Dirigent proteins are members of a class of

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

which dictate the

stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereois ...

of a compound synthesized by other enzymes. The first dirigent protein was discovered in ''

Forsythia intermedia ''Forsythia'' , is a genus of flowering plants in the olive family Oleaceae. There are about 11 species, mostly native to eastern Asia, but one native to southeastern Europe. ''Forsythia'' – also one of the plant's common names – is ...

''. This protein has been found to direct the stereoselective biosynthesis of (+)-pinoresinol from coniferyl alcohol monomers: (+)-Pinoresinol Biosynthesis

Lignan The lignans are a large group of low molecular weight polyphenols found in plants, particularly seeds, whole grains, and vegetables. The name derives from the Latin word for "wood". Lignans are precursors to phytoestrogens. They may play a role ...

biosynthesis is

catalysed Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

oxidative enzyme An oxidative enzyme is an enzyme that catalyses an oxidation reaction. Two most common types of oxidative enzymes are peroxidases, which use hydrogen peroxide, and oxidases, which use molecular oxygen. They increase the rate at which ATP is prod ...

s. In the

test tube A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom. Test tubes are usually placed in s ...

the reaction results in a heteregenous mixture of dimeric compounds. When a dirigent protein is present during the reaction, one

stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...

of one compound is highly enriched. Dirigent proteins appear to possess no oxidative radical forming activity of their own; in the absence of oxidative enzyme, no reaction will occur. Recently, a second, enantiocomplementary dirigent protein was identified in ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

'', which directs enantioselective synthesis of (-)-

pinoresinol Pinoresinol is a tetrahydrofuran lignan found in ''Styrax sp.'', ''Forsythia suspensa, and in Forsythia koreana''. It is also found in the caterpillar of the cabbage butterfly, ''Pieris rapae'' where it serves as a defence against ants. In food, i ...

Activity

lignan The lignans are a large group of low molecular weight polyphenols found in plants, particularly seeds, whole grains, and vegetables. The name derives from the Latin word for "wood". Lignans are precursors to phytoestrogens. They may play a role ...

biosynthesis, oxidative enzymes perform proton coupled electron transfer to remove a hydrogen atom from

monolignol Monolignols, also called lignols, are the source materials for biosynthesis of both lignans and lignin and consist mainly of paracoumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S). These monolignols differ in their degree of meth ...

s, forming a radical intermediate. These intermediates then couple in a radical termination reaction to form one of a variety of dimers, known as

s. ''

In vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

'' reactions of

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 compounds, ...

(a common

) in the presence of oxidative enzymes produce a wide variety of different dimers at varying concentrations. When dirigent protein from "

" is present, production of (+)-

is greatly enriched, and other products are far less abundant. Because this enrichment is so pronounced, the enzyme is hypothesized to produce (+)-

exclusively, and to compete with the non-protein-mediated coupling reaction, which produces a heterologous mix of products. This has been confirmed by analyzing the various mixtures produced with different concentrations of dirigent proteins present. The mechanism by which this stereoselectivity is achieved is not well understood at this time. However, since no reaction proceeds in the absence of oxidative enzymes, dirigent protein does not itself appear to catalyze the oxidation of coniferyl alcohol to form radicals.

The activity of dirigent protein from

is specific to coniferyl alcohol. When other monolignols, such as p-coumaryl alcohol 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 are ...

, are reacted in vitro with oxidative enzymes in the presence of dirigent protein, they produce a heterologous mixture of products indistinguishable from identical experiments in the absence of dirigent protein.

Structure

Circular dichroism experiments have shown that the secondary structure of dirigent protein from

is composed primarily of β-pleated sheets and loop structures. The tertiary structure has not been solved, but the protein has been confirmed to be dimeric. Each dimer has a single binding site for coniferyl alcohol, for a total of two binding sites. One coniferyl alcohol can bind to each site, so that the reaction geometry between the two is confined, increasing the production of (+)-pinoresinol and inhibiting the production of other products.

Biological importance

In the absence of dirigent protein, pinoresinol is a relatively minor product of lignan biosynthesis. When dirigent protein is present, it becomes the major product. The biological significance of (+)-pinoresinol in plants is not fully understood, but it has been found to be effective as a feeding deterrent against ants in caterpillars of the

cabbage butterfly Cabbage white or cabbage butterfly may refer to: * ''Pieris brassicae'', a Palearctic butterfly * ''Pieris rapae ''Pieris rapae'' is a small- to medium-sized butterfly species of the whites-and-yellows family Pieridae. It is known in Europe a ...

, which obtain the compound from their diet. It may serve a similar defensive purpose in

References

{{Reflist Proteins by function Enzymes