Highly thermostable endo-1,3-β -glucanase (laminarinase) Lam A from Thermotoga neapolitana: nucleotide sequence of the gene and characterization of the recombinant gene product

Abstract

The nucleotide sequence of clone pTT26 (3786 bp), containing the gene for 1,3-β -glucanase lamA (laminarinase) from Thermotoga neapolitana, was determined. It contains an ORF encoding a protein of 646 aa (73 328 Da). The central part of the protein is homologous to the complete catalytic domain of bacterial and some eukaryotic endo-1,3-β -D-glucanases and belongs to family 16 of glycosyl hydrolases. This domain is flanked on both sides by one copy on each side of a substrate binding domain homologue (family II). The recombinant laminarinase protein was purified from Escherichia coli host cells in two forms, a 73 kDa and a processed 52 kDa protein, both having high specific activity towards laminarin (3100 and 2600 U mg-1, respectively) and K m values of 2.8 and 2.2 mg ml-1, respectively. Limited activity on 1,3-1,4-β -glucan (lichenan) was detected (90 U mg-1). Laminarin was degraded in an endoglucanase modus, yielding glucose, laminaribiose and -triose as end products. Thus lamA classifies as an endo-1,3(4)-β -glucanase (EC 3.2.1.6). The optimum temperature of the enzymes was 95° (73 kDa) and 85° (52 kDa) at an optimum pH of 6.2. The superior thermostability of the 73 kDa enzyme is demonstrated by incubation without substrate at 100°, where 57% of the initial activity remained after 30 min (82% at 95°). Thus, lamA is the most thermostable 1,3-β -glucanase described to date.