Hemicelluloses are heterogeneous polymers of pentoses (xylose and L-arabinose), hexoses (mannose) and sugar acids. Xylans, major hemicelluloses of many plant materials, are heteropolysaccharides with a homopolymeric backbone chain of 1,4-linked P-D-xylopyranose units. Besides xylose, xylans may contain L-arabinose, D-glucuronic acid or its 4-o-methyl ether, and acetic, p-coumaric, and ferulic acids.

The total hydrolysis of xylan requires endo P-l,4-xylanase (EC 3.2.1.8), P — xylosidase (EC 3.2.1.37), and several accessory enzyme activities such as a-L — arabinosidase (EC 3.2.1.55), a-glucuronidase (EC 3.2.1.1), acetyl xylan esterase (EC 3.2.1.6), feruloyl esterase and p-coumaroyl esterase which are necessary for hydrolyzing various substituted xylans. The endo-xylanase randomly attacks the main chains of xylans and P-xylosidase hydrolyzes xylooligosaccharides to xylose. The a-L — arabinosidase and a-glucuronidase remove the arabinose and 4-O-methyl glucuronic acid substituents, respectively from the xylan backbone. The esterases hydrolyze the ester linkages between xylose units of the xylan and acetic acid (acetyl xylan esterase) or between arabinose side chain residues and phenolic acids such as ferulic acid (feruloyl esterase) and p-coumaric acid (p-coumaroyl esterase). Synergistic action of depolymerizing and side-group cleaving enzymes has been proved using acetylated xylan as substrate (37). Bachmann and McCarthy (38) reported significant synergistic interaction between endo-xylanase, P-xylosidase, a-L-arabinofuranosidase, and acetyl xylan esterase enzymes of the thermophilic actinomycete Thermomonospora fusca. Many xylanases do not cleave glycosidic bonds between xylose units which are substituted. The side chains must be cleaved before the xylan backbone can be completely hydrolyzed (39). On the other hand, several accessory enzymes only remove side-chains from xylooligosaccharides. These enzymes require xylanases to hydrolyze hemicellulose partially before side-chains can be cleaved (40). Although the structure of xylan is more complex than cellulose and requires several different enzymes with different specificities for a complete hydrolysis, the polysaccharide does not form tightly packed crystalline structures and is thus more accessible to enzymatic hydrolysis (41). The yeast-like fungus Aureobasidium is a promising source of xylanase (MW 20 kDa) with an exceptionally high specific activity (2100 U/mg protein) (42). Xylanase represented nearly half the total extracellular protein, with a yield of up to 0.3 g of xylanase per liter (43). A few recent reviews have dealt with the multiplicity, structure and function of microbial xylanases, and molecular biology of xylan degradation (5, 44, 45).

The utilization of hemicellulosic sugars is essential for efficient conversion of lignocellulose to ethanol. The commercial exploitation of the pentose fermenting yeasts for ethanol production from xylose is restricted mainly by their low ethanol tolerance, slow rates of fermentation, difficulty to control the rate of oxygen supply at the optimal level plus sensitivity to microbial inhibitors, particularly those liberated during pretreatment and hydrolysis of lignocellulosic substrates (46, 47). Xylose can also be converted to xylulose using the enzyme xylose isomerase and traditional yeasts can ferment xylulose to ethanol (48, 49). Xylose can be easily converted into xylitol, a potentially attractive sweetening agent by a variety of microorganisms (yeasts, fungi and bacteria) (50).