Binding Site Analysis of Cellulose Binding Domain CBDN1from Endoglucanse C ofCellulomonas fimiby Site-Directed Mutagenesis

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Abstract
Endoglucanase C (CenC), a β1,4 glucanase from the soil bacterium Cellulomonas fimi, binds to amorphous cellulose via two homologous cellulose binding domains, termed CBDN1 and CBDN2. In this work, the contributions of 10 amino acids within the binding cleft of CBDN1 were evaluated by single site-directed mutations to alanine residues. Each isolated domain containing a single mutation was analyzed for binding to an insoluble amorphous preparation of cellulose, phosphoric acid swollen Avicel (PASA), and to a soluble glucopyranoside polymer, barley β-glucan. The effect of any given mutation on CBD binding was similar for both substrates, suggesting that the mechanism of binding to soluble and insoluble substrates is the same. Tyrosines 19 and 85 were essential for tight binding by CBDN1 as their replacement by alanine results in affinity decrements of approximately 100-fold on PASA, barley β-glucan, and soluble cellooligosaccharides. The tertiary structures of unbound Y19A and Y85A were assessed by heteronuclear single quantum coherence (HSQC) spectroscopy. These studies indicated that the structures of both mutants were perturbed but that all perturbations are very near to the site of mutation.