Purification and Cooperative Activity of Enzymes Constituting the Xylan-Degrading System of Thermomonospora fusca

Abstract

The thermophilic actinomycete Thermomonospora fusca produced endoxylanase, alpha-arabinofuranosidase, beta-xylosidase, and acetyl esterase activities maximally during growth on xylan. Growth yields on glucose, xylose, or arabinose were comparable, but production of endoxylanase and beta-xylosidase was not induced on these substrates. The crude xylanase activity was thermostable and relatively resistant to end product inhibition by xylobiose and xylan hydrolysis products. Six proteins with xylanase activity were identified by zymogram analysis of isoelectric focusing gels, but only a 32-kDa protein exhibiting three isomeric forms could be purified by fast protein liquid chromatography. Endoglucanases were also identified in carboxymethylcellulose-grown cultures, and their distinction from endoxylanases was confirmed. alpha-Arabinofuranosidase activity was due to a single dimeric protein of 92 kDa, which was particularly resistant to end product inhibition by arabinose. Three bands of acetyl esterase activity were detected by zymogram analysis, and there was evidence that these mainly consisted of an intracellular 80-kDa protein secreted to yield active 40-kDa subunits in the culture supernatant. The acetyl esterases were found to be responsible for acetyl xylan esterase activity in T. fusca, in contrast to the distinction proposed in some other systems. The addition of purified betaxylosidase to endoxylanase increased the hydrolysis of xylan, probably by relieving end product inhibition. The enhanced saccharification of wheat straw caused by the addition of purified alpha-arabinofuranosidase to T. fusca endoxylanase suggested a truly synergistic relationship, in agreement with proposals that arabinose side groups on the xylan chain participate in cross-linking within the plant cell wall structure.