|Gene:||PSD_Fi1||Accession Number: G-9895 (MetaCyc)|
Species: Forsythia x intermedia
Subunit composition of
dirigent protein psd-Fi1 = [PSD_Fi1]2
dirigent protein subunit = PSD_Fi1
The unique protein that mediates the regio- and stereospecificity of bimolecular phenoxy radical coupling to generate oligomeric lignans and polymeric lignins has been identified and characterized in Forsythia x intermedia [Davin97]. The protein itself does not express any catalytic activity and serves only to bind and orientate the coniferyl alcohol-derived free radicals. Therefore the protein has been coined 'dirigent protein' (DP) [Lewis99]. The generation of the immediate substrate radicals for DP is realized by unspecific oxidases and/or laccases and particular oxidants such as ammonium peroxydisulfate, and FMN radical generating systems [Davin97]. The cDNA encoding for this protein (PSD_FI1) was isolated from Forsythia, recombinant expressed in a eukaryotic Spodoptera frugiperda/baculovirus system and its molecular function to bind and coordinate coniferyl alcohol radicals to form (+)-pinoresinol confirmed [Gang99a]. The functional protein is a homodimer of 49.5 kDa with a predominantly β-sheet structure. The monomer elutes with a molecular weight of 18.3 kDa but is posttranslational glycosylated resulting in a native molecular weight of 26 kDa [Halls02] [Halls04]. The protein is rather specific for E-coniferyl alcohol and does not accept p-coumaryl alcohol or sinapyl alcohol and derivative radicals thereof [Davin97] [Halls04].
The protein is predominantly expressed in lignifying tissues which are established as sites for initiation of macromolecular lignin biosynthesis [Burlat01]. The dirigent protein is not limited to the formation of lignans but has been found to be involved in other biosyntheses of various phenolics employing corresponding radicals. Therefore, DP's are considered as a protein family with unique features not found in other protein classes [Lewis99]. Because of the rather complex reaction mechanism the Km values determined for the formation of the (+)-pinoresinol enantiomer [Davin97] are probably more apparent as the actual rate-limiting processes had not yet been determined. The kinetic parameters obtained in studies with E-coniferyl alcohol radicals and DP point to the mechanism of directed radical-radical coupling with two coniferyl alcohol radical substrates bound per protein dimer, hence favoring the 8,8'-linkage of the two si-si faces of each substrate molecule [Halls02] [Halls04].
Molecular Weight of Polypeptide: 26 kD (experimental) [Halls02 ]
Molecular Weight of Multimer: 49.5 kD (experimental) [Halls02]
pI: 8.5 [Gang99a]
Unification Links: Entrez:AAF25357
Relationship Links: Entrez-Nucleotide:PART-OF:AF210061
Enzymatic reaction of: dirigent protein
EC Number: 1.10.3.-
The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the direction in which it was curated.
The reaction is physiologically favored in the direction shown.
In Pathways: matairesinol biosynthesis
Davin97: Davin LB, Wang HB, Crowell AL, Bedgar DL, Martin DM, Sarkanen S, Lewis NG (1997). "Stereoselective bimolecular phenoxy radical coupling by an auxiliary (dirigent) protein without an active center." Science 275(5298);362-6. PMID: 8994027
Gang99a: Gang DR, Costa MA, Fujita M, Dinkova-Kostova AT, Wang HB, Burlat V, Martin W, Sarkanen S, Davin LB, Lewis NG (1999). "Regiochemical control of monolignol radical coupling: a new paradigm for lignin and lignan biosynthesis." Chem Biol 6(3);143-51. PMID: 10074466
Halls04: Halls SC, Davin LB, Kramer DM, Lewis NG (2004). "Kinetic study of coniferyl alcohol radical binding to the (+)-pinoresinol forming dirigent protein." Biochemistry 43(9);2587-95. PMID: 14992596
Lewis99: Lewis NG, Davin LB (1999). "Lignans: Biosynthesis and function." In: Comprehensive natural products chemistry Vol. 1: Barton Sir DHR, Nakanishi K. (eds.-in-chief), Carbohydrates Polyketides and other Secondary Metabolites including Fatty Acids and their Derivatives. Amsterdam, New York: Elsevier 1999, 639-712.
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