Purification of intact old yellow enzyme using an affinity matrix for the sole chromatographic step

Access to Diarylmethanols by Wittig Rearrangement of ortho-, meta-, and para-Benzyloxy-N-Butylbenzamides

The N-butyl amide group, CONHBu, has been found to be an effective promoter of the [1,2]-Wittig rearrangement of aryl benzyl ethers and thus allow the two-step synthesis of isomerically pure substituted diarylmethanols starting from simple hydroxybenzoic acid derivatives. The method is compatible with a wide range of functional groups including methyl, methoxy, and fluoro, although not with nitro and, unexpectedly, is applicable to meta as well as ortho and para isomeric series.

Investigating Saccharomyces cerevisiae alkene reductase OYE 3 by substrate profiling, X-ray crystallography and computational methods

Saccharomyces cerevisiae OYE 3 and OYE 1 share 80% sequence identity, but sometimes differ in stereoselectivities.

Applications of protein engineering to members of the old yellow enzyme family

In the 20 years since Massey's initial report in 1995, interest in using alkene reductases to prepare chiral intermediates for synthesis has grown rapidly. While native alkene reductases often show very high stereoselectivities toward favorable substrates, these enzymes have somewhat size-restricted active sites that limit their substrate ranges to small alkenes. In addition, most alkene reductases have the same stereoselectivities, which makes it difficult to access the "other" product enantiomers. Protein engineering strategies have been used to address both of these issues and good progress has been made in several cases. This review summarizes published examples through late 2014 and focuses on studies of six enzymes: Saccharomyces pastorianus OYE 1, tomato OPR1, Zymomonas mobilis NCR, Enterobacter cloacae PB2 PETN reductase, Bacillus subtilis YqjM and Pichia stipitis OYE 2.6.

Pichia stipitis OYE 2.6 variants with improved catalytic efficiencies from site-saturation mutagenesis libraries

An earlier directed evolution project using alkene reductase OYE 2.6 from Pichia stipitis yielded 13 active site variants with improved properties toward three homologous Baylis-Hillman adducts. Here, we probed the generality of these improvements by testing the wild-type and all 13 variants against a panel of 16 structurally-diverse electron-deficient alkenes. Several substrates were sterically demanding, and as hoped, creating additional active site volume yielded better conversions for these alkenes. The most impressive improvement was found for 2-butylidenecyclohexanone. The wild-type provided less than 20% conversion after 24h; a triple mutant afforded more than 60% conversion in the same time period. Moreover, even wild-type OYE 2.6 can reduce cyclohexenones with very bulky 4-substituents efficiently.

Residues Controlling Facial Selectivity in an Alkene Reductase and Semirational Alterations to Create Stereocomplementary Variants

A systematic saturation mutagenesis campaign was carried out on an alkene reductase from Pichia stipitis (OYE 2.6) to develop variants with reversed stereoselectivities. Wild-type OYE 2.6 reduces three representative Baylis–Hillman adducts to the corresponding S products with almost complete stereoselectivities and good catalytic efficiencies. We created and screened 13 first-generation, site-saturation mutagenesis libraries, targeting residues found near the bound substrate. One variant (Tyr78Trp) showed high R selectivity toward one of the three substrates, but no change (cyclohexenone derivative) and no catalytic activity (acrylate derivative) for the other two. Subsequent rounds of mutagenesis retained the Tyr78Trp mutation and explored other residues that impacted stereoselectivity when altered in a wild-type background. These efforts yielded double and triple mutants that possessed inverted stereoselectivities for two of the three substrates (conversions >99% and at least 91% ee (R)). To understand the reasons underlying the stereochemical changes, we solved crystal structures of two key mutants: Tyr78Trp and Tyr78Trp/Ile113Cys, the latter with substrate partially occupying the active site. By combining these experimental data with modeling studies, we have proposed a rationale that explains the impacts of the most useful mutations.

Identification of promiscuous ene-reductase activity by mining structural databases using active site constellations

The exploitation of catalytic promiscuity and the application of de novo design have recently opened the access to novel, non-natural enzymatic activities. Here we describe a structural bioinformatic method for predicting catalytic activities of enzymes based on three-dimensional constellations of functional groups in active sites ('catalophores'). As a proof-of-concept we identify two enzymes with predicted promiscuous ene-reductase activity (reduction of activated C-C double bonds) and compare them with known ene-reductases, that is, members of the Old Yellow Enzyme family. Despite completely different amino acid sequences, overall structures and protein folds, high-resolution crystal structures reveal equivalent binding modes of typical Old Yellow Enzyme substrates and ligands. Biochemical and biocatalytic data show that the two enzymes indeed possess ene-reductase activity and reveal an inverted stereopreference compared with Old Yellow Enzymes for some substrates. This method could thus be a tool for the identification of viable starting points for the development and engineering of novel biocatalysts.

