Characterization of a Thermostable L-Arabinose (D-Galactose) Isomerase from the Hyperthermophilic Eubacterium Thermotoga maritima

Abstract
The araA gene encoding L-arabinose isomerase (AI) from the hyperthermophilic bacterium Thermotoga maritima was cloned and overexpressed in Escherichia coli as a fusion protein containing a C-terminal hexahistidine sequence. This gene encodes a 497-amino-acid protein with a calculated molecular weight of 56,658. The recombinant enzyme was purified to homogeneity by heat precipitation followed by Ni2 affinity chromatography. The native enzyme was estimated by gel filtration chromatography to be a homotetramer with a molecular mass of 232 kDa. The purified recombinant enzyme had an isoelectric point of 5.7 and exhibited maximal activity at 90°C and pH 7.5 under the assay conditions used. Its apparent Km values for L-arabinose and D-galactose were 31 and 60 mM, respectively; the apparent Vmax values (at 90°C) were 41.3 U/mg (L-arabinose) and 8.9 U/mg (D-galactose), and the catalytic efficiencies (kcat/Km) of the enzyme were 74.8 mM1 min1 (L-arabinose) and 8.5 mM1 min1 (D-galactose). Although the T. maritima AI exhibited high levels of amino acid sequence similarity (>70%) to other heat-labile mesophilic AIs, it had greater thermostability and higher catalytic efficiency than its mesophilic counterparts at elevated temperatures. In addition, it was more thermostable in the presence of Mn2 and/or Co2 than in the absence of these ions. The enzyme carried out the isomerization of D-galactose to D-tagatose with a conversion yield of 56% for 6 h at 80°C.