particular residue or linkage. To explain these results, it is suggested that the glucanase has a substrate binding site composed of a consecutive series of individual glucosyl residue binding sites or subsites which are so arranged as to correspond spatially with a sequence of β-(1 → 3)-linked glucopyranosyl residues. Substrates were found to be hydrolyzed at a rate proportional to the degree to which their structures correspond to such a sequence of subsites on the enzyme. A similar relationship was found with the ability of these structures to act as inhibitors of the hydrolysis of laminarin, further indicating that the enzyme binds a number of consecutive glucosyl residues on the active site.