NAD(P)H-independent asymmetric C=C bond reduction catalyzed by ene reductases by using artificial co-substrates as the hydrogen donor

To develop a nicotinamide-independent single flavoenzyme system for the asymmetric bioreduction of C=C bonds, four types of hydrogen donor, encompassing more than 50 candidates, were investigated. Six highly potent, cheap, and commercially available co-substrates were identified that (under the optimized conditions) resulted in conversions and enantioselectivities comparable with, or even superior to, those obtained with traditional two-enzyme nicotinamide adenine dinucleotide phosphate (NAD(P)H)-recycling systems.

Overcoming co-product inhibition in the nicotinamide independent asymmetric bioreduction of activated C=C-bonds using flavin-dependent ene-reductases

Eleven flavoproteins from the old yellow enzyme family were found to catalyze the disproportionation (“dismutation”) of conjugated enones. Incomplete conversions, which were attributed to enzyme inhibition by the co-product phenol could be circumvented via in situ co-product removal by scavenging the phenol using the polymeric adsorbent MP-carbonate. The optimized system allowed to reduce an alkene activated by ester groups in a “coupled-substrate” approach via nicotinamide-free hydrogen transfer with >90% conversion and complete stereoselectivity.

Comparative characterization and expression analysis of the four Old Yellow Enzyme homologues from Shewanella oneidensis indicate differences in physiological function

Shewanella oneidensis contains four genes that encode proteins that have high sequence identity with yeast OYE (Old Yellow Enzyme, an NADPH oxidoreductase), the well-studied archetype of the OYE protein family. The present paper describes the first comparative study of OYEs that are present in a single bacterial species, performed to gain insight into their biochemical properties and physiological importance. The four proteins [named SYE1-SYE4 (Shewanella Yellow Enzyme 1-4)] were expressed as glutathione S-transferase fusion proteins in Escherichia coli. The yield of SYE2, however, was too low for further characterization, even after expression attempts in S. oneidensis. The SYE1, SYE3 and SYE4 proteins were found to have characteristics similar to those of other OYE family members. They were identified as flavoproteins that catalyse the reduction of different alpha,beta-unsaturated carbonyl compounds and form charge transfer complexes with a range of phenolic compounds. Whereas the properties of SYE1 and SYE3 were very similar, those of SYE4 were clearly different in terms of ligand binding, catalytic efficiency and substrate specificity. Also, the activity of SYE4 was found to be NADPH-dependent, whereas SYE1 and SYE3 had a preference for NADH. It has been suggested that yeast OYE protects the actin cytoskeleton from oxidative stress. There are indications that bacterial OYEs are also involved in the oxidative stress response, but their exact role is unclear. Induction studies in S. oneidensis revealed that yeast and bacterial OYEs may share a common physiological role, i.e. the protection of cellular components against oxidative damage. As only SYE4 was induced under oxidative stress conditions, however, a functional divergence between bacterial OYEs is likely to exist.

A cytosolic cytochrome b5-like protein in yeast cell accelerating the electron transfer from NADPH to cytochrome c catalyzed by Old Yellow Enzyme

A 410-nm absorbing species which enhanced the reduction rate of cytochrome c by Old Yellow Enzyme (OYE) with NADPH was found in Saccharomyces cerevisiae. It was solubilized together with OYE by the treatment of yeast cells with 10% ethyl acetate. The purified species showed visible absorption spectra in both oxidized and reduced forms, which were the same as those of the yeast microsomal cytochrome b5. At least 14 amino acid residues of the N-terminal region coincided with those of yeast microsomal b5, but the protein had a lower molecular weight determined to be 12,600 by SDS-PAGE and 9775 by mass spectrometry. The cytochrome b5-like protein enhanced the reduction rate of cytochrome c by OYE, and a plot of the reduction rates against its concentration showed a sigmoidal curve with an inflexion point at 6x10(-8) M of the protein.

