Atomic resolution structure of endoglucanase Cel5A in complex with methyl 4,4(II),4(III),4(IV) tetrathio-alpha-cellopentoside highlights the alternative binding modes targeted by substrate mimics

Many three-dimensional structures of retaining beta -D-glycoside hydrolases have been determined, yet oligosaccharide complexes in which the ligand sp ans the catalytic centre are rare. Those that have been reported so far hav e revealed two modes of binding: those in which the substrate adopts a dist orted skew-boat or envelope conformation in the -1 subsite, reflecting the distortion observed during the catalytic cycle, and those which bypass the true catalytic centre and thus lie in a non-productive manner across the -1 subsite. The three-dimensional structure of a retaining endocellulase, Bac illus agaradhaerens Cel5A, in complex with methyl 4,4(II),4(III),4(IV)-tetr athi-alpha -cellopentoside falls into this latter category. The 1.1 Angstro m structure reveals the binding of five pyranosides, all in the C-4(1) chai r conformation, occupying the -3, +2, +1 and +2 subsites whilst evading the catalytic machinery located in the true -1 subsite. Such binding is in mar ked contrast to the structure of another retaining endocellulase, the Fusar ium oxysporum Cel7B, the identical ligand in which displayed a distorted sk ew-boat conformation at the active centre. These two binding modes may refl ect different steps in the binding and catalytic process.