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PubMed2004

Separation-friendly Mitsunobu reactions: a microcosm of recent developments in separation strategies.

The Mitsunobu reaction is famous for its scope and power, but infamous for its separation headaches. Typically, the target product is enticed away from the reagent-derived byproducts by careful chromatography. The use of polymer-bound Mitsunobu reagents solves only half of the problem, because polymer-bound diethyl azodicarboxylate (DEAD) and phosphine reagents cannot be employed simultaneously. This article classifies, compares, and contrasts various emerging strategies for product isolation in Mitsunobu reactions. Because so many different strategies have been used, the Mitsunobu reaction is a microcosm for the new field of strategy level separations.

Alcohols↗
PubMed2004

A novel method for coupling doxorubicin to lactosaminated human albumin by an acid sensitive hydrazone bond: synthesis, characterization and preliminary biological properties of the conjugate.

The expression of the asialoglycoprotein receptor on the cells of the large majority of the well differentiated hepatocellular carcinomas can be exploited to improve the chemotherapy of these tumours by coupling anticancer agents to macromolecules taken up by the receptor. In line with this approach, in previous experiments we coupled doxorubicin (DOXO) to lactosaminated human albumin (L-HSA) using the (6-maleimidocaproyl)hydrazone derivative of the drug as an acid sensitive linker. Encouraging results were obtained in laboratory animals using L-HSA-DOXO. This conjugate, however, has the disadvantage of a difficult synthesis, which requires protein thiolation with iminothiolane and can hinder its preparation on a large scale. Here we describe a very simple method of coupling. The HS-groups required for the reaction with the maleimide moiety of DOXO-EMCH are made available in L-HSA by a cleavage of the protein disulphides achieved with tris(2-carboxyethyl) phosphine (TCEP). Contrary to thiolic reducing agents, the use of TCEP eliminates the need of an inert atmosphere and allows a one-step coupling reaction, without purification of the reduced protein before the addition of DOXO-EMCH. As the previous L-HSA-DOXO conjugate, the new conjugate accomplishes a very efficient liver targeting of the drug. This novel method of synthesis should facilitate the preparation of L-HSA-DOXO in the amounts required for clinical studies.

Amino Sugars↗
PubMed2003

Residues C123 and D58 of the 2-methylisocitrate lyase (PrpB) enzyme of Salmonella enterica are essential for catalysis.

The prpB gene of Salmonella enterica serovar Typhimurium LT2 encodes a protein with 2-methylisocitrate (2-MIC) lyase activity, which cleaves 2-MIC into pyruvate and succinate during the conversion of propionate to pyruvate via the 2-methylcitric acid cycle. This paper reports the isolation and kinetic characterization of wild-type and five mutant PrpB proteins. Wild-type PrpB protein had a molecular mass of approximately 32 kDa per subunit, and the biologically active enzyme was comprised of four subunits. Optimal 2-MIC lyase activity was measured at pH 7.5 and 50 degrees C, and the reaction required Mg(2+) ions; equimolar concentrations of Mn(2+) ions were a poor substitute for Mg(2+) (28% specific activity). Dithiothreitol (DTT) or reduced glutathione (GSH) was required for optimal activity; the role of DTT or GSH was apparently not to reduce disulfide bonds, since the disulfide-specific reducing agent Tris(2-carboxyethyl)phosphine hydrochloride failed to substitute for DTT or GSH. The K(m) of PrpB for 2-MIC was measured at 19 micro M, with a k(cat) of 105 s(-1). Mutations in the prpB gene were introduced by site-directed mutagenesis based on the active-site residues deemed important for catalysis in the closely related phosphoenolpyruvate mutase and isocitrate lyase enzymes. Residues D58, K121, C123, and H125 of PrpB were changed to alanine, and residue R122 was changed to lysine. Nondenaturing polyacrylamide gel electrophoresis indicated that all mutant PrpB proteins retained the same oligomeric state of the wild-type enzyme, which is known to form tetramers. The PrpB(K121A), PrpB(H125A), and PrpB(R122K) mutant proteins formed enzymes that had 1,050-, 750-, and 2-fold decreases in k(cat) for 2-MIC lyase activity, respectively. The PrpB(D58A) and PrpB(C123A) proteins formed tetramers that displayed no detectable 2-MIC lyase activity indicating that both of these residues are essential for catalysis. Based on the proposed mechanism of the closely related isocitrate lyases, PrpB residue C123 is proposed to serve as the active site base, and residue D58 is critical for the coordination of a required Mg(2+) ion.

