Purification and characterization of thermostable pectate-lyases from a newly isolated thermophilic bacterium, Thermoanaerobacter italicus sp. nov.

Abstract

A novel thermophilic spore-forming anaerobic microorganism (strain Ab9) able to grow on citrus pectin and polygalacturonic acid (pectate) was isolated from a thermal spa in Italy. The newly isolated strain grows optimally at 70 degrees C with a growth rate of 0.23 h(-1) with pectin and 0.12 h(-1) with pectate as substrates. Xylan, starch, and glycogen are also utilized as carbon sources and thermoactive xylanolytic (highest activity at 70 degrees - 75 degrees C), amylolytic as well as pullulolytic enzymes (highest activity at 80 degrees - 85 degrees C) are formed. Two thermoactive pectate lyases were isolated from the supernatant of a 300-l culture of isolate Ab9 after growth on citrus pectin. The two enzymes (lyases a and b) were purified to homogeneity by ammonium sulfate treatment, anion exchange chromatography, hydrophobic chromatography and finally by preparative gel electrophoresis. After sodium dodecylsulfate (SDS) gel electrophoresis, lyase a appeared as a single polypeptide with a molecular mass of 135000 Da whereas lyase b consisted of two subunits with molecular masses of 93000 Da and 158000 Da. Both enzymes displayed similar catalytic properties with optimal activity at pH 9.0 and 80 degrees C. The enzymes were very stable at 70 degrees C and at 80 degrees C with a half-life of more than 60 min. The maximal activity of the purified lyases was observed with orange pectate (100%) and pectate-sodium salt (90%), whereas pectin was attacked to a much lesser extent (50%). The Km values of both lyases for pectate and citrus pectin were 0.5 g(-1) and 5.0 g(-1), respectively. After incubation with polygalacturonic acid, mono-, di-, and trigalacturonate were detected as final products. A 2.5-fold increase of activity was obtained when pectate lyases were incubated in the presence of 1 mM Ca2+. The addition of 1 mM ethylenediaminetetraacetic acid (EDTA) resulted in complete inhibition of the enzymes. These heat-stable enzymes represent the first pectate-lyases isolated and characterized from a thermophilic anaerobic bacterium. On the basis of the results of the 16S rRNA sequence comparisons and the observed phenotypic differences, we propose strain Ab9 as a new species of Thermoanaerobacter, namely Thermoanaerobacter italicus sp. nov.