The role of glutamine 114 in old yellow enzyme

Glutamine 114 of OYE1 is a well conserved residue in the active site of the Old Yellow Enzyme family. It forms hydrogen bonds to the O2 and N3 of the flavoprotein prosthetic group, FMN. Glutamine 114 was mutated to asparagine, introducing an R-group that is one methylene group shorter. The resultant enzyme was characterized to determine the effect of the mutation on the mechanistic behavior of the enzyme, and the crystal structure was solved to determine the effect of the mutation on the structure of the protein. The Q114N mutation results in little change in the protein structure, moving the amide group of residue 114 out of H-bonding distance, allowing repositioning of the FMN prosthetic group to form new interactions that replace the lost H-bonds. The mutation decreases the ability to bind ligands, as all dissociation constants for substituted phenols are larger than for the wild type enzyme. The rate constant for the reductive half-reaction with beta-NADPH is slightly greater, whereas that for the oxidative half-reaction with 2-cyclohexenone is smaller than for the wild type enzyme. Oxidation with molecular oxygen is biphasic and involves formation and reaction with O(2), a phenomenon that is more pronounced with this mutation than with wild type enzyme. When superoxide dismutase is added to the reaction, we observe a single-phase reaction typical of the wild type enzyme. Turnover reactions using beta-NADPH with 2-cyclohexenone and molecular oxygen were studied to further characterize the mutant enzyme.

Old yellow enzyme: reduction of nitrate esters, glycerin trinitrate, and propylene 1,2-dinitrate

The reaction of the old yellow enzyme and reduced flavins with organic nitrate esters has been studied. Reduced flavins have been found to react readily with glycerin trinitrate (GTN ) (nitroglycerin) and propylene dinitrate, with rate constants at pH 7.0, 25 degrees C of 145 M(-1)s(-1) and 5.8 M(-1)s(-1), respectively. With GTN, the secondary nitrate was removed reductively 6 times faster than the primary nitrate, with liberation of nitrite. With propylene dinitrate, on the other hand, the primary nitrate residue was 3 times more reactive than the secondary residue. In the old yellow enzyme-catalyzed NADPH-dependent reduction of GTN and propylene dinitrate, ping-pong kinetics are displayed, as found for all other substrates of the enzyme. Rapid-reaction studies of mixing reduced enzyme with the nitrate esters show that a reduced enzyme--substrate complex is formed before oxidation of the reduced flavin. The rate constants for these reactions and the apparent K(d) values of the enzyme--substrate complexes have been determined and reveal that the rate-limiting step in catalysis is reduction of the enzyme by NADPH. Analysis of the products reveal that with the enzyme-catalyzed reactions, reduction of the primary nitrate in both GTN and propylene dinitrate is favored by comparison with the free-flavin reactions. This preferential positional reactivity can be rationalized by modeling of the substrates into the known crystal structure of the enzyme. In contrast to the facile reaction of free reduced flavins with GTN, reduced 5-deazaflavins have been found to react some 4--5 orders of magnitude slower. This finding implies that the chemical mechanism of the reaction is one involving radical transfers.

Old yellow enzyme: stepwise reduction of nitro-olefins and catalysis of aci-nitro tautomerization

The Old Yellow Enzyme has been shown to catalyze efficiently the NADPH-linked reduction of nitro-olefins. The reduction of the nitro-olefin proceeds in a stepwise fashion, with formation of a nitronate intermediate that is freely dissociable from the enzyme. The first step involves hydride transfer from the enzyme-reduced flavin to carbon 2 of the nitro-olefin. The protonation of the nitronate at carbon 1 to form the final nitroalkane product also is catalyzed by the enzyme and involves Tyr-196 as an active site acid/base. This residue also is involved in aci-nitro tautomerization of nitroalkanes, the first example of a nonredox reaction catalyzed by the enzyme.

A homolog of old yellow enzyme in tomato. Spectral properties and substrate specificity of the recombinant protein

A cDNA was isolated and characterized from a tomato shoot cDNA library, the deduced amino acid sequence of which exhibited similarity with yeast Old Yellow Enzymes (OYEs) and related enzymes of bacterial and plant origin. Sequence identity was particularly high with 12-oxophytodienoate 10,11-reductase (OPR) from Arabidopsis thaliana. The cDNA-encoded protein was expressed as a glutathione S-transferase fusion protein in Escherichia coli and was purified from bacterial extracts. The protein was found to be a flavoprotein catalyzing the NADPH-dependent reduction of the olefinic bond of alpha,beta-unsaturated carbonyl compounds, including 12-oxophytodienoic acid. Thus, the tomato enzyme was termed LeOPR. The catalytic efficiency of LeOPR was highest with N-ethylmaleimide followed by 12-oxophytodienoic acid and maleic acid as substrates. Photoreduction of the LeOPR-bound FMN resulted in the formation of a red, anionic semiquinone prior to the formation of the fully reduced flavin dihydroquinone. Spectroscopic characterization of LeOPR revealed the formation of charge transfer complexes upon titration with para-substituted phenolic compounds, a distinctive feature of the enzymes of the OYE family. The ligand binding properties were compared between LeOPR and OYE, and the findings are discussed with respect to structural differences between the active sites of OYE and LeOPR.

