Lignin Formation in Plants. The Dilemma of Linkage Specificity

Supramolecular Self-Assembled Chaos: Polyphenolic Lignin’s Barrier to Cost-Effective Lignocellulosic Biofuels

“…Lignin synthesis is preceded primarily but not exclusively, by the biosynthesis of its basic units, the three monolignols (Figure 1). Lignin formation involves: a) biosynthesis of monolignols; b) transport of monolignols to lignifying sites; c) enzymatic radicalization of monolignols; and, d) non-enzymatic coupling of the radical monomers into the growing lignin polymer [31]. This is a simplified description of lignin formation; several aspects of even these simple steps are controversial.…”

“…Lignin's structural plasticity and its lack of regularity may work to the plant's advantage by serving as a defense against pathogens or attack by enzymes secreted by invading microorganisms. The irregularity of the lignin structure requires a complicated evolutionary pathway in order to generate a single enzyme that is capable of recognizing and breaking all the various types of linkages found in lignin ( Figure 5) [31]. Nature has circumvented this difficulty by endowing microorganisms with a battery of redox and hydrolytic enzymes that work in concert to degrade LCCs including the relatively easily oxidized benzylic ether linkages.…”

“…When a monolignol radical is close to a lignin chain that has not been radicalized, it is possible for the monolignol radical to transfer its higher oxidation state to the lignin chain. The reduced monolignol can then undergo a second round of oxidation by a cell wall-bound peroxidase or laccase and complete the cycle by coupling to the now radicalized lignin chain [28,31,37]. Alternately, RSMs might be involved in further radical generation and for maintaining lignin polymer growth [37,45].…”

“…The first, namely, 'Zutropfverfahren' or 'Zutropf' or Z T DHP, is formed by adding the monolignol slowly and continuously to a reaction mixture resulting in the formation of "end-wise" polymers that resembled natural lignin by containing mostly β-O-4 bonds ( Figure 5). The second type, 'Zulaufverfahren' or 'Zulauf' or Z L DHPs are formed by bulk polymerization where all components (monolignol, H 2 O 2 , peroxidase) are mixed simultaneously, resulting in a larger proportion of β-β and β-5 linkages ( Figure 5) [31,61]. Such model systems suggest that the type of lignin polymers in nature may be regulated by the availability, type, rate of supply of the precursors, and other factors listed above [15,37].…”

Lignification Markers of the Tracheid Walls of Scots Pine (Pinus sylvestris (L.)) in Various Forms of Dead Bark

“…Moreover, lignin facilitates the transportation of water and hinders the degradation of polysaccharides in the cell walls. Their activity can be described as antiseptic towards the pathogenic factors (Monties 1989;Hatfield and Vermerris 2001).…”