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J. Biol. Chem., Vol. 276, Issue 21, 17895-17901, May 25, 2001

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Directed Mutagenesis of Specific Active Site Residues on Fibrobacter succinogenes 1,3-1,4--D-Glucanase Significantly Affects Catalysis and Enzyme Structural Stability^*

Jui-Lin Chen, Li-Chu Tsai^§, Tuan-Nan Wen^¶, Jyh-Bing Tang, Hanna S. Yuan^§, and Lie-Fen Shyur

From the Institutes of  BioAgricultural Sciences, ^§ Molecular Biology, and ^¶ Botany, Academia Sinica, Taipei 115, Taiwan, Republic of China

The functional and structural significance of amino acid residues Met³⁹, Glu⁵⁶, Asp⁵⁸, Glu⁶⁰, and Gly⁶³ of Fibrobacter succinogenes 1,3-1,4--D-glucanase was explored by the approach of site-directed mutagenesis, initial rate kinetics, fluorescence spectroscopy, and CD spectrometry. Glu⁵⁶, Asp⁵⁸, Glu⁶⁰, and Gly⁶³ residues are conserved among known primary sequences of the bacterial and fungal enzymes. Kinetic analyses revealed that 240-, 540-, 570-, and 880-fold decreases in k_cat were observed for the E56D, E60D, D58N, and D58E mutant enzymes, respectively, with a similar substrate affinity relative to the wild type enzyme. In contrast, no detectable enzymatic activity was observed for the E56A, E56Q, D58A, E60A, and E60Q mutants. These results indicated that the carboxyl side chain at positions 56 and 60 is mandatory for enzyme catalysis. M39F, unlike the other mutants, exhibited a 5-fold increase in K_m value. Lower thermostability was found with the G63A mutant when compared with wild type or other mutant forms of F. succinogenes 1,3-1,4--D-glucanase. Denatured wild type and mutant enzymes were, however, recoverable as active enzymes when 8 M urea was employed as the denaturant. Structural modeling and kinetic studies suggest that Glu⁵⁶, Asp⁵⁸, and Glu⁶⁰ residues apparently play important role(s) in the catalysis of F. succinogenes 1,3-1,4--D-glucanase.

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