The Biochemical Control of Monolignol Coupling and Structure During Lignan and Lignin Biosynthesis

Abstract

Over the last six decades or so, various theories and views have emerged to account for lignin and lignan assembly mechanisms. None have yet been able to satisfactorily describe all of the biochemical, structural and anatomical observations associated with formation of either distinct metabolic class in vivo. In this chapter a quite different perspective in terms of substance, approach and conclusion is given which now appears to account for all of the observations hitherto made. It is based primarily on results from the definitive elucidation of monolignol biochemical reactions, established at both the protein and gene levels, which reveals that monomer coupling processes do not occur haphazardly as often depicted. In this context, the first example of regio-and stereochemical control of monolignol bimolecular phenoxy radical coupling was that which gave the dimeric 8-8' linked lignan, (+)-pinoresinol, through the participation of a dirigent protein. The gene encoding this protein has no sequence homology to that of any other of known function, perhaps helping to explain further why stereoselective bimolecular phenoxy radical coupling, as controlled by this new class of proteins, had previously gone undiscovered. It is now contemplated that distinct arrays of dirigent protein sites (or some equivalent) stipulate macromolecular assembly mechanisms in the cell wall leading to the various lignins encountered. That is, to specifically account for the well-defined heterogeneous nature of lignin biopolymers within particular cell-wall types, it is proposed that arrays of dirigent protein sites in the developing wall specify the nature (monomer composition and bond frequency) of the primary lignin chain. There is growing evidence to show that specific lignin-related proteins are translocated to predetermined areas within the pre-lignified cell wall. An array of lignin monomers are then targeted to presumed specific sites on these proteins within the wall at about the same time. Although the precise mechanistic details need to be elucidated, lignification is then envisaged to occur via end-wise polymerization of the aligned monomers thereby generating the initial primary lignin chain(s), with chain replication occurring via a template polymerization effect as proposed by Sarkanen. Such a mechanism for lignin biosynthesis does not, however, readily account for formation of minor (reduced) substructures (e.g. dihydrodehydrodiconiferyl alcohol), supposedly present in the lignin macromolecule, which would suggest pre-or post-coupling modifications. As discussed elsewhere, these are viewed instead to be the products of a distinct pathway to the lignans. 334

Lignin Formation in Plants. The Dilemma of Linkage Specificity

“…Metabolic control over the process is exerted at the level of monolignol synthesis and transport to the wall matrix. Therefore, the random model for lignification is not invalidated, despite such claims (Lewis and Davin, 1998;Davin and Lewis, 2000). What does need to be addressed in more detail is how different tissues within the plant are able to achieve variation in lignin composition.…”

“…Various fragments carefully excised from lignins show no detectable optical activity-they are racemic (Freudenberg et al, 1965;Ralph et al, 1999;Akiyama et al, 2000). To explain this observation, one must envision a second array of dirigent proteins with the opposite stereo-and regiospecificity to generate a racemic mixture of coupling products in the lignin molecule, a notion proposed by the dirigent protein advocates (Lewis and Davin, 1998;Davin and Lewis, 2000). Thus, there would be a need for twice as many dirigent proteins to account for all the bonds and to produce opposite optical activities.…”

“…Thus, there would be a need for twice as many dirigent proteins to account for all the bonds and to produce opposite optical activities. An alternative is that with the template hypothesis, the next polymer formed would result in exactly the opposite optical activity, producing a racemic mixture (Lewis and Davin, 1998).…”

“…The second model, the dirigent protein model, is more recent and suggests that lignification must be under strict regulation of specialized proteins that control the formation of individual bonds (Lewis and Davin, 1998;Davin and Lewis, 2000). This new model for lignin formation stems from the definition of dirigent proteins (Davin et al, 1997).…”

Wet explosion pretreatment of loblolly pine leads to an increase in methoxylation of the lignin

“…A number of studies [3,[22][23][24][25] have indicated that NMR spectroscopy is a powerful technique capable of giving compositional and structural information of biomass lignin. We used loblolly pine, the most dominant softwood indigenous to the Southern United States and constituting over one half of the standing pine volume [26], as biomass substrate for the study.…”