Design and synthesis of novel imidazole derivatives as potent inhibitors of allene oxide synthase(CYP74)

Selective Mono- and Diamination of Ketones in a Combined Copper-Organocatalyst System

Herein, we report a simple and mild protocol for the chemoselective mono- and diamination of ketone using pyrazole as the amine source in a combined copper-organocatalyst system. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy gives an efficient and convenient solution for the synthesis of α-pyrazole and α,α-dipyrazole ketone derivatives. The control experiment demonstrates that in situ generated hydrazone is a key intermediate in the transformation.

Separation of enzymatic functions and variation of spin state of rice allene oxide synthase-1 by mutation of Phe-92 and Pro-430

Rice allene oxide synthase-1 mutants carrying F92L, P430A or F92L/P430A amino acid substitution mutations were constructed, recombinant mutant and wild type proteins were purified and their substrate preference, UV-vis spectra and heme iron spin state were characterized. The results show that the hydroperoxide lyase activities of F92L and F92L/P430A mutants prefer 13-hydroperoxy substrate to other hydroperoxydienoic acids or hydroperoxytrienoic acids. The Soret maximum was completely red-shifted in P430A and F92L/P430A mutants, but it was partially shifted in the F92L mutant. ESR spectral data showed that wild type, F92L and P430A mutants occupied high and low spin states, while the F92L/P430A mutant occupied only low spin state. The extent of the red shift of the Soret maximum increased as the population of low spin heme iron increased, suggesting that the spectral shift reflects the high to low transition of heme iron spin state in rice allene oxide synthase-1. Relative to wild type allene oxide synthase-1, the hydroperoxide lyase activities of F92L and F92L/P430A are less sensitive to inhibition by imidazole with (13S or 9S)-hydroperoxydienoic acid as substrate and more sensitive than wild type with (13S)-hydroperoxytrienoic acid as substrate. Our results suggest that hydroperoxydienoic acid is the preferred substrate for the hydroperoxide lyase activity and (13S)-hydroperoxytrienoic acid is the preferred substrate for allene oxide synthase activity of allene oxide synthase-1.

Tandem photoaffinity labeling of a target protein using a linker with biotin, alkyne and benzophenone groups and a bioactive small molecule with an azide group

A novel linker containing biotin, alkyne and benzophenone groups (1) was synthesized to identify target proteins using a small molecule probe. This small molecule probe contains an azide group (azide probe) that reacts with an alkyne in 1 via an azide–alkyne Huisgen cycloaddition. Cross-linking of benzophenone to the target protein formed a covalently bound complex consisting of the azide probe and the target protein via 1. The biotin was utilized via biotin–avidin binding to identify the cross-linked complex. To evaluate the effectiveness of 1, it was applied in a model system using an allene oxide synthase (AOS) from the model moss Physcomitrella patens (PpAOS1) and an AOS inhibitor that contained azide group (3). The cross-linked complex consisting of PpAOS1, 1 and 3 was resolved via SDS–PAGE and visualized using a chemiluminescent system. The method that was developed in this study enables the effective identification of target proteins.

Dual positional substrate specificity of rice allene oxide synthase-1: insight into mechanism of inhibition by type II ligand imidazole

Phylogenetic and amino acid sequence analysis indicated that rice allene oxide synthase-1 (OsAOS1) is CYP74, and is clearly distinct from CYP74B, C and D subfamilies. Regio- and stereo-chemical analysis revealed the dual substrate specificity of OsAOS1 for (cis,trans)-configurational isomers of 13(S)- and 9(S)-hydroperoxyoctadecadienoic acid. GC-MS analysis showed that OsAOS1 converts 13(S)- and 9(S)-hydroperoxyoctadecadi(tri)enoic acid into their corresponding allene oxide. UV-Visible spectral analysis of native OsAOS1 revealed a Soret maximum at 393 nm, which shifted to 424 nm with several clean isobestic points upon binding of OsAOS1 to imidazole. The spectral shift induced by imidazole correlated with inhibition of OsAOS1 activity, implying that imidazole may coordinate to ferric heme iron, triggering a heme-iron transition from high spin state to low spin state. The implications and significance of a putative type II ligand-induced spin state transition in OsAOS1 are discussed. [BMB Reports 2013; 46(3):151-156]

