Complementary GH54 and GH43 α-L-Arabinofuranosidases from Fusarium oxysporum Enable Complete Side Chain Degradation of Gum Arabic

Abstract

Gum arabic (GA) is a highly branched arabinogalactan-protein complex widely used in food and pharmaceutical industries. Its complex side chains contain L-arabinofuranose residues linked via α-(1→3) and α-(1→4) bonds, which represent the final structural barrier to complete enzymatic degradation of GA. Here, we isolated and characterized two complementary α-L-arabinofuranosidases, FoAF2 and FoAF3, from Fusarium oxysporum 12S. FoAF2, a glycoside hydrolase (GH) family 54 enzyme, preferentially cleaves α-(1→3)-arabinosyl residues while exhibiting weak activity toward α-(1→4) linkages at high enzyme concentrations. In contrast, the GH43_34 enzyme FoAF3 displays strict specificity for α-(1→4)-arabinosyl residues but requires prior removal of neighboring α-(1→3) substituents for efficient catalysis. Structural modeling using AlphaFold 3 revealed that the constrained catalytic pocket of FoAF3 is highly sensitive to steric hindrance from adjacent branches, explaining its dependence on sequential FoAF2 action for complete debranching. This two-enzyme system functions similarly to the bifunctional Bifidobacterium BIAraE, but achieves the same effect using separate enzymes rather than fused domains. Combined with previously characterized F. oxysporum enzymes, FoAF2 and FoAF3 complete a comprehensive toolkit enabling systematic GA degradation from complex side chains to monosaccharides. These findings provide molecular insights into the mechanisms underlying α-L-arabinofuranosidase specificity and establish a foundation for the enzymatic modification of GA for industrial applications.