Substrate Recognition of a Family 10 Xylanase from Streptomyces olivaceoviridis E-86: A Study by Site-directed Mutagenesis to Make an Hindrance around the Entrance toward the Substrate-binding Cleft

Abstract

A family 10 xylanase of Streptomyces olivaceoviridis E-86 (SoXyn10A) is known to have a modular structure where an N-terminal catalytic module and a C-terminal xylan-binding module (XBM) are connected by a Gly/Pro-rich linker. The crystal structure of native SoXyn10A indicates that the XBM is located beyond -3 subsite from the substrate binding cleft. To investigate the mechanism of xylan binding by SoXyn10A, several kinds of cleft mutants of SoXyn10A were constructed. The mutants had an hindrance in the outer side of the (-) subsite or (+) subsite in the substrate binding cleft. Circular dichroism spectral analysis indicated that the mutant enzymes fold as a native enzyme. Kinetic studies for the mutants were performed by using p-nitrophenyl-β-D-xylobioside. The kcat/Km value did not change for the mutants which had a hindrance in the outer side of the (+) subsite while the value significantly decreased for the mutants which had a hindrance in the outer side of the (-) subsite, suggesting that xylan come into the substrate binding cleft of SoXyn10A from the direction of (-) subsites. The activities of the mutants against insoluble xylan were tested with or without XBM. All the mutants possessing XBM showed higher activity than those possessing the catalytic modules without XBM, indicating that the linker sequence connecting the catalytic module and XBM is flexible so that XBM binds to insoluble xylan to increase the concentration around the enzyme.