Lignin Enzymology—Recent Efforts to Understand Lignin Monomer Catabolism

The Organic Synthesis of a Dimeric Lignin Probe Utilizing Förster Resonance Energy Transfer (FRET)

“…2 Cellulose, hemicellulose, and lignin are the founding constituents that provide the recalcitrant foundation of plant cell walls. 3 Cellulose was first discovered by Anselme Payen (1838) by oxidizing wood with nitric acid, producing an insoluble fiber (cellulose) and soluble remnants, which would later be identified as lignin by Schulze (1857). 1.1b, hemicellulose is a cross-linked heterogeneous polymer that is made of various C5 and C6 monosaccharides (e.g., galactose, mannose, arabinose, and xylose, etc.…”

“…5 Lignin, the second most abundant carbon, is a heterogeneous polyphenolic polymer that consists of three monolignols (monomers), p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 1.1c), and undergoes radical polymerization to form lignin. 3,5,6,7 Through covalent and noncovalent interactions, hemicellulose and lignin are chemically interlinked with cellulose microfibrils, which bridge neighboring cell walls and reinforce the mechanical integrity within plant cell walls; primary examples of this strong foundation can be seen in wood and bark. 5,6 Additionally, lignin plays a vital role in the transportation of water among plants by acting as an osmotic buffer.…”

“…8,9 Together, these three main constituents form the strong backbone of trees and terrestrial plants that have allowed them to survive and evolve for over 400 million years. 3,10 Making up 38% of annual biowaste (130 million tons) from the pulp/paper industry 11 , lignin can help alleviate the burdens of climate change by finding a potential use for it instead of remaining underutilized. Climate change is the change in the climate system of the atmosphere, glacier and sea levels, biogeochemical cycles, and ecosystems, which results in global warming.…”

“…The polymerization of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol forms lignin with three distinct structural characterizations: syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H). 3,5,6,7 The differences among the S, G, and H subunits are the presence of two, absence of one, or absence of both meta-positioned methoxy groups, respectively, as shown in Figure 1.2.…”

“…4,6 Laccase, an oxygen dependent enzyme, and peroxidase, a peroxide dependent enzyme, have been shown to oxidize monolignols to form phenolic radicals in vivo and in vitro. 3,6 However, the role of laccase and peroxidase in lignin polymerization remains inconclusive since recent studies indicate nonenzymatic oxidation as another key component to forming nonphenolic radicals. 6,14 It does, however, remain clear in both enzymatic and nonenzymatic cases that the production of radical monolignols that initiate the polymerization is done through oxidative chemistry.…”