Lignin is a sturdy organic compound that is difficult to break down. It fortifies and protects the cell walls of plants. Lignin supports saplings as they climb and compete for sunlight, and wards off many forms of microbial attack. It plays several other important roles, such as transporting water in vascular plant tissue, storing and providing an abundant renewable carbon source, and locking atmospheric carbon away inside the living tissues of woody plants. It also has the potential to serve as an oil substitute for several petrochemical-based products. However, the realization of lignin’s benefits is contingent on better understanding its degradation processes and products. Therefore, a team of researchers in Argonne’s Biosciences Division is investigating how some microorganisms can promote lignin degradation.
In nature, lignin is broken down by enzymes in certain bacteria and fungi. While not all organisms can initiate breakdown of the large and resilient polymer molecules, many species can use the broken-apart monomeric lignin products as a carbon source. Bacteria employ a common metabolic strategy that transforms a large assortment of lignin degradation products into a smaller number of simpler compounds. These compounds then proceed through central degradation pathways as building blocks for a variety of useful chemicals. The Argonne team’s past work focused on how benzoate-like compounds fed directly into central degradation pathways. Current work turns to the less well-understood proteins involved in the transport or metabolism of non-benzoate compounds, called phenylpropanoids.
Researchers employed structural and functional characterization methods to understand the import of phenylpropanoids into bacterial cells. And while gaining a better view of the transport and metabolism of these compounds, they noted other diverse biological roles extending beyond their use as carbon sources, namely an emerging role in quorum sensing. Quorum sensing is a pheromone-based awareness that permits bacteria to detect the density of their neighbors. Bacteria use this mechanism to coordinate biological responses such as pathogenicity, spore formation, DNA uptake and exchange, and biofilm formation.
New functional information from Argonne’s team aids the identification of specific enzymes and regulatory proteins from lignin’s peripheral metabolic pathways. It also supports DNA sequence-based methods to predict the function of these poorly understood proteins. Their work is helping to unravel complex lignin molecules, and to identify the fascinating roles of proteins involved in this process.
“Structural and functional characterization of solute binding proteins for aromatic compounds derived from lignin: p-coumaric acid and related aromatic acids,” Kemin Tan, Changsoo Chang, Marianne Cuff, Jerzy Osipiuk, Elizabeth Landorf, Jamey C. Mack, Sarah Zerbs, Andrzej Joachimiak, and Frank R. Collart. Proteins: Structure, Function and Bioinformatics, 2013 Apr 22. doi: 10.1002/prot.24305.