Characterization of a modular enzyme of exo-1,5-alpha-L-arabinofuranosidase and arabinan binding module from Streptomyces avermitilis NBRC14893.

A gene encoding an alpha-L-arabinofuranosidase, designated SaAraf43A, was cloned from Streptomyces avermitilis. The deduced amino acid sequence implies a modular structure consisting of an N-terminal glycoside hydrolase family 43 module and a C-terminal family 42 carbohydrate-binding module (CBM42). The recombinant enzyme showed optimal activity at pH 6.0 and 45 degrees C and was stable over the pH range of 5.0-6.5 at 30 degrees C. The enzyme hydrolyzed p-nitrophenol (PNP)-alpha-L-arabinofuranoside but did not hydrolyze PNP-alpha-L-arabinopyranoside, PNP-beta-D-xylopyranoside, or PNP-beta-D-galactopyranoside. Debranched 1,5-arabinan was hydrolyzed by the enzyme but arabinoxylan, arabinogalactan, gum arabic, and arabinan were not. Among the synthetic regioisomers of arabinofuranobiosides, only methyl 5-O-alpha-L-arabinofuranosyl-alpha-L-arabinofuranoside was hydrolyzed by the enzyme, while methyl 2-O-alpha-L-arabinofuranosyl-alpha-L-arabinofuranoside and methyl 3-O-alpha-L-arabinofuranosyl-alpha-L-arabinofuranoside were not. These data suggested that the enzyme only cleaves alpha-1,5-linked arabinofuranosyl linkages. The analysis of the hydrolysis product of arabinofuranopentaose suggested that the enzyme releases arabinose in exo-acting manner. These results indicate that the enzyme is definitely an exo-1,5-alpha-L-arabinofuranosidase. The C-terminal CBM42 did not show any affinity for arabinogalactan and debranched arabinan, although it bound arabinan and arabinoxylan, suggesting that the CBM42 bound to branched arabinofuranosyl residues. Removal of the module decreased the activity of the enzyme with regard to debranched arabinan. The CBM42 plays a role in enhancing the debranched arabinan hydrolytic action of the catalytic module in spite of its preference for binding arabinofuranosyl side chains.