Peroxidases Bound to the Growing Lignin Polymer Produce Natural Like Extracellular Lignin in a Cell Culture of Norway Spruce.
Lignin, an important component of plant cell walls, is a polymer of monolignols derived from the phenylpropanoid pathway. Monolignols are oxidized in the cell wall by oxidative enzymes (peroxidases and/or laccases) to radicals, which then couple with the growing lignin polymer. We have investigated the characteristics of the polymerization reaction by producing lignin polymersin vitrousing different oxidative enzymes and analyzing the structures formed with NMR. The ability of the enzymes to oxidize high-molecular-weight compounds was tested using cytochromecas a substrate. The results support an idea that lignin structure is largely determined by the concentration ratios of the monolignol (coniferyl alcohol) and polymer radicals involved in the coupling reaction. High rate of the lignin polymer oxidation compared to monolignol oxidation leads to a natural-like structure. The high relative rate can be achieved by an open active site of the oxidative enzyme, close proximity of the enzyme with the polymeric substrate or simply by high enzymatic activity that consumes monolignols rapidly. Monolignols, which are oxidized efficiently, can be seen as competitive inhibitors of polymer oxidation. Our results indicate that, at least in a Norway spruce (Picea abiesL. Karst.) cell culture, a group of apoplastic, polymer-oxidizing peroxidases bind to the lignin polymer and are responsible for production of natural-like lignin in cell suspension culturesin vivo, and alsoin vitro. The peroxidases bound to the extracellular lignin had the highest ability to bind to various cell wall polymersin vitro. Extracellular lignin contains pectin-type sugars, making them possible attachment points for these cationic peroxidases.
Department of Agricultural Sciences, Viikki Plant Science Centre, University of Helsinki Finland.
Tino
T
Warinowski
Department of Food and Environmental Chemistry, University of Helsinki Finland.
Sanna
S
Koutaniemi
Department of Agricultural Sciences, Viikki Plant Science Centre, University of Helsinki Finland.
Anna
A
Kärkönen
Department of Agricultural Sciences, Viikki Plant Science Centre, University of Helsinki Finland.
Ilari
I
Sundberg
Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki Finland.
Merja
M
Toikka
Department of Biosciences, University of Helsinki Finland.
Liisa Kaarina
LK
Simola
Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki Finland.
Ilkka
I
Kilpeläinen
Department of Agricultural Sciences, Viikki Plant Science Centre, University of Helsinki Finland.
Teemu H
TH
Teeri
dehydrogenation polymer; lignin polymerization; lignin-carbohydrate complex; oxidative enzymes; peroxidase; polymer oxidation