The α-galactosidase type A gene aglA from Aspergillus niger encodes a fully functional α-N-acetylgalactosaminidase

Two genes in the genome of Aspergillus niger, aglA and aglB, have been assigned to encode for alpha-D-galactosidases variant A and B. However, analyses of primary and 3D structures based on structural models of these two enzymes revealed significant differences in their active centers suggesting important differences in their specificity for the hydrolyzed carbohydrates. To test this unexpected finding, a large screening of libraries from 42 strains of filamentous fungi succeeded in identifying an enzyme from A. niger CCIM K2 that exhibited both alpha-galactosidase and alpha-N-acetylgalactosaminidase activities, with the latter activity predominating. The enzyme protein was sequenced, and its amino acid sequence could be unequivocally assigned to the enzyme encoded the aglA gene. Enzyme activity measurements and substrate docking clearly demonstrated the preference of the identified enzyme for alpha-N-acetyl-D-galactosaminide over alpha-D-galactoside. Thus, we provide evidence that the alpha-galactosidase type A gene aglA from A. niger in fact encodes a fully functional alpha-N-acetylgalactosaminidase using a retaining mechanism.