Interactions of flavins with amino acid residues: assessments from spectral and photochemical studies

Equivalence between inverted regions of the energy gap law and inverted regions of donor–acceptor distances in photoinduced electron transfer processes in flavoproteins

In the present work, we discuss about the relationship between the energy gap law and extended Dutton law in flavoproteins. The extend Dutton law is defined herein as the dependence of logarithmic rates (ln Rate) of photoinduced electron transfer (ET) from aromatic amino acids to excited isoalloxazine (Iso*) on donor–acceptor distances (Rcs). Both functions of ln Rate vs. negative values of the standard free energy gap and ln Rate vs. Rc display a parabolic behavior, when the ET rates are ultrafast. The negative values of the standard free energy gap at peaks of ln Rate [X_m(ES)] were obtained for FMN-binding protein, wild-type pyranose 2-oxidase, T169S (Thr169 is replaced by Ser) pyranose 2-oxidase, and medium-chain acyl-CoA dehydrogenase. The values of Rc at peaks of ln Rate [X_m(Rc)] were also obtained for these flavoproteins. The negative values of the standard free energy gap decreased with approximate linear functions of Rc. The negative values of standard free energy gap [X_m(ESRc)] at Rc = X_m(Rc) were evaluated using the linear functions of the negative standard free energy gap with Rc. The values of X_m(ESRc) were mostly in very good agreement with the values of X_m(ES). This implies that the energy gap law and the extend Dutton law are equivalent. X_m(ES) values in ET donors displaying the linear extend Dutton law with Rc were obtained by energy gap law, and then X_m(Rc) values were evaluated with the negative standard free energy gap. Thus, the obtained X_m(Rc) values were much smaller than the Rc range obtained by the method of molecular dynamics simulation. This suggests that ET processes with linear profiles of the extend Dutton law could be parabolic when Rc becomes much shorter than the Rc range obtained by the method of molecular dynamics simulation.

Relationship between EXDL and SEGL.

Fluorescence Properties of Flavin Semiquinone Radicals in Nitronate Monooxygenase

Fluorescent cofactors like flavins can be exploited to probe their local environment with spatial and temporal resolution. Although the fluorescence properties of the oxidized and two-electron-reduced states of flavins have been studied extensively, this is not the case for the one-electron-reduced state. Both the neutral and anionic semiquinones have proven particularly challenging to examine, as they are unstable in solution and are transient, short-lived species in many catalytic cycles. Here, we report that the nitronate monooxygenase (NMO) from Pseudomonas aeruginosa PAO1 is capable of stabilizing both semiquinone forms anaerobically for hours, thus enabling us to study their spectroscopy in a constant protein environment. We found that in the active site of NMO, the anionic semiquinone exhibits no fluorescence, whereas the neutral semiquinone radical shows a relatively strong fluorescence, with a behavior that violates the Kasha-Vavilov rule. These fluorescence properties are discussed in the context of time-dependent density functional theory calculations, which reveal low-lying dark states in both systems.

Sensitive fluorescence-based detection of magnetic field effects in photoreactions of flavins

Magnetic field effect studies have been conducted on a variety of flavin-based radical pair systems chosen to model the magnetosensitivity of the photoinduced radical pairs found in cryptochrome flavoproteins. Cryptochromes are blue-light photoreceptor proteins which are thought to mediate avian magnetoreception, an hypothesis supported by recent in vitro observations of magnetic field-dependent reaction kinetics for a light-induced radical pair in a cryptochrome from the plant Arabidopsis thaliana. Many cryptochromes are difficult to express in large quantities or high concentrations and are easily photodegraded. Magnetic field effects are typically measured by spectroscopic detection of the transient radical (pair) concentrations. Due to its low sensitivity, single-pass transient absorption spectroscopy can be of limited use in such experiments and much recent work has involved development of other methodologies offering improved sensitivity. Here we explore the use of flavin fluorescence as the magnetosensitive probe and demonstrate the exceptional sensitivity of this technique which allows the detection of magnetic field effects in flavin samples at sub-nanomolar concentrations and in cryptochromes.

