Template Polymerization in Lignin Biosynthesis

The primary structures of lignins are determined by the sequences not of the constituent monomer residues, which are very similar to one another, but rather of the interunit linkages, which are about ten in number. This configurational determinant has usually been thought to vary randomly among macromolecular lignin chains: the final step in the assembly of the biopolymer, namely the coupling of radical intermediates produced during the dehydrogenative polymerization of monolignols, is not under direct enzymatic control. However, it has now been found that, without covalent participation in radical coupling, lignin macromolecules in vitro cogently promote formation of high molecular weight species from the enzyme-catalyzed dehydropolymerization of monolignols. In lignifying tissues such an effect could provide the organization necessary for replicating sequences of interunit linkages along macromolecular lignins chains by a direct template polymerization mechanism. The configurations of the progenitorial lignin chains are presumably established by proteins which proffer arrays of dirigent sites comparable to those that prescribe regio-and stereoselectivity in the phenoxy-radical coupling processes responsible for lignan formation. Among the major families of structural proteins so far described in plant cell walls, the proline-rich proteins (PRPs) would most closely approach the plausible manifestation of such characteristics.