Enhanced expression and activity yields of Clostridium thermocellum xylanases without non-catalytic domains

Abstract

Two major xylanase components, XynC and XynZ from the anaerobic thermophilic bacterium, Clostridium thermocellum cellulosome were cloned and expressed with and without non-catalytic domains in E. coli. Two constructs of XynC, one with its cellulose binding domain and the catalytic domain (pXynC-BC) and the other with only the catalytic domain (pXynC-C) were produced. For XynZ the constructs produced were pXynZ-BDC, which included the dockerin domain, and pXynZ-C, which did not. E. coli cells transformed with pXynC-BC or pXynZ-BDC gave xylanase expression of 30% and 25% total cell proteins, respectively. Transformation of E. coli cells with the constructs carrying only the catalytic domains gave expression levels of ∼45% in each case. The specific activities of XynC with and without the non-catalytic domains were similar, but for XynZ the specific activity of the enzyme without the non-catalytic domains was ∼5-fold greater than that of the intact enzyme. The total activity increased from 1925 U l −1 OD 600 −1 for XynC-BC to 3050 U l −1 OD 600 −1 for XynC-C. However, the overall increase in activity was ∼9-fold higher for XynZ-C (32,900 U l −1 OD 600 −1) versus XynZ-BDC (3665 U l −1 OD 600 −1). Both the enzymes with and without non-catalytic domains were found to be quite stable over a broad pH range (pH 4–9). XynZ-C was more thermostable than XynZ-BDC as it retained 87% of xylanase activity when incubated at 70 °C for 2 h as compared to 42% for XynZ-BDC. However, XynC-BC retained 70% activity on incubation at 70 °C for 2 h but XynC-C lost all activity under the same conditions. K m values for XynC-BC and XynC-C determined on soluble xylan were 3.1 and 3.6 mg ml −1, respectively, whereas these values for XynZ-BDC and XynZ-C were 33.3 and 15.4 mg ml −1, respectively. Thus the production of xylanase activity by expressing only the catalytic domains of XynC and XynZ is significantly enhanced.