Amino Acid Sequence↗
PubMed2003

Synthesis and characterization of new copper(I) complexes containing 4-(diphenylphosphane)benzoic acid and "scorpionate" ligands with "in vitro" superoxide scavenging activity.

New copper(I) complexes have been synthesised from the reaction of CuCl with 4-(diphenylphosphane)benzoic acid and lithium tris(1H-pyrazol-1-yl)methanesulfonate, Li(SO(3))C(pz)(3), sodium hydrotris(3-trifluoromethyl-1H-pyrazol-1-yl)borate, NaHB[3-(CF(3))pz](3), potassium dihydrobis(1H-1,2,4-triazol-1-yl)borate, KH(2)B(tz)(2), hydrotris(1H-1,2,4-triazol-1-yl)borate, KHB(tz)(3), sodium hydrotris(1H-pyrazol-1-yl)borate, NaHB(pz)(3), potassium hydrotris(3,5-dimethyl-1H-pyrazol-1-yl)borate KHB(3,5-Me(2)Pz)(3) or potassium hydrotris(4-bromo-1H-pyrazol-1-yl)borate KHB(4-Brpz)(3). The complexes obtained have been characterized by elemental analyses and FT-IR in the solid state, and by NMR (1H and 31P[(1)H]) spectroscopy and conductivity measurements in solution. The solution data are consistent with partial dissociation of the sterically hindered complexes by way of breaking of Cu-P and Cu-N bonds. Electrospray mass spectrometry has been used to investigate the relative properties of the 4-(diphenylphosphane)benzoic acid and of the "scorpionate" ligands towards copper(I) ions. Chemiluminescence technique was used to evaluate the superoxide scavenging activity of these new copper complexes.

Acridines↗
PubMed2007

9-fluorenylphosphines for the Pd-catalyzed sonogashira, suzuki, and Buchwald-Hartwig coupling reactions in organic solvents and water.

The lithiation/alkylation of fluorene leads to various 9-alkyl-fluorenes (alkyl=Me, Et, iPr, -Pr, -C18H25) in>95% yields, for which lithiation and reaction with R2PCl (R=Cy, iPr, tBu) generates 9-alkyl, 9-PR2-fluorenes which constitute electron-rich and bulky phosphine ligands. The in-situ-formed palladium-phosphine complexes ([Na2PdCl4], phosphonium salt, base, substrates) were tested in the Sonogashira, Suzuki, and Buchwald-Hartwig reactions of aryl chlorides and aryl bromides in organic solvents. The Sonogashira coupling of aryl chlorides at 100-120 degrees C leads to>90% yields with 1 mol% of Pd catalyst. The Suzuki coupling of aryl chlorides typically requires 0.05 mol% of Pd catalyst at 100 degrees C in dioxane for quantitative product formation. To carry out "green" cross-coupling reactions in water, 9-ethylfluorenyldicyclohexylphosphine was reacted in sulphuric acid to generate the respective 2-sulfonated phosphonium salt. The Suzuki coupling of activated aryl chlorides by using this water-soluble catalyst requires only 0.01 mol% of Pd catalyst, while a wide range of aryl chlorides can be quantitatively converted into the respective coupling products by using 0.1-0.5 mol% of catalyst in pure water at 100 degrees C. Difficult substrate combinations, such as naphthylboronic acid or 3-pyridylboronic acid and aryl chlorides are coupled at 100 degrees C by using 0.1-0.5 mol% of catalyst in pure water to obtain the respective N-heterocycles in quantitative yields. The copper-free aqueous Sonogashira coupling of aryl bromides generates the respective tolane derivatives in>95% yield.

