Probable involvement of sulfhydryl groups and a metal as essential components of the cellulase of Clostridium thermocellum |
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Authors: | Eric A. Johnson Arnold L. Demain |
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Affiliation: | (1) Fermentation Microbiology Laboratory, Department of Nutrition and Food Science, Massachusetts Institute of Technology, 02139 Cambridge, MA, USA |
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Abstract: | The crude extracellular cellulase from Clostridium thermocellum was oxidatively inactivated by air and inhibited by sulfhydryl reagents. Activity-loss was prevented and reversed by the addition of a high concentration (10 mM) dithiothreitol (DDT) at zero time and up to 24 h respectively. In the presence of a low concentration (0.4 mM) of DTT, the enzyme was more rapidly inactivated than in air alone. This was probably due to autoxidation of the low DTT concentration to H2O2 as shown by its prevention by a high DTT concentration, exclusion of air, or catalase; and by the oxidative inactivation of the enzyme by H2O2. The inactivation by H2O2 could be prevented by a high concentration of DTT but not by air exclusion. EDTA protected the enzyme from inactivation in air by a low concentration of DTT or by H2O2. This is presumably due to the role of metals in oxidation of SH groups. Furthermore, copper (5 M) also caused inactivation and this was prevented by the presence of a high DTT concentration. Even in the protective atmosphere of a high DTT concentration, cellulase was inactivated by certain apolar chelating agents such as o-phenanthroline and -1-dipyridyl, such inactivation being preventable by the prior incubation of the chelator with a mixture of Fe2+ and Fe3+. These data suggest that the clostridial cellulase, unlike the enzyme from aerobic fungi, contains essential sulfhydryl groups and is stimulated by iron. The endo--glucanase component of the cellulase complex was not susceptible to oxidative inactivation.Abbreviations DTT dithiothreitol - CMC carboxymethylcellulose - DTNB 5,5-dithiobis-(2-nitrobenzoic acid) - NEM N-ethylmaleimide - p-CMB p-chloromercuribenzoic acid |
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Keywords: | Cellulase Clostridium thermocellum Extracellular enzymes Anaerobiosis Sulthydryl oxidation Metalloenzyme |
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