Study of the Active Site Residues of a Glycoside Hydrolase Family 8 Xylanase

• Published in: Journal of molecular biology Vol. 354; no. 2; pp. 425 - 435
• Main Authors: Collins, T., De Vos, D., Hoyoux, A., Savvides, S.N., Gerday, C., Van Beeumen, J., Feller, G.
• Format: Journal Article Web Resource
• Language: English
• Published: England Elsevier Ltd 25.11.2005; Academic Press Ltd Elsevier Science Ltd
• Subjects: Binding Sites; Biochemistry, biophysics & molecular biology; Biochimie, biophysique & biologie moléculaire; Catalysis; catalytic site residues; Crystallization; Crystallography, X-Ray; Endo-1,4-beta Xylanases - chemistry; Endo-1,4-beta Xylanases - genetics; Endo-1,4-beta Xylanases - metabolism; Enzyme Stability; glycoside hydrolase family 8; Hot Temperature; Hydrogen-Ion Concentration; Kinetics; Life sciences; Models, Molecular; Mutagenesis, Site-Directed; Mutation - genetics; Protein Binding; Protein Conformation; Sciences du vivant; xylanase; DSC; WT; catalytic site residues; glycoside hydrolase family 8; xylanase
• ISSN: 0022-2836; 1089-8638; 1089-8638
• DOI: 10.1016/j.jmb.2005.09.064

Site-directed mutagenesis and a comparative characterisation of the kinetic parameters, pH dependency of activity and thermal stability of mutant and wild-type enzymes have been used in association with crystallographic analysis to delineate the functions of several active site residues in a novel glycoside hydrolase family 8 xylanase. Each of the residues investigated plays an essential role in this enzyme: E78 as the general acid, D281 as the general base and in orientating the nucleophilic water molecule, Y203 in maintaining the position of the nucleophilic water molecule and in structural integrity and D144 in sugar ring distortion and transition state stabilization. Interestingly, although crystal structure analyses and the pH-activity profiles clearly identify the functions of E78 and D281, substitution of these residues with their amide derivatives results in only a 250-fold and 700-fold reduction in their apparent k cat values, respectively. This, in addition to the observation that the proposed general base is not conserved in all glycoside hydrolase family 8 enzymes, indicates that the mechanistic architecture in this family of inverting enzymes is more complex than is conventionally believed and points to a diversity in the identity of the mechanistically important residues as well as in the arrangement of the intricate microenvironment of the active site among members of this family.