Aspergillus aculeatus β-1,4-Galactanase: Substrate Recognition and Relations to Other Glycoside Hydrolases in Clan GH-A

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Abstract
The three-dimensional structure of Aspergillus aculeatus β-1,4-galactanase (AAGAL), an enzyme involved in pectin degradation, has been determined by multiple isomorphous replacement to 2.3 and 1.8 Å resolution at 293 and 100 K, respectively. It represents the first known structure for a polysaccharidase with this specificity and for a member of glycoside hydrolase family 53 (GH-53). The enzyme folds into a (β/α)8 barrel with the active site cleft located at the C-terminal side of the barrel consistent with the classification of GH-53 in clan GH-A, a superfamily of enzymes with common fold and catalytic machinery but diverse specificities. Putative substrate−enzyme interactions were elucidated by modeling of β-1,4-linked galactobioses into the possible substrate binding subsites. The structure and modeling studies identified five potential subsites for the binding of galactans, of which one is a pocket suited for accommodating the arabinan side chain in arabinogalactan, one of the natural substrates. A comparison with the substrate binding grooves of other Clan GH-A enzymes suggests that shape complementarity is crucial in determining the specificity of polysaccharidases.

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