Glycoside
hydrolase family 7 (GH7) cellobiohydrolases (CBH) play a key role in biomass
recycling in Nature. They are typically the most abundant enzymes expressed by
potent cellulolytic fungi, and are also responsible for the majority of
hydrolytic potential in enzyme cocktails for industrial processing of plant
biomass. The thermostability of the enzyme is an important parameter for
industrial utilization. In this study, GH7 CBHs from different fungi were
expressed in a fungal host and assayed for thermostability, including Hypocrea jecorina Cel7A as reference. The most stable of
the homologs, Humicola grisea
var. thermoidea Cel7A (HgtCel7A), exhibits 10 °C higher melting
temperature (Tm 72.5
°C), showed 4-5 times higher initial hydrolysis rate than H. jecorina Cel7A on phosphoric acid-swollen cellulose, and the
best performance of the tested enzymes on pretreated cornstover at elevated
temperature (65 °C). The enzyme shares 57% sequence identity with H. jecorina Cel7A. The crystal structure of
the HgtCel7A catalytic module shows
that the celluose-binding path is highly conserved, but indicates higher
flexibility of tunnel-defining loops compared to the less thermostable H. jecorina Cel7A (top figure), and reveals a new
conformation near the active centre, not observed previously in GH7 CBHs (bottom
figure). Potentially stabilizing interactions were recognized at the base of
certain loops, i.e. where they
connect to the secondary-structure framework, which may contribute to the
higher thermostability. In conclusion, HgtCel7A
is a promising GH7 CBH candidate with potential for exploitation in
biomass-conversion applications.
