Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 24, 1998
Publication Date: N/A
Interpretive Summary: Forages cannot be completely digested by animals such as dairy cows. This is due to the poor digestibility of forage fiber (plant cell walls). Improving fiber digestibility would provide positive benefits to dairy production by supplying increased energy, improving protein utilization, and decreasing manure waste. Plant fiber is made up of structural polysaccharides, proteins, and lignin that interact with each other to produce a strong composite wall (like the materials used to construct a wall of a building) to support the plant. We have used molecular modeling, a computer program, to simulate interactions among plant wall components. Such modeling approaches allow us to visualize how the cell wall may be put together and what impact it would have on digestibility. Plant fiber is thought to be tied together with a connecting agent called ferulic acid. The formation of diferulic acid (joining two ferulic acid molecules together) would tie two polysaccharides together strengthening the cell wall (like crossbeams in the wall of a building) and thereby decreasing wall digestibility. If two ferulic acids were on the same polysaccharide and then coupled together, there would be no reinforcement of the cell wall and less structural strength. Our modeling results show that diferulic acids form almost exclusively between two different polysaccharides always adding to the structural strength of the wall. This work gives researchers and plant breeders a better picture of cell wall structure and indicates structural elements within the wall that can be targeted for change to alter wall properties to fit our needs for maximizing plant utilization (increased digestibility or increased strength for biomass production).
Technical Abstract: Molecular modeling is a useful tool for predicting and visualizing molecular interactions such as characteristics of polysaccharide cross- linking. Recently it was shown that grasses contain extensive dehydrodiferulates suggesting a greater prominence of cross-linking among wall polysaccharides, thus changing the physical characteristics of the wall matrix. However, cross-linking would not be as extensive if some dehydrodiferulates arose from intramolecular instead of intermolecular coupling. Molecular modeling was used to evaluate the feasibility of intramolecular diferulate formation. Two ferulates were positioned at various locations along the backbone of an arabinoxylan (16 xylose residues) and the optimized structure generated using MM2 parameters. For ferulates separated by several xylose residues, diferulate formation occurs only if the xylan backbone relaxes allowing the two ferulates close enough for bonding. In positions that allow ferulate overlap, one or both ferulates must rotate along the xylan backbone for radical coupling. High- energy barriers prevented the complete rotation to bonding positions except when three xylose residues separate the ferulated arabinosyl units. In this configuration ferulate rotation can occur allowing formation of the 5-5- linked diferulate with no relaxation of the xylan backbone. This suggests restricted positioning of ferulates for 5-5-coupling and may explain why 5-5-coupling is more prominent in grasses than would be predicted from in vitro coupling studies. It seems unlikely for intramolecular dehydrodiferulates to readily form within grass cell walls.