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Two thermostable endocellulases, CelA and CelB, were purified fromThermotoga neapolitana . CelA (molecular mass, 29 kDa; pI 4.6) is optimally active at pH 6.0 at 95°C, while CelB (molecular mass, 30 kDa; pI 4.1) has a broader optimal pH range (pH 6.0 to 6.6) at 106°C. Both enzymes are characterized by a high level of activity (highV max value and low apparentKm value) with carboxymethyl cellulose; the specific activities of CelA and CelB are 1,219 and 1,536 U/mg, respectively. Withp -nitrophenyl cellobioside theV max values of CelA and CelB are 69.2 and 18.4 U/mg, respectively, while theKm values are 0.97 and 0.3 mM, respectively. The major end products of cellulose hydrolysis, glucose and cellobiose, competitively inhibit CelA, and CelB. TheKi values for CelA are 0.44 M for glucose and 2.5 mM for cellobiose; theKi values for CelB are 0.2 M for glucose and 1.16 mM for cellobiose. CelB preferentially cleaves larger cellooligomers, producing cellobiose as the end product; it also exhibits significant transglycosylation activity. This enzyme is highly thermostable and has half-lives of 130 min at 106°C and 26 min at 110°C. A single clone encoding thecelA andcelB genes was identified by screening aT. neapolitana genomic library inEscherichia coli . ThecelA gene encodes a 257-amino-acid protein, whilecelB encodes a 274-amino-acid protein. Both proteins belong to family 12 of the glycosyl hydrolases, and the two proteins are 60% similar to each other. Northern blots ofT. neapolitana mRNA revealed thatcelA andcelB are monocistronic messages, and both genes are inducible by cellobiose and are repressed by glucose.

Purification, Characterization, and Molecular Analysis of Thermostable Cellulases CelA and CelB from Thermotoga neapolitana