|Li, Xin Liang|
Submitted to: Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2005
Publication Date: 9/9/2005
Citation: Ximenes, E.A., Chen, H., Kataeva, I.A., Cotta, M.A., Felix, C.R., Ljungdahl, L.G., Li, X. 2005. A mannanase, ManA, of the polycentric anaerobic fungus Orpinomyces sp. strain PC-2 has carbohydrate binding and docking modules. Canadian Journal of Microbiology. 51:559-568. Interpretive Summary: Efficient conversion of lignocellulosic biomass such as rice straw and corn stalk to fermentable sugars has been recognized as the major bottleneck for the economical production of biofuels and feedstock chemicals from the almost infinite renewable resources. Cellulose, hemicellulose, and lignin are three major constitutes commonly found in lignocellulosic biomass. Biological degradation of the three constitutes requires many different enzymes to work together as a consortium. The most needed enzymes are those which tackle celluloses and hemicelluloses. Galactomannan and glucomannan are hemicelluloses commonly found in plant cell wall structures. Anaerobic fungi are a group of fungi isolated from the gastrointestinal tracts of herbivores. The fungi are highly capable in breaking down plant biomasses. This is attributed to the production by the fungi of a complete set of hydrolytic enzymes such as cellulases, xylanases, esterases, beta-glucanase, and mannanases. We report here a mannanase that has multi-modular structure. The enzyme presents the first enzyme containing cellulose binding and docking modules besides a catalytic module. The broad pH profile of the enzyme demonstrates potential for the enzyme being useful for feed, food, and biomass conversion applications.
Technical Abstract: The anaerobic fungus Orpinomyces sp. strain PC-2 produces a broad spectrum of glycoside hydrolases, most of which are components of a high molecular mass cellusomal complex. Here we report about a cDNA (manA) having 1,924 bp isolated from the fungus and found to encode a polypeptide of 579 amino acid residues. Analysis of the deduced sequence revealed that it had a mannanase catalytic module (CM), a family 1 carbohydrate-binding module (CBM), and a noncatalytic docking module (NCDM). The CM was homologous to aerobic fungal mannanases belonging to family 5 glycoside hydrolases, but unrelated to the previously isolated mannanases (family 26) of the anaerobic fungus Piromyces. No mannanase activity could be detected in Escherichia coli harboring a manA containing plasmid. The manA was expressed in Saccharomyces cerevisiae, and ManA was secreted into the culture medium in multiple forms. The purified extracellular heterologous mannanase hydrolyzed several types of mannan, but lacked activity against cellulose, chitin, and beta-glucan. The enzyme had high specific activity toward locust bean mannan and extremely broad pH profile. It was stable for several hours at 50 deg C, but rapidly inactivated at 60 deg C. The CBM of ManA produced separately in E. coli bound preferably to insoluble lignocellulosic substrates, suggesting that it might play an important role in the complex enzyme system of the fungus for lignocellulose degradation.