Abstract
The intact crystal structure of family 10 xylanase (SoXyn10A) from Streptomyces olivaceoviridis indicates that the catalytic domain of SoXyn10A consists of nine α-helices (α0-8) and eight β-sheets (β1-8). Interaction in the α-helices of N-terminal (α0) and C-terminal (α8) of catalytic domain of SoXyn10A by 3 hydrogen bonds and 8 hydrophobic interactions is observed and predicted to playing an important role for the stability of the molecule. Therefore, the importance for the stability and folding of SoXyn10A were examined by using C-terminal truncated mutants of SoXyn10A. The thermostability was gradually decreased when the C-terminal was shortened; however, the enzyme activities were not influenced by the length of the C-terminal. The investigation of the stability using guanidine hydrochloride agreed with the expected results; namely the hydrophobic core was completed at Leu-300 and the resulting stability was not changed if the C-terminal was longer than it. The thermostabilty slightly decreased when Asn-252 was replaced with Ala, suggesting hydrogen bonding of Gly-303 with Asn-252 is also important for the stability of the molecule. When the effect of the interaction was observed in chimeric xylanases, which have a slight distortion in the structure, the C-terminal 4 amino acids certainly increased the thermostability of the chimeric enzymes. However, the N- and C-terminal of CfXyn10A from Cellulomonas fimi displayed by that of SoXyn10A decreased thermostability at the same degree as SoXyn10A, suggesting that the limit temperature of the interaction agrees with that of SoXyn10A and the distortion of both terminals make denaturating of the protein easy.
