Biochem. J. (2008) 413
(175–183) (Printed in Great Britain)
The role of double covalent flavin binding in chito-oligosaccharide oxidase from Fusarium graminearum
Dominic P. H. M. HEUTS, Remko T. WINTER, Gerke E. DAMSMA, Dick B. JANSSEN and Marco W. FRAAIJE1
Laboratory of Biochemistry, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
ChitO (chito-oligosaccharide oxidase) from Fusarium graminearum catalyses the regioselective oxidation of N-acetylated oligosaccharides. The enzyme harbours an FAD cofactor that is covalently attached to His94 and Cys154. The functional role of this unusual bi-covalent flavin–protein linkage was studied by site-directed mutagenesis. The double mutant (H94A/C154A) was not expressed, which suggests that a covalent flavin–protein bond is needed for protein stability. The single mutants H94A and C154A were expressed as FAD-containing enzymes in which one of the covalent FAD–protein bonds was disrupted relative to the wild-type enzyme. Both mutants were poorly active, as the kcat decreased (8.3- and 3-fold respectively) and the Km increased drastically (34- and 75-fold respectively) when using GlcNac as the substrate. Pre-steady-state analysis revealed that the rate of reduction in the mutant enzymes is decreased by 3 orders of magnitude when compared with wild-type ChitO (kred=750 s−1) and thereby limits the turnover rate. Spectroelectrochemical titrations revealed that wild-type ChitO exhibits a relatively high redox potential (+131 mV) and the C154A mutant displays a lower potential (+70 mV), while the H94A mutant displays a relatively high potential of approximately +164 mV. The results show that a high redox potential is not the only prerequisite to ensure efficient catalysis and that removal of either of the covalent bonds may perturb the geometry of the Michaelis complex. Besides tuning the redox properties, the bi-covalent binding of the FAD cofactor in ChitO is essential for a catalytically competent conformation of the active site.
Key words: bi-covalent FAD, oligosaccharides, oxidase, pre-steady-state kinetics, redox potential.
Abbreviations used: AknOx, aclacinomycin oxidoreductase; BBE, berberine bridge enzyme; ChitO, chito-oligosaccharide oxidase; EMT, enzyme-monitored turnover; GOOX, gluco-oligosaccharide oxidase; MBP, maltose-binding protein; NCBI, National Center for Biotechnology Information; OTTLE, optically transparent thin-layer electrode; SHE, standard hydrogen electrode; VAO, vanillyl-alcohol oxidase.
1To whom correspondence should be addressed (email m.w.fraaije@rug.nl).
Received 26 November 2007/27 February 2008; accepted 19 March 2008
Published as BJ Immediate Publication 19 March 2008, doi:10.1042/BJ20071591
© The Authors Journal compilation © 2008 Biochemical Society
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