Synthesis of 2RS,4RS-1-[2-phenyl-4-[2-(2-trifluromethoxy-phenoxy)-ethyl]-1,3-dioxolan-2-yl-methyl]-1H-1,2,4-triazole derivatives as potent inhibitors of brassinosteroid biosynthesis

Brassinosteroids are important phytohormones that affect many aspects of plant growth and development. In order to manipulate brassinosteroid levels in plant tissues by using specific biosynthesis inhibitors, we have carried out a systemic search for specific inhibitors of brassinosteroid biosynthesis. Synthesis of triazole derivatives based on the ketoconazole scaffold revealed a series of novel brassinosteroid biosynthesis inhibitors (the YCZ series). To explore the structure-activity relationships of this synthetic series, we now report the synthesis of new triazole derivatives with different aromatic structures at position 2 of 1,3-dioxolane skeleton. We found that the variation of aromatic substituent significantly affect the inhibitory potency. Structure-activity relationships studies indicated that 4-chlorophenyl analogue is the most potent inhibitor of BR biosynthesis with an IC₅₀ value approximately 0.12 ± 0.04 µM, while a bulky biphenyl group exhibited a great negative effect on promoting the inhibitory potency with an IC₅₀ larger than 10 µM.

The allene oxide cyclase family of Arabidopsis thaliana: localization and cyclization

Jasmonates are derived from oxygenated fatty acids (oxylipins) via the octadecanoid pathway and are characterized by a pentacyclic ring structure. They have regulatory functions as signaling molecules in plant development and adaptation to environmental stress. Recently, we solved the structure of allene oxide cyclase 2 (AOC2) of Arabidopsis thaliana, which is, together with the other three AOCs, a key enzyme in the biosynthesis of jasmonates, in that it releases the first cyclic and biologically active metabolite -- 12-oxo-phytodienoic acid (OPDA). On the basis of models for the bound substrate, 12,13(S)-epoxy-9(Z),11,15(Z)-octadecatrienoic acid, and the product, OPDA, we proposed that a conserved Glu promotes the reaction by anchimeric assistance. According to this hypothesis, the transition state with a pentadienyl carbocation and an oxyanion is stabilized by a strongly bound water molecule and favorable pi-pi interactions with aromatic residues in the cavity. Stereoselectivity results from steric restrictions to the necessary substrate isomerizations imposed by the protein environment. Here, site-directed mutagenesis was used to explore and verify the proposed reaction mechanism. In a comparative analysis of the AOC family from A. thaliana involving enzymatic characterization, in vitro import, and transient expression of AOC-enhanced green fluorescent protein fusion proteins for analysis of subcellular targeting, we demonstrate that all four AOC isoenzymes may contribute to jasmonate biosynthesis, as they are all located in chloroplasts and, in concert with the allene oxide synthase, they are all able to convert 13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid into enantiomerically pure cis(+)-OPDA.

Preparative enzymatic solid phase synthesis of cis(+)-12-oxo-phytodienoic acid - physical interaction of AOS and AOC is not necessary