Journal Article↗
PubMed2001

Crystal structure of the stromelysin-3 (MMP-11) catalytic domain complexed with a phosphinic inhibitor mimicking the transition-state.

Stromelysin-3 (ST3) is a matrix metalloproteinase (MMP-11) whose proteolytic activity plays an important role in tumorigenicity enhancement. In breast cancer, ST3 is a bad prognosis marker: its expression is associated with a poor clinical outcome. This enzyme therefore represents an attractive therapeutic target. The topology of matrix metalloproteinases (MMPs) is remarkably well conserved, making the design of highly specific inhibitors difficult. The major difference between MMPs lies in the S(1)' subsite, a well-defined hydrophobic pocket of variable depth. The present crystal structure, the first 3D-structure of the ST3 catalytic domain in interaction with a phosphinic inhibitor mimicking a (d, l) peptide, clearly demonstrates that its S(1)' pocket corresponds to a tunnel running through the enzyme. This open channel is filled by the inhibitor P(1)' group which adopts a constrained conformation to fit this pocket, together with two water molecules interacting with the ST3-specific residue Gln215. These observations provide clues for the design of more specific inhibitors and show how ST3 can accommodate a phosphinic inhibitor mimicking a (d, l) peptide. The presence of a water molecule interacting with one oxygen atom of the inhibitor phosphinyl group and the proline residue of the Met-turn suggests how the intermediate formed during proteolysis may be stabilized. Furthermore, the hydrogen bond distance observed between the methyl of the phosphinic group and the carbonyl group of Ala182 mimics the interaction between this carbonyl group and the amide group of the cleaved peptidic bond. Our crystal structure provides a good model to study the MMPs mechanism of proteolysis.

Amino Acid Sequence↗
PubMed2005

Mono- and beta,beta-double-Heck reactions of alpha,beta-unsaturated carbonyl compounds in aqueous media.

Optimized reaction conditions for the mono- and beta,beta-diarylation of electron-deficient alkenes in aqueous media catalyzed either by a p-hydroxyacetophenone oxime-derived palladacycle or by palladium(II) acetate under phosphine-free conditions and in the presence of (dicyclohexyl)methylamine as base are described. Regioselective monoarylation of unsubstituted and substituted alpha,beta-unsaturated carbonyl compounds takes place with aryl iodides at 120 degrees C in water. Aqueous N,N-dimethylacetamide (DMA), tetra-n-butylammonium bromide (TBAB) as additive, and the palladacycle as catalyst are the most efficient conditions for the coupling with aryl bromides, good stereoselectivities being also obtained in the arylation of crotonates and itaconates, whereas cinnamic derivatives afford lower steroselectivity, with the exception of cinnamic acid and nitrile. beta,beta-Diarylation of unsubstituted alpha,beta-unsaturated carbonyl compounds can be controlled by using higher loading of the palladacycle and can be performed in refluxing water for aryl iodides, whereas DMA must be used for aryl bromides. Microwave irradiation can be used in the monoarylation of tert-butyl acrylate with aryl iodides in water or the coupling between ethyl cinnamate and aryl bromides in aqueous DMA.

Journal Article↗
PubMed2005

Mild and room temperature C-C bond forming reactions of nucleoside C-6 arylsulfonates.

