Engineered thermostable fungal Cel6A and Cel7A cellobiohydrolases hydrolyze cellulose efficiently at elevated temperatures

Thermostability is an important feature in industrial enzymes: it increases biocatalyst lifetime and enables reactions at higher temperatures, where faster rates and other advantages ultimately reduce the cost of biocatalysis. Here we report the thermostabilization of a chimeric fungal family 6 cellobiohydrolase (HJPlus) by directed evolution using random mutagenesis and recombination of beneficial mutations. Thermostable variant 3C6P has a half-life of 280min at 75 degrees C and a T50 of 80.1 degrees C, a approximate to 15 degrees C increase over the thermostable Cel6A from Humicola insolens (HiCel6A) and a approximate to 20 degrees C increase over that from Hypocrea jecorina (HjCel6A). Most of the mutations also stabilize the less-stable HjCel6A, the wild-type Cel6A closest in sequence to 3C6P. During a 60-h Avicel hydrolysis, 3C6P released 2.4 times more cellobiose equivalents at its optimum temperature (Topt) of 75 degrees C than HiCel6A at its Topt of 60 degrees C. The total cellobiose equivalents released by HiCel6A at 60 degrees C after 60h is equivalent to the total released by 3C6P at 75 degrees C after approximate to 6h, a 10-fold reduction in hydrolysis time. A binary mixture of thermostable Cel6A and Cel7A hydrolyzes Avicel synergistically and released 1.8 times more cellobiose equivalents than the wild-type mixture, both mixtures assessed at their respective Topt. Crystal structures of HJPlus and 3C6P, determined at 1.5 and 1.2 angstrom resolution, indicate that the stabilization comes from improved hydrophobic interactions and restricted loop conformations by introduced proline residues. Biotechnol. Bioeng. 2013; 110: 1874-1883. (c) 2013 Wiley Periodicals, Inc.