1. Field of the Invention
The present invention relates to isolated polypeptides having ferulic acid esterase activity and isolated polynucleotides encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods of producing and using the polypeptides.
2. Description of the Related Art
Plant cell wall polysaccharides constitute 90% of the plant cell wall and can be divided into three groups: cellulose, hemicellulose, and pectin. Cellulose represents the major constituent of call wall polysaccharides. Hemicelluloses are the second most abundant constituent of plant cell walls. The major hemicellulose polymer is xylan. The structure of xylans found in cell walls of plants can differ significantly depending on their origin, but they always contain a beta-1,4-linked D-xylose backbone. The beta-1,4-linked D-xylose backbone can be substituted by various side groups, such as L-aribinose, D-galactose, acetyl, feruloyl, p-coumaroyl, and glucuronic acid residues.
The biodegradation of the xylan backbone depends on two classes of enzymes: endoxylanases and beta-xylosidases. Endoxylanases (EC 3.2.1.8) cleave the xylan backbone into smaller oligosaccharides, which can be further degraded to xylose by beta-xylosidases (EC 3.2.1.37). Other enzymes involved in the degradation of xylan include acetylxylan esterase, arabinase, alpha-glucuronidase, p-coumaric acid esterase and ferulic acid esterase (ferouloyl esterase).
A feature of plant cell wall polysaccharides is that they are able to cross-link and such cross-links can include phenolic groups represented by ferulic acid (feruloyl) and p-coumaric acid. While p-coumarate has been identified mainly in the straws of grasses and cereals, feruloyl has been isolated ester-linked to xylans, pectins and xyloglucans in the cell walls of species such as cereals, sugar beet, spinach, bamboo and Chinese water chestnut, and constitutes the polyaromatic domain of suberin. The ferulic acid units can be oxidatively cross-linked by cell wall peroxidases into other polysaccharides, proteins and lignin. This cross-linking increases the plants resistance to microbial degradation.
The enzymes responsible for cleaving the ester-link between the polysaccharide main chain of xylans and either monomeric or dimeric feruloyl are the ferulic acid esterases (EC 3.1.1.73). The breakage of one or both ester bonds from dehydrodimer cross-links between plant cell wall polymers is essential for optimal action of carbohydrases on substrates such as cellulosic biomass. It is the object of the present invention to provide new ferulic acid esterases suitable for use in processes comprising conversion of cellulosic biomass into useful products including ethanol.
A ferulic acid esterase from Penicillium funiculosum (GENPEPT: AJ312296) has 44% identity to the ferulic acid esterase shown in SEQ ID NO:2.