[reaction: see text] Palladium catalyzed cross coupling of nucleoside arylsulfonates and arylboronic acids has been accomplished under mild conditions and at room temperature. Among three structurally similar ligands that differ in their steric and electronic properties, one yielded an effective catalyst in conjunction with Pd(OAc)2. Of the nucleoside arylsulfonates evaluated, the O6-(2,4,6-trimethylphenyl)sulfonate proved optimal, but other alkyl and alkoxy derivatives were also reasonably reactive. On the other hand, a 2-nitrophenyl and a 2-thienyl derivative were ineffective substrates. PhMe and THP were suitable as solvents, yielding good results in several cases, although reactions of some arylboronic acids were faster in PhMe. In contrast, reactions of arylboronic acids bearing strongly electron-withdrawing groups proceeded more successfully in THP. Interplay between several factors that include substituents on the nucleoside arylsulfonate, ligand substituents, and solvent is responsible for successful cross coupling. Using 31P NMR, an initial investigation has been conducted to study the interaction of Pd(OAc)2 with the ligand. At a 1:1 stoichiometry of ligand and Pd(OAc)2, a predominant species, likely a cyclopalladation product, was obtained. At a 2:1 ratio of ligand and Pd(OAc)2, a different species bearing chemically distinct phosphine ligands was observed. Both complexes display catalytic activity, although the 2:1 species may be superior.

Arylsulfonates↗
PubMed2001

Asymmetric Pt(II)-catalyzed ene reactions: counterion-dependent additive and diphosphine electronic effects.

[reaction: see text]. Catalysis of the glyoxylate-ene reaction by dicationic P2Pt(II) complexes is subject to anion-dependent additive effects. For [((S)-MeOBiphep)Pt](OTf)2 catalysts, acidic phenols such as 3-CF3-C6H3OH or C6F5OH provide substantial rate increases but do not affect the more active SbF6-based catalysts. Enantioselectivity and reactivity also increased with diphosphine basicity, with 4-t-Bu-substituted MeOBiphep ligands yielding the highest enantioselectivities.

Anions↗
PubMed2005

Synthesis and incorporation into DNA of a chemically stable, functional abasic site analogue.

[reaction: see text] The abasic site building block 7 for DNA synthesis, containing a methylenephosphinic acid group at C3', was prepared in six steps and was incorporated into DNA via a combination of H-phosphonate and phosphoramidite chemistry. Corresponding oligodeoxynucleotides were shown to be chemically stable under basic conditions and fully functional at the respective hemiacetal center.

DNA↗
PubMed2003

Enhanced stereoselectivity in internucleotidic bond formation by the use of the chiral ribose moiety of thymidine.

This paper deals with the synthesis of new cyclic thymidine 3'-phosphoramidite building blocks having a covalent linker between the trityl type 5'-hydroxyl protecting group and the phosphorus atom attached to the 3'-hydroxyl group of thymidine. The ring structures were designed to reduce the conformational freedom around the phosphorus center so that the stereoselectivity in the internucleotide linkage formation would be improved. The linkers were also designed to be removed readily by treatment with aqueous ammonia. These building blocks were synthesized in good yield by one-pot cyclization of the diol precursors with dichloro(N,N-diisopropylamino)phosphine, despite their large-membered ring. Various activators having 1H-tetrazole, imidazole, and triazole structures were investigated to find the best selectivity in the synthesis of thymidylyl(5'-3')thymidine phosphorothioate. It turned out that our cyclic phosphoramidites gave preferentially the R(p) diastereoisomer in high coupling yield applicable to the solid-phase synthesis of oligodeoxynucleotides. It should be noted that high stereoselectivity was achieved without any chiral sources other than the 2'-deoxyribose moiety itself. The mechanistic studies revealed the importance of the steric bulk and the acidity of the activators. It was also found that the steric bulk of the alcoholic nucleophile was an important factor that determined the stereoselectivity in our systems.

Catalysis↗
PubMed2006

Predicting the stereochemistry of diphenylphosphino benzoic acid (DPPBA)-based palladium-catalyzed asymmetric allylic alkylation reactions: a working model.