Enhancement of photophysical and photosensitizing properties of flavin adenine dinucleotide by mutagenesis of the C-terminal extension of a bacterial flavodoxin reductase

The role of the mobile C-terminal extension present in Rhodobacter capsulatus ferredoxin-NADP(H) reductase (RcFPR) was evaluated using steady-state and dynamic spectroscopies for both intrinsic Trp and FAD in a series of mutants in the absence of NADP(H). Deletion of the six C-terminal amino acids beyond Ala266 was combined with the replacement A266Y to emulate the structure of plastidic reductases. Our results show that these modifications of the wild-type RcFPR produce subtle global conformational changes, but strongly reduce the local rigidity of the FAD-binding pocket, exposing the isoalloxazine ring to the solvent. Thus, the ultrafast charge-transfer quenching of (1) FAD* by the conserved Tyr66 residue was absent in the mutant series, producing enhancement of the excited singlet- and triplet-state properties of FAD. This work highlights the delicate balance of the specific interactions between FAD and the surrounding amino acids, and how the functionality and/or photostability of redox flavoproteins can be modified.

Classification of the mechanisms of photoinduced electron transfer from aromatic amino acids to the excited flavins in flavoproteins

Emission wavelength-dependence of the relationship between logarithmic ET rate vs. donor–acceptor distance in pyranose 2-oxidase.

Photoinduced electron transfer modeling to simulate flavoprotein fluorescence decay

A method of analysis is described on the photoinduced electron transfer (PET) from aromatic amino acids as tryptophans (Trp) and tyrosines (Tyr) to the excited isoalloxazine (Iso*) in FMN-binding proteins (FBP) from Desulfovibrio vulgaris (strain, Miyazaki F). Time-dependent geometrical factors as the donor-acceptor distances are determined by means of a molecular dynamics simulation (MDS) of the proteins. Fluorescence decays of the single mutated isoforms of FBP are used as experimental data. The electrostatic (ES) energy between the photoproducts and ionic groups in the proteins is introduced into the Kakitani and Mataga (KM) model, which is modeled for an electron transfer process in solution. The PET parameters contained in the KM rate are determined by means of a nonlinear least square method, according to the Marquardt algorithm. The agreement between the observed and calculated decays is quite good, but not optimal. Characteristics on PET in flavoproteins, obtained by the present method, are described. Possible improvements of the method are discussed.

Micronutrient cofactor research with extensions to applications

Following identification of essential micronutrients, there has been a continuum of research aimed at revealing their absorption, transport, utilization as cofactors, and excretion and secretion. Among those cases that have received our attention are vitamin B6, riboflavin, biotin, lipoate, ascorbate, and certain metal ions. Circulatory transport and cellular uptake of the water-soluble vitamins exhibit relative specificity and facilitated mechanisms at physiological concentrations. Isolation of enzymes and metabolites from micro–organisms and mammals has provided information on pathways involved in cofactor formation and metabolism. Kinases catalysing phosphorylation of B6and riboflavin have a preference for Zn2+in stereospecific chelates with adenosine triphosphate. The synthetase for flavin adenine dinucleotide prefers Mg2+. The flavin mononucleotide-dependent oxidase that converts the 5′–phosphates of pyridoxine and of pyridoxamine to pyridoxal phosphate is a connection between B6and riboflavin and is a primary control point for conversion of B6to its coenzyme. Sequencing and cloning of a side–chain oxidase for riboflavin was achieved. Details on binding and function have been delineated for some cofactor systems, especially in several flavoproteins. There is both photochemical oxidation and oxidative catabolism of B6and riboflavin. Both biotin and lipoate undergo oxidation of their acid side chains with redox cleavage of the rings. Applications from our findings include the development of affinity absorbents, enhanced drug delivery, delineation of residues in biopolymer modification, pathogen photoinactivation in blood components, and input into human dietary recommendations. Ongoing and future research in the cofactor arena can be expected to add to this panoply. At the molecular level, the way in which the same cofactor can participate in diverse catalytic reactions resides in interactions with surrounding enzyme structures that must be determined case by case. At the level of human intake, more knowledge is desirable for making micronutrient recommendations based on biochemical indicators, especially for the span between infancy and adulthood.

