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The crystal structure of the catalytic core domain of endoglucanase I from Trichoderma reesei at 3.6 Å resolution, and a comparison with related enzymes 1 1Edited by K.Nagai

1997; Elsevier BV; Volume: 272; Issue: 3 Linguagem: Inglês

10.1006/jmbi.1997.1243

1089-8638

Gerard J. Kleywegt, Jin-Yu Zou, Christina Divne, G.J. Davies, Irmgard Sinning, Jerry Ståhlberg, Tapani Reinikainen, Malee Srisodsuk, Tuula T. Teeri, T. Alwyn Jones,

Studies on Chitinases and Chitosanases

Cellulose is the most abundant polymer in the biosphere. Although generally resistant to degradation, it may be hydrolysed by cellulolytic organisms that have evolved a variety of structurally distinct enzymes, cellobiohydrolases and endoglucanases, for this purpose. Endoglucanase I (EG I) is the major endoglucanase produced by the cellulolytic fungus Trichoderma reesei, accounting for 5 to 10% of the total amount of cellulases produced by this organism. Together with EG I from Humicola insolens and T. reesei cellobiohydrolase I (CBH I), the enzyme is classified into family 7 of the glycosyl hydrolases, and it catalyses hydrolysis with a net retention of the anomeric configuration. The structure of the catalytic core domain (residues 1 to 371) of EG I from T. reesei has been determined at 3.6 Å resolution by the molecular replacement method using the structures of T. reesei CBH I and H. insolens EG I as search models. By employing the 2-fold non-crystallographic symmetry (NCS), the structure was refined successfully, despite the limited resolution. The final model has an R-factor of 0.201 (Rfree 0.258). The structure of EG I reveals an extended, open substrate-binding cleft, rather than a tunnel as found in the homologous cellobiohydrolase CBH I. This confirms the earlier proposal that the tunnel-forming loops in CBH I have been deleted in EG I, which has resulted in an open active site in EG I, enabling it to function as an endoglucanase. Comparison of the structure of EG I with several related enzymes reveals structural similarities, and differences that relate to their biological function in degrading particular substrates. A possible structural explanation of the drastically different pH profiles of T. reesei and H. insolens EG I is proposed.