The role of threonine 37 in flavin reactivity of the old yellow enzyme

Threonine 37 is conserved among all the members of the old yellow enzyme (OYE) family. The hydroxyl group of this residue forms a hydrogen bond with the C-4 oxygen atom of the FMN reaction center of the enzyme [Fox, K. M. & Karplus, P. A. (1994) Structure 2, 1089-1105]. The position of Thr-37 and its interaction with flavin allow for speculations about its role in enzyme activity. This residue was mutated to alanine and the mutant enzyme was studied and compared with the wild-type OYE1 to evaluate its mechanistic function. The mutation has different effects on the two separate half-reactions of the enzyme. The mutant enzyme has enhanced activity in the oxidative half-reaction but the reductive half-reaction is slowed down by more than one order of magnitude. The peaks of the absorption spectra for enzyme bound with phenolic compounds are shifted toward shorter wavelengths than those of wild-type OYE1, consistent with its lower redox potential. It is suggested that Thr-37 in the wild-type OYE1 increases the redox potential of the enzyme by stabilizing the negative charge of the reduced flavin through hydrogen bonding with it.

On the active site of Old Yellow Enzyme. Role of histidine 191 and asparagine 194

Old Yellow Enzyme (OYE) binds phenolic ligands forming long wavelength (500-800 nm) charge-transfer complexes. The enzyme is reduced by NADPH, and oxygen, quinones, and alpha,beta-unsaturated aldehydes and ketones can act as electron acceptors to complete catalytic turnover. Solution of the crystal structure of OYE1 from brewer's bottom yeast (Fox, K. M., and Karplus, P. A. (1994) Structure 2, 1089-1105) made it possible to identify histidine 191 and asparagine 194 as amino acid residues that hydrogen-bond with the phenolic ligands, stabilizing the anionic form involved in charge-transfer interaction with the FMN prosthetic group. His-191 and Asn-194 are also predicted to interact with the nicotinamide ring of NADPH in the active site. Mutations of His-191 to Asn, Asn-194 to His, and a double mutation, H191N/N194H, were made of OYE1. It was not possible to isolate the N191H mutant enzyme, but the other two mutant forms had the expected effect on phenolic ligand binding, i.e. decreased binding affinity and decreased charge-transfer absorbance. Reduction of the H191N mutant enzyme by NADPH was similar to that of OYE1, but the reduction rate constant for NADH was greatly decreased. The double mutant enzyme had an increased rate constant for reduction by NADPH, but the reduction rate constant with NADH was lower by a factor of 15. The reactivity of OYE1 and the mutant enzymes with oxygen was similar, but the reactivity of 2-cyclohexenone was greatly decreased by the mutations. The crystal structures of the two mutant forms showed only minor changes from that of the wild type enzyme.

The oxidative half-reaction of Old Yellow Enzyme. The role of tyrosine 196

Tyrosine 196 in Old Yellow Enzyme (OYE) was mutated to phenylalanine, and the resulting mutant enzyme was characterized to evaluate the mechanistic role of the residue. The residue demonstrates little effect on ligand binding and the reductive half-reaction, but a dramatic slowing by nearly 6 orders of magnitude of its oxidative half-reaction with 2-cyclohexenone. Observation of the oxidative half-reaction with a series of substrates allows us to propose a model describing the mechanism of the oxidative half-reaction. In addition, the curtailed reactivity with enones allows for characterization of the manner in which reduced enzyme primes the substrate for the redox reaction by observation of the Michaelis complex with reduced enzyme bound to substrate.