Palladium-catalyzed asymmetric allylic alkylation has proven to be a powerful method for the preparation of a wide variety of chiral compounds and the rapid assembly of complex molecular architecture from simple starting materials. While many types of catalyst systems have been successfully employed with certain systems, diphenylphosphino benzoic acid (DPPBA) based ligands have found use over a broad range of substrate classes. This Account highlights the mechanistic aspects considered when designing reactions with DPPBA-based ligands and presents a working model for the a priori prediction of their stereochemical outcome.

Alkylation↗
PubMed2006

Photochemical generation of dihydrogen complexes of chromium and tungsten.

Photolysis of solutions of M(CO)(6) (M = Cr, W) at low temperature in the presence of hydrogen gas affords Cr(CO)(5)(H(2)) (1) and W(CO)(5)(H(2)) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T(1) values for the hydride resonances and the observation of a large HD coupling in the HD derivatives. Irradiation of a phosphine-substituted derivative (PMe(3))Cr(CO)(5) in the presence of hydrogen gas gave similar results. Thus cis-(PMe(3))Cr(CO)(4)(H(2)) (3) and trans-(PMe(3))Cr(CO)(4)(H(2)) (4) were prepared and characterized by (1)H and (31)P NMR spectroscopy. When the photolysis reactions were carried out in methylene chloride, solvent binding competitive with hydrogen binding was observed. This was not observed in less coordinating solvents such as alkanes. Subsequent displacement of solvent by H(2) leads to the dihydrogen complexes. Complexes 1 and 2 are moderately acidic, with deprotonation effected by mild bases.

Journal Article↗
PubMed2005

Ruthenium-catalyzed propargylic substitution reactions of propargylic alcohols with oxygen-, nitrogen-, and phosphorus-centered nucleophiles.

The scope and limitations of the ruthenium-catalyzed propargylic substitution reaction of propargylic alcohols with heteroatom-centered nucleophiles are presented. Oxygen-, nitrogen-, and phosphorus-centered nucleophiles such as alcohols, amines, amides, and phosphine oxide are available for this catalytic reaction. Only the thiolate-bridged diruthenium complexes can work as catalysts for this reaction. Results of some stoichiometric and catalytic reactions indicate that the catalytic propargylic substitution reaction proceeds via an allenylidene complex formed in situ, whereby the attack of nucleophiles to the allenylidene C(gamma) atom is a key step. Investigation of the relative rate constants for the reaction of propargylic alcohols with several para-substituted anilines reveals that the attack of anilines on the allenylidene C(gamma) atom is not involved in the rate-determining step and rather the acidity of conjugated anilines of an alkynyl complex, which is formed after the attack of aniline on the C(gamma) atom, is considered to be the most important factor to determine the rate of this catalytic reaction. The key point to promote this catalytic reaction by using the thiolate-bridged diruthenium complexes is considered to be the ease of the ligand exchange step between a vinylidene ligand on the diruthenium complexes and another propargylic alcohol in the catalytic cycle. The reason why only the thiolate-bridged diruthenium complexes promote the ligand exchange step more easily with respect to other monoruthenium complexes in this catalytic reaction should be that one Ru moiety, which is not involved in the allenylidene formation, works as an electron pool or a mobile ligand to another Ru site. The catalytic procedure presented here provides a versatile, direct, and one-step method for propargylic substitution of propargylic alcohols in contrast to the so far well-known stoichiometric and stepwise Nicholas reaction.

Journal Article↗
PubMed2005

Inorganic-organic hybrids of the p,p'-diphenylmethylenediphosphinate, pcp2-. Synthesis, characterization, and XRPD structures of [Sn(pcp)] and [Cu(pcp)].

