RESUMEN
Cellobiose 2-epimerase (CE) catalyzes the reversible epimerization of cellobiose to 4-O-ß-D-glucopyranosyl-D-mannose. By using a PCR-based metagenomic approach, 71 ce-like gene fragments were obtained from wide-ranging environmental samples such as sheep rumen, soils, sugar beet extracts, and anaerobic sewage sludge. The frequency of isolation of the fragments similar to known sequences varied depending on the nature of the samples used. The ce-like genes appeared to be widely distributed in environmental bacteria belonging to the phyla Bacteroidetes, Chloroflexi, Dictyoglomi, Firmicutes, Proteobacteria, Spirochaetes, and Verrucomicrobia. The phylogenetic analysis suggested that the cluster of CE and CE-like proteins was functionally and evolutionarily separated from that of N-acetyl-D-glucosamine 2-epimerase (AGE) and AGE-like proteins. Two ce-like genes containing full-length ORFs, designated md1 and md2, were obtained by PCR and expressed in Escherichia coli. The recombinant mD1 and mD2 exhibited low K m values and high catalytic efficiencies (k cat/K m) for mannobiose compared with cellobiose, suggesting that they should be named mannobiose 2-epimerase, which is involved in a new mannan catabolic pathway we proposed.
Asunto(s)
Bacterias/enzimología , Proteínas Bacterianas/genética , Celobiosa/metabolismo , Metagenómica , Racemasas y Epimerasas/genética , Rumen/microbiología , Secuencia de Aminoácidos , Animales , Bacterias/clasificación , Bacterias/genética , Bacterias/aislamiento & purificación , Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismo , Cinética , Datos de Secuencia Molecular , Filogenia , Reacción en Cadena de la Polimerasa , Racemasas y Epimerasas/química , Racemasas y Epimerasas/metabolismo , Alineación de Secuencia , Ovinos , Microbiología del Suelo , Especificidad por SustratoRESUMEN
We have proposed a new mannan catabolic pathway in Bacteroides fragilis NCTC 9343 that involves a putative mannanase ManA in glycoside hydrolase family 26 (BF0771), a mannobiose and/or sugar transporter (BF0773), mannobiose 2-epimerase (BF0774), and mannosylglucose phosphorylase (BF0772). If this hypothesis is correct, ManA has to generate mannobiose from mannans as the major end product. In this study, the BF0771 gene from the B. fragilis genome was cloned and expressed in Escherichia coli cells. The expressed protein was found to produce mannobiose exclusively from mannans and initially from manno-oligosaccharides. Production of 4-O-ß-D-glucopyranosyl-D-mannose or 4-O-ß-D-mannopyranosyl-D-glucose from mannans was not detectable. The results indicate that this enzyme is a novel mannobiose-forming exo-mannanase, consistent with the new microbial mannan catabolic pathway we proposed.