12-Oxophytodienoate-10,11-reductase: occurrence of two isoenzymes of different specificity against stereoisomers of 12-oxophytodienoic acid

The reduction of 12-oxophytodienoic acid (OPDA) to 3-oxo-2(2'[Z]-pentenyl)-cyclopentane-1-octanoic acid is catalyzed by 12-oxophytodienoate-10,11-reductase (OPR). Analysis of the isomer preference of OPR has indicated that the activity is composed of two isoenzymes exhibiting different stereoselectivities. The two isoforms of OPR have been separated, using protein extracts of Rock Harlequin (Corydalis sempervirens) as the starting material. OPRI, the enzyme reported earlier from the same species and corresponding to the cloned OPR from Arabidopsis, utilized 9R,13R-OPDA >> 9S, 13R-OPDA but not the 13S-configured isomers, whereas the new activity, OPRII, effectively reduced all four OPDA isomers, including the natural 9S,13S-OPDA (cis-[+]-OPDA). OPRII activity is characterized in detail. The enzyme's enzymatic, biochemical, and immunological properties prove that it is a close relative of OPRI. The roles of OPRI and OPRII in octadecanoid biology are discussed.

Synthesis and properties of 8-CN-flavin nucleotide analogs and studies with flavoproteins

A high potential analog of riboflavin with a cyano function at the 8-position was synthesized by employing novel reaction conditions, starting from 8-amino-riboflavin. This was converted to the FAD level with FAD synthetase. The reduced 8-CN-riboflavin, unlike normal reduced flavin, has a distinctive absorption spectrum with two distinctive peaks in the near ultraviolet region. The oxidation-reduction potential of the new flavin was determined to be -50 mV, approximately 160 mV more positive than that of normal riboflavin. The 8-CN-riboflavin and 8-CN-FMN were found to be photoreactive and need to be protected from exposure to light. However such complications were not encountered with protein-bound flavins. The apoproteins of flavodoxin and Old Yellow Enzyme (OYE) were reconstituted with the 8-CN-FMN and apoDAAO was reconstituted with 8-CN-FAD. Spectral properties of the enzyme-bound neutral and anionic semiquinones were determined from these reconstituted proteins. In the case of 8-CN-FMN-OYE I, it was shown that the comproportionation reaction of a mixture of reduced and oxidized enzyme bound flavin is very rapid, compared with the same reaction with native protein, resulting in approximately 100% thermodynamically stable anionic semiquinone. In the case of 8-CN-OYE I, it was shown that the rate of reduction of the enzyme bound flavin by NADPH is approximately 40 times faster, and the rate of reoxidation of reduced enzyme bound flavin by oxygen is an order of magnitude slower than with the normal FMN enzyme. This is in accord with the high oxidation-reduction potential of the flavin, which thermodynamically stabilizes the reduced enzyme.

Molecular cloning and characterization of 12-oxophytodienoate reductase, an enzyme of the octadecanoid signaling pathway from Arabidopsis thaliana. Structural and functional relationship to yeast old yellow enzyme

Using partial amino acid sequence information for 12-oxophytodienoate-10,11-reductase obtained from Corydalis sempervirens we have cloned the homologous enzyme from Arabidopsis thaliana. The open reading frame of the cDNA encodes a polypeptide of 372 amino acids (Mr = 41,165) with significant similarity to the sequence of Old Yellow Enzyme from Saccharomyces carlsbergensis (Saito, K., Thiele, D. J., Davio, M., Lockridge, O., and Massey, V. (1991) J. Biol. Chem. 266, 20720-20724), a flavin (FMN)-protein catalyzing the NADPH-dependent reduction of the olefinic bond of alpha,beta-unsaturated carbonyls. Specifically, all residues required for binding of FMN in Old Yellow Enzyme are conserved in the A. thaliana sequence, as are all residues associated with catalytic activity. The enzyme was functionally expressed from its cDNA in Escherichia coli and thus proven to encode OPDA reductase. Further similarities of OPDA reductase and yeast Old Yellow Enzyme include their binding to and elution by reductant from N-(4-hydroxybenzoyl)aminohexyl-Sepharose, the immunoreactivity of yeast Old Yellow Enzyme with an antiserum raised against plant OPDA reductase and the demonstration that Old Yellow Enzyme is an active OPDA reductase. It is thus conceivable that the physiological role of Old Yellow Enzymes now known from bacteria, yeasts, and higher plants, is in oxylipin metabolism.

Serratia liquefaciens as a new host superior for overproduction and purification using the N-acetylneuraminate lyase gene of Escherichia coli

Serratia liquefaciens was screened as a host strain for effective gene expression and easy purification of the target protein. A model gene, N-acetylneuraminate lyase gene (nanA), fused with the promoter region of Escherichia coli lac operon successfully overproduced the protein independently from the inducer. Since S. liquefaciens grew at lower temperature than E. coli and its proteins were more heat sensitive than those of E. coli, simple incubation at 60 degrees C could inactivate most enzymes but the nanA protein. Subsequent column works for purification, then, became simple and rapid.

