12-oxophytodienoate reductase (OPRs) is an enzyme of the family of Old Yellow Enzymes (OYE).^[2] OPRs are grouped into two groups: OPRI and OPRII – the second group is the focus of this article, as the function of the first group is unknown, but is the subject of current research.^[3] The OPR enzyme utilizes the cofactor flavin mononucleotide (FMN) and catalyzes the following reaction in the jasmonic acid synthesis pathway:^[4]

This reaction occurs in peroxisomes in plants.^[5] Several isozymes have been discovered, with varying substrate stereospecificity: three in Solanum lycopersicum, 13 in Oryza sativa, and five in Arabidopsis thaliana.^[6] The OPR3 isozyme is most extensively studied because it can reduce all 4 stereoisomers of the substrate, OPDA and because it has shown to be the most significant enzyme in the jasmonic acid synthesis pathway.^[7][4]

12-oxophytodienoate reductase structure resembles OYE enzymes and has been elucidated by x-ray crystal structures.^[1] The cDNA encodes 372 amino acids for this enzyme.^[2] It exhibits a barrel fold of eight parallel beta-strands surrounded by eight alpha-helices to create a barrel shape.^[6] Turns at the N-terminus loops of the beta-strands have been shown to contain three to four amino acid residues and the C-terminus loops range between three and 47 amino acid residues.^[6] The C-terminus loops largely make up the active site and the larger range of the amount of residues is due to the diversity in the different isozyme active sites.^[6]

OPR3, the most extensively studied isoform of 12-oxophytodienoate reductase, has a wider binding pocket than OPR1, which is enantioselective for only one OPDA substrate enantiomer.^[1] The residues Tyr78 and Tyr246 that are at the mouth of the active site are responsible for the higher enantioselectivity of OPR1.^[8][1] OPR1 and OPR3 have identical substrate binding residues, but the difference in the width of the mouth of the active site determines the OPR1 specificity.^[8][1]

12-oxophytodienoate reductase has also been shown to practice self-inhibition by dimerization.^[6] This is the only flavoprotein known to dimerize for inhibition and this dimerization is thought to be regulated by phosphorylation.^[6] The dimerization occurs by the mutual binding of two loops into the two active sites.^[6] These loops are highly evolutionarily conserved, indicating the dimerization is purposeful and significant in regulation.^[6]

The reduction mechanism employed has been shown to be a ping-pong, bi-bi mechanism.^[6] The FMN cofactor is first reduced by NADPH, the substrate is then bound, and finally the substrate is reduced by a hydride transfer from NADPH to the substrate’s beta carbon.^[6] The Km of OPR3 in Zea mays was found to be 190 micromolar for its substrate OPDA.^[9]

The reaction catalyzed by 12-oxophytodienoate reductase is in the jasmonic acid biosynthesis pathway. Jasmonic acid is known for its importance as a gene regulator for development and defense.^{[4][10][11][12]}

OPR3 is shown to be induced by touch, wind, UV light, application of detergent, wounding, and brassinosteroids.^[4] In wound response, its activity has been shown to partially depend on jasmonic acid perception.^[4] It is also shown to have greater enzyme efficiency than OPR1 and OPR2 in Arabidopsis thaliana, showing it is the significant enzyme in the jasmonic acid biosynthesis pathway.^[4]

This enzyme is of interest in plant biology research because the disrupted OPR3 gene has been shown to cause male sterility in Arabidopsis thaliana.^[13] This is a point of interest in understanding the factors surrounding viable pollen development, a focus of research in the agriculture industry.^[13]

OPR has shown to also function in the reduction of explosive 2,4,6-trinitrotoluene (TNT).^[14] Because TNT is a known toxic, environmental pollutant that is difficult to degrade, the use of phytoremediation to clean up sites contaminated with TNT is of significant interest.^[14] OPR1 degraded TNT faster and with greater amount of degraded products than other isozymes.^[14] This enzyme could therefore be used in phytoremediation.^[14]

A phylogenetic analysis studying the structural evolution and functional divergence of the various OPR paralogues found seven conserved sub-families and suggested expansion of the OPR families occurred in land plants.^[15] A total of 74 OPR genes in 11 species from six major plant lineages were found.^[15] Surprisingly, introns were found to differ in length and number, but conserved in position, indicating successive intron loss.^[15] The study also indicated that the substrate binding loop and the alpha-helices, but not the beta-sheets, were critical for functional divergence after sub-families were established and are therefore important in the OPR proteins.^[15]