Photo-CIDNP NMR spectroscopy of amino acids and proteins

Photo-chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance (NMR) phenomenon which, among other things, is exploited to extract information on biomolecular structure via probing solvent-accessibilities of tryptophan (Trp), tyrosine (Tyr), and histidine (His) amino acid side chains both in polypeptides and proteins in solution. The effect, normally triggered by a (laser) light-induced photochemical reaction in situ, yields both positive and/or negative signal enhancements in the resulting NMR spectra which reflect the solvent exposure of these residues both in equilibrium and during structural transformations in "real time". As such, the method can offer - qualitatively and, to a certain extent, quantitatively - residue-specific structural and kinetic information on both the native and, in particular, the non-native states of proteins which, often, is not readily available from more routine NMR techniques. In this review, basic experimental procedures of the photo-CIDNP technique as applied to amino acids and proteins are discussed, recent improvements to the method highlighted, and future perspectives presented. First, the basic principles of the phenomenon based on the theory of the radical pair mechanism (RPM) are outlined. Second, a description of standard photo-CIDNP applications is given and it is shown how the effect can be exploited to extract residue-specific structural information on the conformational space sampled by unfolded or partially folded proteins on their "path" to the natively folded form. Last, recent methodological advances in the field are highlighted, modern applications of photo-CIDNP in the context of biological NMR evaluated, and an outlook into future perspectives of the method is given.

Reverse structural genomics: an unusual flavin-binding site in a putative protease from Bacteroides thetaiotaomicron

The structure of a putative protease from Bacteroides thetaiotaomicron features an unprecedented binding site for flavin mononucleotide. The flavin isoalloxazine ring is sandwiched between two tryptophan residues in the interface of the dimeric protein. We characterized the recombinant protein with regard to its affinity for naturally occurring flavin derivatives and several chemically modified flavin analogs. Dissociation constants were determined by isothermal titration calorimetry. The protein has high affinity to naturally occurring flavin derivatives, such as riboflavin, FMN, and FAD, as well as lumichrome, a photodegradation product of flavins. Similarly, chemically modified flavin analogs showed high affinity to the protein in the nanomolar range. Replacement of the tryptophan by phenylalanine gave rise to much weaker binding, whereas in the tryptophan to alanine variant, flavin binding was abolished. We propose that the protein is an unspecific scavenger of flavin compounds and may serve as a storage protein in vivo.

Donor−Acceptor Distance-Dependence of Photoinduced Electron-Transfer Rate in Flavoproteins

Ultrafast fluorescence quenching of flavin in flavodoxin from Megasphaera elsdenii was investigated by means of a fluorescence up-conversion method. Fluorescence lifetimes of flavodoxin from M. elsdenii were estimated to be tau(1) approximately 165 fs (0.97%) and tau(2) approximately 10 ps (0.03%). Correlation of photoinduced electron-transfer rates (k(ET)) with averaged distances (D(av)) between isoalloxazine and nearby tryptophan or tyrosine was examined and obtained an empirical equation of ln k(ET) vs D(av) by means of a nonlinear least-squares method using reported data together with flavodoxin from M. elsdenii. The values of D(av) were calculated from X-ray structures of the flavoproteins. The ln k(ET) was approximately linear at D(av) shorter than 7 A. The model free empirical equation was expressed as ln k(ET) = 29.7 + (-0.327 D(av) + 2.84 x 10(-5))/(0.698 - D(av)(2)). We also analyzed the observed values of ln k(ET) with Marcus theory, but could not obtain reasonable results. Our analysis suggests that the average distance, rather than the shortest (edge to edge) distance or interplanar angles between the aromatics rings, is the key factor in the process of the photoinduced electron transfer in these flavoproteins.

