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Abstract
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Endoglucanase Cel9A from Alicyclobacillus acidocaldarius (AaCel9A) is a monomeric enzyme with 537 residues. This enzyme has an Ig-like domain in the N-terminus of the catalytic domain In this study, the role of the Ig-like domain on the activity, stability and structural rigidity of AaCel9A and the effect of calcium on enzyme activity and stability were examined by comparing a truncated enzyme with deletion of the Ig-like domain (AaCel9AΔN) to the wild-type enzyme. Our results showed that the deletion of the Ig-like domain increased the catalytic efficiency of the truncated enzyme up to three folds without any significant changes in the Km of the enzyme. Furthermore, pH and temperature optimum for activity were shifted from 6.5 to 7.5 and from 65˚C to 60˚C, respectively, by deletion of the Ig-like domain. The thermal stability and fluorescence quenching results indicated that the stability and rigidity of the truncated enzyme have been more than that of the wild-type enzyme. Calcium similarly increased the catalytic efficiency of the enzymes (up to 40%) and remarkably raised the stability of the AaCel9A compared to the AaCel9AΔN. This shows that Ig-like domain has a role in the increase of the enzyme stability by Calcium in the wild-type enzyme.
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