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α-Glucuronidase and Other Hemicellulase Activities of Fibrobacter succinogenes S85 Grown on Crystalline Cellulose or Ball-Milled Barley Straw

Fibrobacter succinogenes produces an α-glucuronidase which cleaves 4-O-methyl-α-d-glucuronic acid from birch wood 4-O-methyl-α-d-glucuronoxylan. Very low levels of α-glucuronidase activity were detected in extracellular enzyme preparations of F. succinogenes on birch wood xylan substrate. The release of 4-O-methyl-α-d-glucuronic acid was enhanced when the birch wood xylan substrate was predigested by either a purified Schizophyllum commune xylanase or a cloned F. succinogenes S85 xylanase. These data suggest that the α-glucuronidase is unable to cleave 4-O-methyl-α-d-glucuronic acid from intact xylan but can act on unique low-molecular-weight glucuronoxylan fragments created by the cloned F. succinogenes xylanase. The cloned xylanase presumably must account for a small proportion of the indigenous xylanase activity of F. succinogenes cultures, since this xylanase source does not support high glucuronidase activity. The α-glucuronidase and associated hemicellulolytic enzymes exhibited higher activities in culture fluid from cells grown on ball-milled barley straw than in that of cellulose-grown cells. The profile of xylanases separated by isoelectric focusing (zymogram) of culture filtrate from cells grown on barley straw was more complex than that of culture filtrates from cells grown on cellulose. These data demonstrate that F. succinogenes produces an α-glucuronidase with an exacting substrate specificity which enables extensive cleavage of glucuronic acid residues from xylan as a consequence of synergistic xylanase action.