Submitted to: European Journal of Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/19/1999
Publication Date: N/A
Citation: Interpretive Summary: Considerable nutrition is provided to ruminant animals by the plant cell wall. Cellulose and other polysaccharides are broken down by rumen microorganisms to simple sugars which are digestible. Polysaccharides in the cell walls of all grasses, however, are interconnected (or "cross- linked") by so-called ferulates. The ferulate molecule is attached to a polysaccharide chain and can cross-link in a variety of ways with another ferulate on another chain, thereby tying the two chains together. This cross-linking significantly limits the digestion of the cell wall since microorganisms have less access to the polysaccharide chains when they are cross-linked. An enzyme from a fungus (Aspergillus niger) has been known to attack these cross-links freeing the polysaccharide chains and improving digestibility. This basic study aimed to understand the mechanism of its attack on the cross-links and to determine the rate at which the cross- links could be cleaved. It was found that some types of these cross-links were cleaved efficiently, while others were quite resistant. Also, cleaving the link to the second polysaccharide is much slower than cleaving the first link, but cleaving only one link is all that is required to free both polysaccharide chains. The basic studies are aimed at improving dairy sustainability through more efficient utilization of plant resources by ruminant animals.
Technical Abstract: A feruloyl esterase (FAEA) from Aspergillus niger releases ferulic acid and 5-5 and 8-O-4 dehydrodiferulic acids from plant cell walls. 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 these substrates, but it is not known if feruloyl esterases can break these cross slinks by hydrolyzing one of the ester linkages without release of the free dimer. The catalytic properties of FAEA (Aspergillus niger) were examined using a range of synthetic esterified dehydrodimers (5-5, 8-5 Benzofuran and 8-O-4) and two 5-5 diferulate oligosaccharides isolated from maize bran. The results show: (1) FAEA more efficiently hydrolyzed one ester bond from the 5-5 and the 8-5 diesterified diferulate substrates than esterified ferulate. (2) The initial product derived from the hydrolysis of diester substrates is always a monoester. (3) The 5-5 monoester is hydrolyzed to form 5-5 diferulic acid but the catalytic efficiency is 10-fold lower than for the diester. (4) The 8-O-4 and 8-5 benzofuran monoesters were not significant substrates. These results show that the hydrolysis of diferulates to free acids occurs in two discrete steps involving dissociation of a monoesterified intermediate. FAEA from Aspergillus niger is more specific for hydrolysis of dehydrodiferulates than ferulates but does not always form the free acid dimer suggesting a key role for this esterase in the degradation of plant cell walls through cleavage of at least one ester bond from the diferulate cross-links which exist between wall polymers.