19F NMR studies with 2'-F-2'-deoxyarabinoflavoproteins

Apoproteins of several flavoproteins were reconstituted with 2'-F-2'-deoxyarabinoflavins and studied by 19F NMR and absorption spectroscopy. Extensive protein-fluorine interactions were observed by large chemical shift changes on binding to the apoprotein of Old Yellow Enzyme (apoOYE) and apoflavodoxin, whereas binding to apoglucose oxidase and apo -amino acid oxidase (apoDAAO) resulted in minimal interactions. Modification at the flavin 2'-position in OYE resulted in a substantial decrease in the binding affinity of the flavin, possibly from the disruption of two important hydrogen bonds to Pro-35 and Arg-243. 19F NMR studies of complexes of OYE with testosterone, cyclohexenone, and beta-estradiol suggest that phenols and alpha,beta-unsaturated ketones orient differently at the active site on binding. The two separate one-electron potentials for the EFlox/EFlsq and EFlsq/EFlred couples were different for the reconstituted OYE. With native enzyme, there is 15-20% thermodynamic stabilization of the anionic flavin semiquinone, while no detectable amount of semiquinone was observed with modified OYE. This change in potential was further substantiated by blue shifts for the maxima of the modified protein-phenol charge transfer complexes. In accordance with the crystal structure of the OYE-p-OH-benzaldehyde complex (Fox, K.M. & Karplus, P.A. (1994) Structure 2, 1089-1105), 19F NMR studies with the modified OYE-2,4-F2-phenol suggest strong interaction between the para-fluorine of the phenol and Tyr-375.

A new old yellow enzyme of Saccharomyces cerevisiae

In 1993, the first gene of Old Yellow Enzyme (OYE) of Saccharomyces cerevisiae was cloned (Stott, K., Saito, K., Thiele, D. J., and Massey, V. (1993) J. Biol. Chem. 268, 6097-6106) and named OYE2 to distinguish it from the first OYE gene cloned from Saccharomyces carlsbergenesis (Saito, K., Thiele, D. J., Davio, M., Lockridge, O., and Massey, V. (1991) J. Biol. Chem. 266, 20720-20724). The analysis of an OYE2 deletion mutant suggested that S. cerevisiae had at least two OYE genes. In the present study, we cloned a new OYE species named OYE3 and analyzed the OYE3 protein expressed in Escherichia coli. OYE3 consists of 400 amino acid residues and its molecular mass calculated by electrospray mass spectrometry is 44,788 daltons, in good agreement with the value of 44,920 daltons predicted from the amino acid sequence derived from the DNA sequence. In the downstream region of the OYE3 gene, the cytochrome oxidase (COX10) gene exists with a 426-base pair intermediate sequence. Some of the physicochemical and kinetic properties of OYE2 and OYE3 have been determined. Although the two enzymes are clearly closely related, they show differences in ligand binding properties and in their catalytic activities with oxygen and cyclohexen-2-one as acceptors.

19F NMR studies on 8-fluoroflavins and 8-fluoro flavoproteins

The 19F NMR spectra of the oxidized and reduced forms of 8-fluororiboflavin, 8-fluoro-FAD, and the 8-fluoroflavin-reconstituted flavoproteins flavodoxin, riboflavin binding protein, D-amino acid oxidase, p-hydroxybenzoate hydroxylase, Old Yellow Enzyme, anthranilate hydroxylase, general acyl-CoA dehydrogenase, glucose oxidase, and L-lactate oxidase were measured. For the proteins studied the oxidized resonances appeared over a 10.1-ppm range, while the reduced resonances were spread over 10.3 ppm. Reduction caused an upfield shift of about 27 ppm for the free 8-fluoroflavins and most of the 8-fluoro flavoproteins. The notable exception was 8-fluoro-FMN flavodoxin, which was shifted 37.6 ppm, indicating an unusually high electron density in the benzene ring. Ligand binding to the oxidized 8-fluoro flavoproteins caused either upfield or downfield shifts of 1.5-5 ppm, depending on the protein/ligand combination. The 8-fluoro-FAD anthranilate hydroxylase resonance was shifted downfield and split into two peaks in the presence of anthranilate. The 8-fluoro-FMN Old Yellow Enzyme resonance was shifted upfield upon complexation with charge-transfer-forming, para-substituted phenolates. The upfield shift increased from less than 1 to 5 ppm as the electron-donating capacity of the phenolate increased. Complexation of native Old Yellow Enzyme with 2,4-difluorophenol caused the fluorine resonances of the ligand to shift and split into two pairs of signals. Each pair of signals was associated with a different isozyme of Old Yellow Enzyme.

