Characterization of the individual components of the xylanolytic enzyme system of Talaromyces emersonii

Abstract

Talaromyces emersonii produces a complete xylandegrading enzyme system when grown on appropriate substrates (Tuohy & Coughlan, 1992). The individual components are all markedly thermostable glycoproteins with half-life values at pH 4–5 in excess of 8 days at 50°C and, depending on the enzyme, from 5–250 min at 80°C. The system includes at least 13 endo-β-1,4- d-xylanases (EC3.2.1.8), one β- d-xylosidase (EC 3.2.1.37), one α- l-arabinofuranosidase (EC 3.2.1.55), three acetylxylan esterases, while α- d-glucuronidase, ferulic and p-coumaric acid esterase activities are also present. Most of these enzymes have been purified to apparent homogeneity and characterized as individual components and in various combinations. The xylanases have M r values ranging from 57000–131 000, pI values from pH 3.5–5.5, pH optima from pH 3.5–4.7, temperature optima from 67–80°C. K m values with the soluble fraction of oat spelts xylan as substrate range from 0.4 to 13.3 mg ml −1 and k cat values from 216–9213 s −1. All but two of the xylanases are active against a variety of xylooligosaccharides and against substituted and unsubstituted xylans (Tuohy et al., 1993 a; b). Products of hydrolysis of a variety of xylooligosaccharides, substituted and unsubstituted xylans vary with the substrate and the enzyme used. There is evidence for transferase, as well direct hydrolase, activity in the case of several components. The two unusual xylanases catalyze the hydrolysis of artificial xylosides, xylooligosaccharides and unsubstituted xylans. Prior removal of arabinosyl residues is a prerequisite to activity against wheat-straw arabinoxylan. The β-xylosidase is a dimer (M r of 181 000; pI 8.9) with pH and temperature optima of 2.5 and 60°C, respectively. The K m value with pNPX as substrate is 0.13 m m and the k cat is 426 s −1. It is active against p- and chloronitrophenyl-β-xylosides and against unsubstituted xylooligosaccharides from X 1 to X 1 with X 1 being the main end-product of hydrolysis. Activity decreases with increasing DP and there is little or no activity against polymeric xylan. The α-arabinofuranosidase is a dimer (M r 210 000) with pH and temperature optima of 3.2 and 70°C, respectively. The k cat is approximately 10.5 s −1. Arabinose is the sole end-product of long-term hydrolysis of arabinoxylan. Examples of homeo- and heterosynergistic reactions (Coughlan et al., 1993) have been noted between the various components in the hydrolysis of xylans.