Lignin and Lignan Biosynthesis: Distinctions and Reconciliations

Abstract

Before 1996, the framework within which lignin biosynthesis was understood at the molecular level had not fundamentally changed for 4 decades. During the same period nothing at all had been explicitly proposed about the mechanistic basis for lignan formation in vivo. The associated deficit in plant biochemistry was not minor: lignins and lignans together account for roughly 30% of the organic carbon in vascular plants. On the other hand, the biochemical transformations in phenylpropanoid metabolism leading, via the shikimate-chorismate pathway through phenylalanine or tyrosine, to the so-called monolignols (namely, the monomeric lignin/lignan precursors) came to be reasonably well documented. Indeed, attention has more recently been drawn to the identification of de facto rate-limiting steps in the various metabolic segments of the pathway as potential control-points for biotechnological manipulation. The most curious characteristic usually attributed to the lignin/lignan biosynthetic pathway is that the monolignol-derived radical coupling processes leading to lignans are regio-and stereospecific, whereas those resulting in lignin macromolecules ostensibly are not. Now that the molecular basis for the dehydrogenative dimerization of monolignols to lignans has been unraveled, however, it appears likely that an analogous mechanism may be operative in the dehydrogenative polymerization of monolignols to lignins. The investigation of this possibility has indeed become a central concern in the field of lignin biosynthesis.It has been almost three decades (7) since a comprehensive attempt had been made to summarize and evaluate contending views about lignin biosynthesis. Accordingly, the present volume draws together the current, yet conceptually vastly differing, ideas about how lignin composition and structure are established in living plants. It differs from all prior contributions, not only in introducing fresh evidence to support a compelling new paradigm that seeks to understand how lignin structure is established in vivo, but also in including lignan biosynthesis for comparative purposes. Indeed it appears that the first committed step in lignan biosynthesis bears an important relationship to the manner in which the configurations of lignins are specified in vivo.

Lignin Chemistry: Biosynthetic Study and Structural Characterisation of Coniferyl Alcohol Oligomers Formed In Vitro in a Micellar Environment

“…However, since the polymerisation reaction of the monomeric lignin precursors cannot be studied in vivo, many theories on lignin structure and biosynthesis still rely upon in vitro experiments. [2][3][4][5] A major milestone in lignin chemistry was Freudenbergs success, almost sixty years ago, in polymerising coniferyl alcohol into a lignin-like dehydrogenative polymer (DHP) by using a fungal laccase and other oxidative enzymes. [6][7][8] Only coniferyl alcohol, the most abundant lignin constituent, was used as the starting material to simplify the structural investigations on the dehydrogenation products.…”

“…It is also worth pointing out that, although DHPs were found to be good models for lignins, their structures, which are strongly dependent on the polymerisation conditions, may be quite different from the natural polymer, [2][3][4][5]18] principally because the wood cell environment, in which the native lignins are assembled, is dramatically different from the in vitro environment. In addition, as we have already pointed out, the oligolignols once formed in vitro subtract themselves very rapidly from a possibly higher degree of polymerisation, thus conferring to DHPs very low degrees of polymerisation.…”

Evolution of Plant Defense Mechanisms

“…hancinone 14 and tetrahydrodehydrodiconiferyl alcohol tetraacetate 13 (Fig. 3), an observation that cannot be explained simply on the basis of random coupling (64).…”

Polymerization of Monolignols by Redox Shuttle–Mediated Enzymatic Oxidation

“…The most striking difference is that the ␤ -O -4 linkage is much more common in lignin than in DHP, whereas carbon-carbon links such as 5-5 bonds are much more common in DHP than in native lignin (Lewis et al, 1998). However, a very slow addition of the monolignol to the polymerization mixture can give DHP, or at least dimers, with a very large proportion of ␤ -O -4 linkages (Syrjanen and Brunow, 2000).…”