Lignin biosynthesis via shikimate-cinnamate pathways in plants, and the biosynthetic differences of guaiacyl-and syringyl lignins between gymnosperms and angiosperms have been elucidated by tracer experiments using 14C labeled precursors and the following enzyme reactions. The formation of guaiacyl lignin but not syringyl lignin in gymnosperms was attributed to the following factors; absence of ferulate-5-hydroxylase, poor affinity of O-methyltransferase toward 5-hydroxyferulate, and lack of activation and/or reduction of sinapatc. A mechanism of lignin-carbohydrate complexes formation in wood cell walls was elucidated based on the reaction of the quinone methide of guaiacylglycerol-β-guaiacyl ether with sugars, and the analysis of DHP-polysaccharide complexes. The main cleavage mechanisms of side chains and aromatic rings of lignin model compounds and synthetic lignin (DHP) by white-rot fungi and their enzymes, lignin peroxidase and laccase have been elucidated using 2H, 13C and 18O-labeled lignin substructure dimcrs with 18O2 and H2
18O. Side chains and aromatic rings of these substrates were cleaved via aryl cation radical and phenoxy radical intermediates, in reaction mediated only by lignin peroxidase/H2O2 and laccase/O2.

Lignin biochemistry: Biosynthesis and biodegradation *

We have investigated the abilities of extracellular enzymes from dark-grown cell-suspension cultures of sycamore maple (Acer pseudoplatanus L.) to oxidize monolignols, the precursors for lignin biosynthesis in plants, as well as a variety of other lignin-related compounds. Laccase and peroxidase both exist as a multiplicity of isoenzymes in filtrates of spent culture medium, but their abilities to produce water-insoluble, dehydrogenation polymers (DHPs) from the monolignols (in the presence of hydrogen peroxide for the peroxidase reaction) appear identical whether or not the enzymes are purified from the concentrated filtrates or left in a crude mixture. The patterns of bonds formed in these DHPs are identical to those found in DHPs synthesized using horseradish peroxidase or fungal laccase, and many of these bonds are found in the natural lignins extracted from different plant sources. On the other hand, sycamore maple laccase is very much less active on phenolic substrates containing multiple aromatic rings than is sycamore maple peroxidase. We suggst that whereas laccase may function during the early stages of lignification to polymerize monolignols into oligo-lignols, cell-wall peroxidases may function when H2O2 is produced during the later stages of xylem cell development or in response to environmental stresses.

Extracellular laccases and peroxidases from sycamore maple (Acer pseudoplatanus) cell-suspension cultures. Reactions with monolignols and lignin model compounds

Laccase (p-diphenolrOi oxidoreductase, EC 1.10.3.2) was the first enzyme used to polymerize a synthetic lignin (DHP) in vitro. Based on that work, laccase was proposed to perform the same function in vivo. However, various biochemical and histological studies eventually led most researchers to believe that peroxidases, not laccases, were principally responsible for monolignol polymerization in vascular plants. Recent evidence from several laboratories necessitates a re-evaluation of the relative roles played by oxidases and peroxidases in lignin deposition. In this paper, we review the current literature regarding plant laccases, äs well äs selected fungal laccases. From this body of evidence, we have developed a model for the evolution of lignin äs a structural polymer, and we describe how this model may be used to develop new questions regarding the origins of lignin and vascular plants.

Laccase and the Deposition of Lignin in Vascular Plants

Electrochemical Study of Kinetics of a Chemical Reaction Coupled between Two Charge Transfer Reactions. Potentiostatic Reduction of p-Nitrosophenol.

The role of the cinnamate pathway in monolignol biosynthesis based on feeding experiments with lignifying plant stems and characterization of the enzymes in the pathway, O-methyltransferase (OMT), cinnamyl alcohol dehydrogenase (CAD), etc. is discussed. Monolignol biosynthesis via metabolic grids according to newly characterized enzymes in the pathway is also reviewed and discussed. The cleavage mechanisms of side chains and aromatic rings by lignin peroxidase and laccase elucidated by using 18O, 2H, and 13C labeled lignin substructure dimers and DHP with 18O2 and H2
 18O are reviewed. Finally, the prospects of lignin biochemistry in the wood and paper industries are discussed according to the recent progress on gene technology on wood formation and microbial degradation of lignin.

/pdf/look-back-over-the-studies-of-lignin-biochemistry-3fcu1qdvbs.pdf

Look back over the studies of lignin biochemistry

The electrolytic current due to the superposition of an alternating potential upon a constant potential applied to the electrode is approximately derived for the case of reversible reaction. The parameter which determines the reversibility of the a.c. electrolysis is also given. Using the above derived results, the instateneous electrolytic current as well as current-voltage curves of reversible Breyer’s and Fournier’s polarogram are shown.

Studies on A.C. Polarography. V. Theory of Reversible Wave

We have been studying on the genesis of lignin and the changes of some chemical constituents contained in the gourd fruits which were taken in turn in certain periods after flowering.The results obtained are as follows: Lignin was at first generated on the fifteenth-day after flowering, and since then increases rapidly and finally occupies the whole fruit. The distributions of peroxidase and phenoloxidase in the tissue of fruit agreed with the lignification, and, it is suggested that these enzymes have relations to the biosynthesis of lignin.Lignin is mainly synthesized at the tissue of the shell-part of fruit. The content of lignin was only 2.3% on the fifteenth-day but it reached to 32% in hardened fruit. The methoxyl content of lignin was only 2% on the fifteenth-day but it increased along with the increase of lignin reaching to a level of 12% on the fifteenth-day, and to 15.3% after drying. In addition to these changes of lignin, other components except lignin also showed characteristic changes.In ord...

Studies on the Biosynthesis of Lignin

(1) Isonicotinic acid shows a single polaro-graphic reduction wave. It is found from the standpoint of the wave analysis as well as of A.C. polarography that the electrode process is irreversible. (2) The wave height at pH 5.0 is proportional to the concentration of isonicotinic acid in the concentration range from 10−4 to 10−3 mol per liter. (3) Since the single reduction wave is considered to be due to the reduction of undissociated molecules, the i-pH. curve shows an apparent polarographic dissociation curve. (4) The kinetic current due to the recombination of dissociated anions with proton is investigated, and the recombination velocity constant and the thickness of reaction layer are determined according to J. Koutecký and R. Brdicka’s equations.

Isamu Tachi

Papers

Studies on A.C. Polarography. V. Theory of Reversible Wave

Studies on the Biosynthesis of Lignin

Polarographic Studies on Heterocyclic Compounds. I. Kinetic Current of Isonicotinic Acid

A.C. Polarography of Flavin Mononucleotide

Polarographic current of stepwise electrode process involving chemical reaction