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PubMed2006

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.

Amino Acid Sequence↗
PubMed1993

Structure of the product from a novel cyclization reaction involving a C(6)-substituted uridine analog.

5,6,7,7a,9,10,14b,14c-Octahydro-4-[2,3-O-(1-methylethylidene )-beta-D- ribofuranosyl]cyclopenta-[4,5]pyrimido[5',4':3,4]pyrrolo[2,1- a]isoquinoline-1,3(2H,4H)-dione, C25H31N3O6, M(r) = 469.54, tetragonal, P4(3)2(1)2, a = 12.577 (2), b = 12.577 (2), c = 29.893 (4) A, V = 4729 (1) A3, Z = 8, Dx = 1.319 g cm-3, lambda (Cu K alpha) = 1.5418 A, mu = 7.9 cm-1, F(000) = 2000, T = 298 K, final R = 0.038, wR = 0.064 for all 1884 independent reflections and 432 variables. The crystal structure shows a syn conformation around the N(1)--C(1') single bond, consistent with other C(6)-substituted uridine analogs, and an unusual O(1') endo conformation of the ribose ring. The stereochemistries of the three newly created chiral centers are 14cR, 4aR, 7aS.

Crystallography, X-Ray↗
PubMed1979

Synthesis of oxytocin using iodine for oxidative cyclization and silica gel adsorption chromatography for purification.

Oxytocin (OT) was synthesized employing the solid phase method. Resins made of copolymers of polystyrene-1%-crosslinked with divinylbenzene gave better yields (73-95%) of Z-Cys(Bzl)-Tyr(Bzl)-Ile-Gln-Asn-Cys(Bzl)-Pro-Leu-Gly-NH2 (I) than 2%-crosslinked resins (10--56%). Reduction of I with Na-liq.NH3 and oxidation with I2-MeOH at -40 degrees minimized dimer and polymer formation, and resulted in good yields (49--54%) of OT. The large volumes of MeOH required when several grams of I are reduced and then oxidized were rapidly evaporated in vacuo, and the residue was desalted by dissolving the peptide in a small volume of glacial acetic acid and filtering to remove the salt. OT was purified by adsorption chromatography on a silica gel column with combinations of MeOH-CHCl3 of graded polarity. Oxytocin elutes with 33% MeOH-CHCl3. After two purification steps by adsorption chromatography, the resulting OT was found to be homogeneous. The hormone was characterized chemically and found to be active biologically.

Adsorption↗
PubMed2000

Studies on Lewis acid-mediated intramolecular cyclization reactions of allene-ene systems.

The Lewis acid-mediated reactions of allene-ene compounds, derived from 3-methylcitronellal or dimethyl malonate, were carried out using various Lewis acids such as ethylaluminum dichloride, diethylaluminum chloride, titanium chloride, zinc chloride etherate, or boron trifluoride etherate, affording unexpectedly intramolecular [2+2]cycloaddition products under some particular reaction conditions without any formation of intramolecular ene reaction products.

Aldehydes↗