Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 10, 1997
Publication Date: N/A
Interpretive Summary: Fiber makes up 15 to 80% of the dry weight of plants. Fiber is composed primarily of structural polysaccharides that are potentially an important source of energy for humans and for livestock. Structural polysaccharides may also be converted into ethanol for use in automobile fuels or other purposes. To be a source of food or fuel energy, structural polysaccharides smust be broken down by digestive enzymes into simple sugars but this process is limited by structural barriers in fiber. For years, scientists have thought that one of these structural barriers was a molecule called diferulic acid. Diferulic acid interconnects (cross-links) structural polysaccharides and it is possible that these cross-links act as a barrier preventing penetration of digestive enzymes into fiber. We ran experiments to determine if diferulate cross-links limit the enzymatic degradation of structural polysaccharides into simple sugars. To do this, we isolated fiber from plant cells that were composed primarily of structural polysaccharides. The fiber was then treated in a way that either limited or stimulated the formation of diferulate cross-links. We found that diferulate cross-links controlled the rate at which structural polysaccharides were degraded into sugars by digestive enzymes. Based on these results we recommend that biotechnology be used to develop plants that have low amounts of diferulate cross-links. Reducing diferulate cross- links in fiber should lower the cost and environmental impact of converting fibrous crops into food and fuel.
Technical Abstract: We assessed the effect of ferulate substitution and diferulate cross- linking of xylans on the hydrolysis of cell walls by two fungal hydrolase mixtures, one of which contained feruloyl esterase activity. Nonlignified cell suspensions of maize (Zea mays) were grown with 0 or 40 deg micromolar 2-aminoindan-2-phosphonic acid to produce walls with normal (17.2 mg g-1) or reduced (5.1 mg g-1) ferulate concentrations. Walls were incubated with mercaptoethanol to inhibit diferulate formation or with hydrogen peroxide to stimulate diferulate formation by wall bound peroxidases. Increasing dehydrodimerization of ferulates from 18 to 40% reduced carbohydrate release by 94 to 122 mg g-1 after 3-h and by 0 to 48 mg g-1 after 54-h of enzymatic hydrolysis. Diferulate cross-links impeded the release of all sugars from walls, particularly that of xylose. These results provide compelling evidence that diferulate cross-links reduce the rate, and to a lesser degree, the extent of wall hydrolysis by fungal enzymes. Wall hydrolysis was not affected by varying ferulate substitution of xylans. Feruloyl esterases released only small quantities of diferulates and their activity did not significantly enhance the hydrolysis of walls by fungal carbohydrases.