High-resolution structure of exo-arabinanase from Penicillium chrysogenum

Arabinanase Abnx from Penicillium chrysogenum 31B, which belongs to the GH93 family, releases arabinobiose from the nonreducing terminus of alpha-1,5-L-arabinan, which is distributed in the primary cell walls of higher plants. Crystal structures of Abnx and of its complex with arabinobiose were determined at the high resolutions of 1.14 angstrom to an R-work of 10.7% (R-free = 12.8%) and 1.04 angstrom to an R-work of 10.4% (R-free = 12.5%). Abnx has a six-bladed beta-propeller fold with a typical ring-closure mode called 'Velcro', in which the last four-stranded beta-sheet is completed by the incorporation of a strand from the N-terminus. Catalytic residues which act as a nucleophile and an acid/base were proposed from the structures and confirmed by site-directed mutagenesis. The substrate-binding groove is enclosed at one end by two residues, Glu64 and Tyr66, which contribute to the recognition of the nonreducing chain end of the polysaccharide. A comparison with the related enzyme Arb93A which has a quite similar overall structure suggested that Abnx has different mechanisms to funnel substrates to the active site and/or to stabilize the transition state.