Monolignols, also called lignols, are the source materials for

biosynthesis Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...

of both lignans and

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

and consist mainly of paracoumaryl alcohol (H),

coniferyl alcohol Coniferyl alcohol is an organic compound with the formula HO(CH3O)C6H3CH=CHCH2OH. A colourless or white solid, it is one of the monolignols, produced via the phenylpropanoid biochemical pathway. When copolymerized with related aromatic compound ...

(G) and

sinapyl alcohol Sinapyl alcohol is an organic compound structurally related to cinnamic acid. It is biosynthetized via the phenylpropanoid biochemical pathway, its immediate precursor being sinapaldehyde. This phytochemical is one of the monolignols, which a ...

(S). These monolignols differ in their degree of methoxylation of the aromatic ring. The monolignols are derived from the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...

via the

phenylpropanoid The phenylpropanoids are a diverse family of organic compounds that are biosynthesized by plants from the amino acids phenylalanine and tyrosine in the shikimic acid pathway. Their name is derived from the six-carbon, aromatic phenyl group and ...

pathway involving various enzymes. Phenylalanine is first converted to paracoumaryl alcohol (H), which is subsequently elaborated to

(G) and

(S). This reaction happens in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

, while the polymerization of the monolignols occurs in the apoplast to which the monolignols have to be transported through the

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. The monolignols have been found as monolignol-4-O-β-d- glucosides, which might be their major way of storage. Another theory for this conversion is that is improving the transportation of the monolignols. The polymerization consists of oxidative coupling reactions, which occur between the propenyl substituens, two aromatic rings or a propenyl substituent and a ring. The difference between lignans and lignin is the number of monolignols they are composed of. Lignans are typically dimers and therefore soluble and susceptible to biodegradation. Lignin is a polymer with an inert nature that forms the structures of woody plants. The ratio of the three monolignols as well as their linkages varies depending on the

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

species. For example,

Norway spruce ''Picea abies'', the Norway spruce or European spruce, is a species of spruce native to Northern, Central and Eastern Europe. It has branchlets that typically hang downwards, and the largest cones of any spruce, 9–17 cm long. It is very clo ...

lignin is almost entirely derived from

, whereas paracoumaryl alcohol is the main monomer of lignin in

grass Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and spe ...

es. Even within one plant, the monomer composition of lignin can vary depending on the part of the plant and other phenolic compounds can be found as monomers in lignin. The phenylpropenes are derived from the monolignols.

References

{{Phenylpropanoids