Two new inorganic-organic polymeric hybrids [Sn(pcp)] and [Cu(pcp)], pcp=CH2(PhPO2)2(2-), have been synthesized and structurally characterized. The tin derivative has been obtained by reaction of the p,p'-diphenylmethylenediphosphinic acid (H2pcp) in water with SnCl2.2H2O, while the copper derivative has been synthesized through a hydrothermal reaction from the same H2pcp acid and Cu(O2CMe)2.H2O. The structures of these compounds have been solved "ab initio" by X-ray powder diffraction (XRPD) data. [Sn(pcp)] has a ladder-like polymeric structure, with tin(II) centers bridged by diphenylmethylenediphosphinate ligands, and alternating six- and eight-membered rings. The hemilectic coordination around the metal shows the tin(II) lone pair to be operative, resulting in significant interaction mainly with a C-C bond of one phenyl ring. The [Cu(pcp)] complex displays a polymeric columnar structure formed by two intersecting sinusoidal ribbons of copper(II) ions bridged by the bifunctional phosphinate ligands. The intersections of the ribbons are made of dimeric units of pentacoordinated copper ions. Crystal data for [Sn(pcp)]: monoclinic, space group P2(1)/c, a=11.2851(1), b=15.4495(6), c=8.6830(1) A, beta=107.546(1) degrees, V=1443.44(9) A, Z=4. Crystal data for [Cu(pcp)]: triclinic, space group P, a=10.7126(4), b=13.0719(4), c=4.9272(3) A, alpha=92.067(5), beta=95.902(7), gamma=87.847(4) degrees, V=685.47(7), Z=2. The tin compound has been characterized by 119Sn MAS NMR (magic-angle spinning NMR), revealing asymmetry in the valence electron cloud about tin. Low-temperature magnetic measurements of the copper compound have indicated the presence of weak antiferromagnetic interactions below 50 K.

Journal Article↗
PubMed2002

Divalent osmium complexes: synthesis, characterization, strong red phosphorescence, and electrophosphorescence.

We report new divalent osmium complexes that feature strong red metal-to-ligand-charge-transfer (MLCT) phosphorescence and electrophosphorescence. The general formula of the complexes is Os(II)(N-N)(2)L-L, where N-N is either a bipyridine or a phenanthroline and L-L is either a phosphine or an arsine. New polypyridyl ligands synthesized are 4,4'-di(biphenyl)-2,2'-bipyridine (15) and 4,4'-di(diphenyl ether)-2,2'-bipyridine (16), and the 1,10-phenanthroline derivatives synthesized are 4,7-bis(p-methoxyphenyl)-1,10-phenanthroline (17), 4,7-bis(p-bromophenyl)-1,10-phenanthroline (18), 4,7-bis(4'-phenoxybiphen-4-yl)-1,10-phenanthroline (19), and 4,7-bis(4-naphth-2-ylphenyl)-1,10-phenanthroline (20). 4,4'-Diphenyl-2,2'-bipyridine (21) and 4,7-diphenyl-1,10-phenanthroline (22) were also used in these studies. Strong pi-acid ligands used were 1,2-bis(diphenylarseno)ethane (23), cis-1,2-bis(diphenylphosphino)ethylene (24), and cis-1,2-vinylenebis(diphenylarsine) (25). Ligand 25 is used for the first time in these types of luminescent osmium complexes. These compounds feature strong MLCT absorption bands in the visible region and strong red phosphorescent emission ranging from 611 to 651 nm, with quantum efficiency up to 45% in ethanol solution at room temperature. Red organic light-emitting diodes (OLEDs) were successfully fabricated by doping the Os(II) complexes into blend of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD). Brightness over 1400 cd/m(2) for a double-layer device has been reached, with a turn-on voltage of 8 V. The maximum external quantum efficiency was 0.64%. Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (x, y) of the red electrophosphorescence from the complexes are (0.65, 0.34), which indicates pure red emission.

Journal Article↗
PubMed1998

Pyramidal Inversion at Phosphorus Facilitated by the Presence of Proximate Lewis Acids. Coordination Chemistry of Group 13 Elements with the Macrocyclic Bis(amidophosphine) Ligand [P(2)N(2)] ([P(2)N(2)] = [PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh]).