Autocatalytic quinone methide formation from mitomycin c

Mitomycin c in the presence of NADPH and brewers' yeast NADPH: (acceptor) oxidoreductase (Old Yellow enzyme, EC 1.6.99.1) is transformed, at pH 8.0 and with anaerobicity, to two major mitosene products (the cis- and trans-1-hydroxy-2,7-diaminomitosenes; respective yields of 45 and 30%). These arise by covalent trapping by solvent of a quinone methide intermediate, obtained by rearrangement of the mitomycin c hydroquinone. At lower pH (6.5), the major product of this reaction is 2,7-diaminomitosene, which arises by covalent trapping of the quinone methide by H+. In the former instance the quinone methide acts as an electrophile and in the latter as a nucleophile. A detailed kinetic analysis indicates that the role of the NADPH and Old Yellow enzyme is to initiate an autocatalytic reaction, propagated by mitomycin c reduction by the mitosene hydroquinones (arising by the electrophilic pathway). The evidence supporting this conclusion is the formation of oxidized mitosene products, under the rigorously anaerobic reaction circumstance, the nonstoichiometric participation of NADPH, a dependence of the velocity on the total mitomycin c concentration, the pH dependence of the reaction, and the accurate simulation of the overall kinetic course with a mathematical model of the autocatalytic pathway. These observations indicate that the autocatalytic pathway may be dominant during in vitro mitomycin c anaerobic reductive activation by other reducing agents and that (as with anthracycline reductive activation) oxidation of the mitosene hydroquinone obtained from nucleophile addition to the quinone methide may be a necessary event for the formation of stable covalent adducts in vivo.

Nuclear magnetic resonance studies of the old yellow enzyme. 2. 13C NMR of the enzyme recombined with 13C-labeled flavin mononucleotides

The apoenzyme of NADPH oxidoreductase, 'old yellow enzyme', was reconstituted with selectively 13C-enriched flavin mononucleotides and investigated by 13C NMR spectroscopy. The 13C NMR results confirm the results obtained by 15N NMR spectroscopy and yield additional information about the coenzyme-apoenzyme interaction. A strong deshielding of the C(2) and C(4) atoms of enzyme-bound FMN both in the oxidized and reduced state is observed, which is supposed to be induced by hydrogen-bond formation between the protein and the two carbonyl groups at C(2) and C(4) of the isoalloxazine ring system. The chemical shifts of all 13C resonances of the flavin in the two-electron-reduced state indicate that the N(5) atom is sp3-hybridized. From 31P NMR measurements it is concluded that the FMN phosphate group is not accessible to bulk solvent. The unusual 31P chemical shift of FMN in old yellow enzyme seems to indicate a different binding mode of the FMN phosphate group in this enzyme as compared to the flavodoxins. The 13C and 15N NMR data on the old-yellow-enzyme--phenolate complexes show that the atoms of the phenolate are more deshielded whereas the atoms of the enzyme-bound isoalloxazine ring are more shielded upon complexation. A non-linear correlation exists between the chemical shifts of the N(5) and the N(10) atoms and the pKa value of the phenolate derivative bound to the protein. Since the chemical shifts of N(5), N(10) and C(4a) are influenced most on complexation it is suggested that the phenolate is bound near the pyrazine ring of the isoalloxazine system. 15N NMR studies on the complex between FMN and 2-aminobenzoic acid indicate that the structure of this complex differs from that of the old-yellow-enzyme--phenolate complexes.