The pathway of jasmonic acid (JA) biosynthesis was established in the 1980s by Vick and Zimmerman but, until now, the preparative biosynthesis of the jasmonic acid precursors 12-oxo-phytodienoic acid (OPDA) and 3-oxo-2-[2'-pentenyl]-cyclopentan-1-octanoic acid (OPC-8:0) in their endogenous and biologically relevant cis(+)-configuration was only possible in small amounts and had to put up with high costs. This was mainly due to the lack of high amounts of pure and enzymatically active allene oxide cyclase (AOC), which is a key enzyme in the biosynthesis of jasmonates in that it releases, in a coupled reaction with allene oxide synthase (AOS), the first cyclic and biological active metabolite - OPDA. We describe here the expression and purification of AOS and AOC and their subsequent coupling to solid matrices to produce an enantioselective, reusable bioreactor for octadecanoid production. With the method described here it is possible to produce optically pure enantiomers of octadecanoids in high amounts in a cost- and time-efficient manner. Furthermore, it could be demonstrated that a physical interaction of AOS and AOC, hitherto postulated to be required for substrate channeling from AOS to AOC, is not necessary for the in vitro cyclization of the unstable epoxide generated by the AOS reaction.

The crystal structure of Arabidopsis thaliana allene oxide cyclase: insights into the oxylipin cyclization reaction

We describe the crystallization and structure elucidation of Arabidopsis thaliana allene oxide cyclase 2 (AOC2), a key enzyme in the biosynthesis of jasmonates. In a coupled reaction with allene oxide synthase, AOC2 releases the first cyclic and biologically active metabolite, 12-oxo-phytodienoic acid (OPDA). AOC2 (AT3G25770) folds into an eight-stranded antiparallel beta-barrel with a C-terminal partial helical extension. The protein forms a hydrophobic binding cavity with two distinct polar patches. AOC2 is trimeric in crystals, in vitro and in planta. Based on the observed folding pattern, we assigned AOC2 as a low molecular weight member of the lipocalin family with enzymatic activity in plants. We determined the binding position of the competitive inhibitor vernolic acid (a substrate analog) in the binding pocket. Based on models for bound substrate 12,13-epoxy-9,11,15-octadecatrienoic acid and product OPDA, we propose a reaction scheme that explains the influence of the C15 double bond on reactivity. Reaction is promoted by anchimeric assistance through a conserved Glu residue. The transition state with a pentadienyl carbocation and an oxyanion is stabilized by a strongly bound water molecule and favorable pi-pi interactions with aromatic residues in the cavity. Stereoselectivity results from steric restrictions to the necessary substrate isomerizations imposed by the protein.

Characterization of novel imidazole derivative, JM-8686, a potent inhibitor of allene oxide synthase

The inhibitory properties of a first synthetic jasmonic acid biosynthesis inhibitor, JM-8686, were investigated. Steady-state kinetic analysis indicates that the compound is a competitive inhibitor of allene oxide synthase (AOS) with a K(i) value of approximate 0.62+/-0.15 microM. Dialysis experiment indicates that AOS inactivation by JM-8686 is reversible. The optical difference spectroscopy analysis of JM-8686 and AOS interaction indicates that JM-8686 induced type II binding spectra with a K(d) value of approximate 1.6+/-0.2 microM, suggesting that JM-8686 binds to the prosthetic heme iron of AOS. Comparison of the inhibitory potency of the compound against HPL (CYP74B) from tomato revealed that JM-8686 was a highly selective inhibitor for AOS.

Allene oxide synthase from Arabidopsis thaliana (CYP74A1) exhibits dual specificity that is regulated by monomer-micelle association

We investigate the effects of detergent on the kinetics and oligomeric state of allene oxide synthase (AOS) from Arabidopsis thaliana (CYP74A1). We show that detergent-free CYP74A1 is monomeric and highly water soluble with dual specificity, but has relatively low activity. Detergent micelles promote a 48-fold increase in k(cat)/K(m) (to 5.9 x 10(7)M(-1)s(-1)) with concomitant changes in the spin state equilibrium of the haem-iron due to the binding of a single detergent micelle to the protein monomer, which is atypical of P450 enzymes. This mechanism is shown to be an important determinant of the substrate specificity of CYP74A1. CYP74A1 may be suited for structural resolution of the first plant cytochrome P450 and its 9-AOS activity and behaviour in vitro has implications for its role in planta.