About Lignin

Lignin Biosynthesis

Lignin, hemicelluloses,  and cellulose are the three major chemical components of wood, in approximate proportions of 1:1:2.1-4 In the formation of secondary xylem (wood), lignin is deposited between the cells (compound middle lamella, CML, and cell corners, CC) and in the layers of the secondary cell walls (of the three layers, the S2 layer is predominant) (Figure 1). Lignin surrounds the preformed hemicelluloses and cellulose to form a strong and hydrophobic secondary wall. Lignin is typically polymerized from three phenylpropanoid monomers (Figure 2), p-coumaryl (20), coniferyl (22) and sinapyl (24) alcohols, also known as the H, G and S monolignols. In dicotyledons, lignin is polymerized from S and G monolignols and low levels of H and other intermediates (light color, Figure 2).1,5-7 In wood of Populus, the S/G ratio is approximately 2.2,7-9 Knowledge of the enzymes and metabolites of the lignin pathway stems from tracer studies by Brown, Neish and Higuchi in the 1950’s.10-18 Ten enzyme families convert phenylalanine (1) to monolignols, with a principal path (heavy color) through a metabolic grid (Figure 2).2,7,8,19-30Monolignols may be exported to the cell wall via Golgi-mediated vesicle fusion with the plasma membrane31-33 or partition-based membrane diffusion.34,35In the wall, monolignols are irreversibly oxidized to phenoxy radicals (Figure 2) by H2O2-dependent class III peroxidases (POs) through an electron oxidoreduction cycle.1,2,5,6,8,10,36-46 Phenoxy radicals in monolignols are delocalized to form mesomeric structures capable of coupling with each other or with the resonance-stabilized radicals in a growing lignin polymer.5-8,47 The biosynthesis of lignin is primarily the addition of a monomer to a polymer. The propensity for cross-coupling by radicals of both coupling partners at specific positions is dictated by regiochemistry, forming specific chemical linkages.7,8,47-49 The process is “combinatorial”.7,8,47 β-O-4, β-5 and β-β are typical G and S interunit linkages (Figure 2).1,2,5-8,10,36,47,50