Investigations on the preparation of four- and five-coordinate aluminum and gallium bis(amidophosphine) derivatives are reported. The reaction of the macrocyclic ligand precursor anti-Li(2)(THF)(2)[P(2)N(2)] ([P(2)N(2)] = [PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh]) with AlCl(3) or GaCl(3) in toluene at 25 degrees C leads to the formation of the four-coordinate species anti-MCl[P(2)N(2)] (M = Al (1), Ga (2)). An X-ray diffraction study of anti-GaCl[P(2)N(2)] shows it to be monomeric with a distorted tetrahedral geometry at Ga; only one of the phosphine donors of the [P(2)N(2)] ligand binds to the gallium, resulting in the retention of the anti-configuration. The solution NMR spectra are consistent with C(s)() symmetry. The addition of AlCl(3) or GaCl(3) to the macrocyclic ligand precursor syn-Li(2)(dioxane)[P(2)N(2)] in toluene at 25 degrees C yields the five-coordinate complexes syn-MCl[P(2)N(2)] (M = Al (3), Ga (4)). The X-ray crystal structure of syn-GaCl[P(2)N(2)] reveals a trigonal bipyramidal geometry about the metal atom, necessitating the coordination of both phosphorus atoms. The solution NMR spectra are consistent with a C(2)(v)() symmetric complex. Heating the anti complexes results in the clean conversion to the syn complexes, with pyramidal inversion observed at phosphorus. The kinetics of this inversion were studied by (1)H NMR spectroscopy and found to be first-order. Barriers to pyramidal inversion (DeltaG()) were calculated to be 29.1 and 30.1 kcal mol(-)(1) for the aluminum and gallium complexes, respectively; these barriers are approximately 2-3 kcal mol(-)(1) lower than that determined for the metal-free, protonated compounds anti- and syn-H(2)[P(2)N(2)]. It is suggested that the role that the metals play in this inversion, based on the values of DeltaG(), involves the large negative entropies of activation and thus help organize the transition state.

Journal Article↗
PubMed1998

Human thioredoxin reductase from HeLa cells: selective alkylation of selenocysteine in the protein inhibits enzyme activity and reduction with NADPH influences affinity to heparin.

Human thioredoxin reductase (TR) contains selenocysteine (Secys) in a redox center [cysteine (Cys)-497,Secys-498] near the C-terminus. The essential role of Secys in TR isolated from HeLa cells was demonstrated by the alkylation studies. Reaction of native NADPH reduced enzyme with bromoacetate at pH 6.5 inhibited enzyme activity 99%. Of the incorporated carboxymethyl (CM) group, 1.1 per subunit, >90% was in CM-Secys-498. Alkylation at pH 8 increased the stoichiometry to 1.6 per subunit with additional modification of the Cys-59, Cys-64 disulfide center. A minor tryptic peptide containing both CM-Cys-497 and CM-Secys-498 was isolated from enzyme alkylated at pH 6.5 or at pH 8. Preparations of TR isolated from HeLa cells grown in a fermentor under high aeration contained selenium-deficient enzyme species that had 50% lower activity. Decreasing oxygen to an optimal level increased cell yield, and fully active TR containing one Se per subunit was present. Reduction of fully active enzyme with tris-(2-carboxyethyl) phosphine converted it from a low to a high heparin affinity form. The tris-(2-carboxyethyl) phosphine-reduced enzyme was oxygen-sensitive and lost selenium and catalytic activity unless maintained under strictly anaerobic conditions. This enzyme could be converted to an oxygen-insensitive species by addition of NADPH, indicating that bound pyridine nucleotide is important for enzyme stability. An induced enzyme conformation in which the essential Secys is shielded from oxidative damage could explain these effects.

Alkylation↗