Structure-based engineering of glucose specificity in a family 10 xylanase from Streptomyces olivaceoviridis E-86

Substrate specificity is one of the most important functional property of enzymes. We use family 10 xylanase from Streptomyces olivaceoviridis as a model for substrate specificity of glycoside hydrolases. Seven variants were initially designed to change the preference from xylose to glucose at substrate binding subsites −2 and −1. The known mobility of Trp at the −1 subsite and the influence of its environment, which is different in subset 1 and subset 2 family 10 enzymes, were taken into account in variant design. Q88A/R275A had the best ratio of p-nitrophenyl cellobioside vs p-nitrophenyl xylobioside hydrolyzing activity in the first series of variants. The crystal structure shows a movement of Trp274 compared to the native, as a result of loss of interaction with the long side chain of Arg275. The movement creates extra space for the hydroxymethyl of glucose, resulting in improved Km on glucose derived substrates, while the negative effect on kcat is compensated by the Q88A mutation, which also contributes to a further reduction of Km. Further mutagenesis based on the Q88A/R275A variant resulted in 5.2 times improvement compared to the wild-type p-nitrophenyl cellobioside hydrolyzing activity, which is the best improvement obtained so far for an engineered xylanase.