Specificity of Microbial α-Amylase and Amyloglucosidase Hydrolysis of Methyl Amyloses

Abstract

Methyl amyloses prepared under various conditions were exhaustively digested by means of α-amylase (B. licheniformis) and amyloglucosidase (A. niger). The amount of glucose that was released by the action of the enzymes was determined. Degradation products were further investigated as their O-methyl-O-deuteromethyl derivatives by mass spectrometry. The substituent distribution in the reduced and O-methylated-O-ethylated oligosaccharides was determined, differentiating between non-reducing terminal glucosyl residues, 1→4-linked inner glucosyl units and reducing glucose end groups. The portion of glucose that could be liberated by the enzymes decreased with increasing degree of substitution (DS) but at the same DS it was considerably higher for heterogeneously prepared amylose ethers than for those prepared under homogeneous conditions. Mass spectrometry (fast atom bombardment, FAB- and matrix-assisted laser desorption ionisation, MALDI-MS) gave evidence of different oligomer patterns and average values of degree of substituent/degree of polymerisation (DS/DP) in dependence on the methylation conditions applied. More detailed analysis of the O-methylated positions of the oligosaccharides showed that the reducing glucose end group was usually unsubstituted, while the non-reducing glucosyl residues could be 2-, 6- or even 2,6-di-O-substituted. In contrast, all 3-O-methyl groups were located in the inner 1,4-linked glucosyl units, indicating inhibition of both enzymes. The results of this selective partial degradation are compared with those obtained after chemical random degradation and mass spectrometry.