Nuclear magnetic resonance studies of the old yellow enzyme. 1. 15N NMR of the enzyme recombined with 15N-labeled flavin mononucleotides

The apoenzyme of NADPH oxidoreductase, 'old yellow enzyme', was reconstituted with specifically 15N-labeled flavin mononucleotide and investigated by 15N NMR spectroscopy in the oxidized and reduced state. The results indicate that in the oxidized state a hydrogen bond is formed between the N(5) atom and the apoprotein. In addition, hydrogen bonds exist between the N(1) and N(3) atoms of FMN and the apoprotein. The resonance position of N(10) indicates that this atom is somewhat sp3-hybridized, i.e. lifted out of the molecular plane of the isoalloxazine ring system. In the reduced state the N(1) atom is negatively charged and the N(3) atom forms a hydrogen bond with the apoprotein. The N(10) atom in protein-bound FMN exhibits about the same hybridization state as in free anionic reduced FMN, i.e. it is located in the plane of the isoalloxazine ring. The chemical shift of the N(5) resonance indicates that this atom is almost completely sp3-hybridized. This interpretation can also be derived from the 15N(5)-1H coupling constant. Among the flavoproteins thus far studied by NMR techniques, old yellow enzyme is the only protein that shows a conformation of the reduced prosthetic group with the N(5) atom lifted out of the molecular plane. The isoelectric focussing properties of old yellow enzyme and a new easy method for the preparation of the apoprotein are also reported.

Reaction mechanism of flavin-dependent hydroxylation. Evolution of a non-imitable enzyme

Analysis of the partially refined X-ray crystallographic structure of the active site of p-hydroxybenzoate hydroxylase shows clearly that the enzyme is complementary to a flavin C4,C4a-dioxetane derivative. A mechanism is proposed based on such an intermediate as the oxenoid transferring agent. A non-enzymatic analogue of this reaction is not known in organic chemistry. A possible evolutionary pathway for such a non-imitable enzyme is discussed.

On the enigma of old yellow enzyme's spectral properties

Old yellow enzyme (NADPH oxidoreductase) in the free and complexed state was thoroughly investigated by the following techniques: absorption, circular dichroism, fluorescence/phosphorescence and electron paramagnetic resonance spectroscopy and fluorescence/phosphorescence decay measurements, applied over a wide range of temperature (7-293K). The data obtained were interpreted by comparison with results from similar measurements on free FMN, existing spectral data on isoalloxazine model systems and theoretical data. The results clearly demonstrate the inadequacy of a simple phenolate-FMN donor-acceptor charge-transfer complex to explain the phenomena occurring upon the addition of phenols to old yellow enzyme. Instead it was found that the phenolate anion interferes strongly with an existing tight complex between FMN and the apoprotein, probably an H-bonded structure in which FMN is tautomerized and interacts with an L-chiral center. This is concluded from a separate electronic transition with an origin at 496 nm, thus far not recognized as such, and the circular dichroism observed. The emission is dominated by that of free FMN, although protein-bound FMN seems also to become luminescent in glassy solution at 143 K. A second fluorescence/phosphorescence emission appears upon excitation of both native and complexed old yellow enzyme in the ultraviolet. This emission is quenched by the addition of phenol to the enzyme, shows a large (3000-cm-1) blue shift on going to a low-temperature glass and is tentatively assigned to excimers of nucleic acids. Long-wavelength excitation with a synchronously pumped, mode-locked Rhodamine 6-G dye laser revealed a third, extremely weak emission in both native old yellow enzyme and its complexes. It decays with a lifetime of about 3 ns at 143 K. Electron paramagnetic resonance spectra revealed the presence of a low amount of an unpaired spin in old yellow enzyme. Owing to an unusual relaxational behaviour it could only be observed below 15 K and, again, the signal was measured in both the native enzyme and its complexes. Possible assignment and consequences of this observation are discussed. In frozen aqueous solutions of the enzyme-phenolate complex, a phase transition was discovered at which the colour of the complex reverted to that of the native enzyme. Subsequent melting restored the original colour. The observed phenomena and existing literature data lead to the conclusion that the only model from which no apparent inconsistencies emerge is that of a very complicated network of hydrogen-bonded structures in the protein. These involve several, partly unknown, chromophores. Phenols interfere with this network, leading to the formation of the long-wavelength absorption band in old yellow enzyme.

Resonance Raman spectra for flavin derivatives modified in the 8 position

Spontaneous resonance Raman or resonance-enhanced AC-coupled inverse Raman spectra were obtained for 8-mercaptoriboflavin, 8-(dimethylamino) riboflavin, 8-hydroxyriboflavin, and 8-aminoriboflavin all free in solution. These Raman spectra were all similar to one another and markedly different from that of riboflavin. In addition, the Raman spectra of 8-mercaptoflavin bound to the apoproteins of old yellow enzyme, glucose oxidase, and L-lactate oxidase were determined. The Raman spectra of these protein-bound flavins were distinctly different from both those of the above 8-substituted riboflavins and that of riboflavin. An argument is presented in favor of assigning a quinonoid electronic structure to these protein-bound flavins.