Syntheses, pi-stacking interactions and base-pairings of uracil pyridinium salts and uracilyl betaines with nucleobases

Reaction of 6-chlorouracil with 4-(dimethylamino)pyridine, 4-methylpyridine, and pyridin-4-yl-morpholine yielded pyridinium-substituted uracils as chlorides which were converted into pyridinium uracilates by deprotonation. These heterocyclic mesomeric betaines are cross-conjugated and thus possess separate cationic (pyridinium) and anionic (uracilate) moieties. Calculations and X-ray single crystal analyses were performed in order to characterize these systems and to compare the salts with the betaines. (1)H NMR experiments in D(2)O proved pi-interactions between the uracilyl betaines and adenine, adenosine, as well as adeninium. No pi-stacking interactions were detected between the betaines and guanosine. The acidic N8-H group of the uracil pyridinium salts caused acid-base reactions which were observed in parallel to pi-stacking interactions. Self-complementarity of the modified uracils was detected by (1)H NMR experiments in DMSO-d(6) and electrospray ionisation mass spectrometry (ESIMS). Ab initio calculations predicted base-pairings of the modified uracils with adeninium, cytosine, and guanine. Several geometries of hydrogen-bonded associates were calculated. Hoogsteen pairings between the uracil-4-(dimethylamino)pyridinium salt and adeninium, as well as associates between the corresponding betaine plus cytosine, and the betaine plus guanine were calculated, and the most stable conformations were determined. In the ESI mass spectra, prominent peaks of associates between the modified uracils and adeninium, cytosine, cytidine, guanosine and d(CpGp) were detected.

Hydrogen-bonding dynamics of free flavins in benzene and FAD in electron-transferring flavoprotein upon excitation

The dynamic natures of two hydrogen-bonding model systems, riboflavin tetrabutylate (RFTB)-trichloroacetic acid (TCA) and RFTB-phenol in benzene, and of electron-transferring flavoprotein (ETF) from pig kidney upon excitation of flavins was investigated by means of steady state and time-resolved fluorescence spectroscopy. In both model systems fluorescence intensities of RFTB decreased as TCA or phenol was added. The spectral characteristics of ETF under steady state excitation were quite similar to those of the RFTB-TCA system, but not to those of the RFTB-phenol system. The observed fluorescence decay curves of ETF fit well with the calculated decay curves with two lifetime components, as in the model systems. Averaged lifetime was 0.9 ns. The time-resolved fluorescence spectrum of ETF shifted toward longer wavelength with time after pulsed excitation, which was also observed in the RFTB-TCA system. In the RFTB-phenol system the emission spectrum did not shift at all with time. These results reveal that the dynamic nature of ETF can be ascribed to aliphatic hydrogen-bonding(s) of the isoalloxazine ring with surrounding amino acid(s). From the fluorescence characteristics of ETF in comparison with the model systems, human ETF and other flavoproteins, it was suggested that ETF from pig kidney does not contain Tyr-16 in the beta subunit, unlike human ETF.

Mechanisms of reactions of flavin mononucleotide triplet with aromatic amino acids

Chemical reactions between the photoexcited triplet state of flavin mononucleotide and the aromatic amino acids, N-acetyl tryptophan (TrpH), N-acetyl tyrosine (TyrOH), and N-acetyl histidine (HisH) in aqueous solution have been studied in the pH range 2-12. Across the whole pH range, the principal mechanism of reaction of both TrpH and TyrOH is shown to be electron transfer. For HisH, the mechanism and rate of the reaction depend on the protonation state of the reactants. In acidic conditions (pH < 4), reaction does not occur. At 4 < pH < 11, the reaction proceeds via hydrogen atom abstraction with a rate constant varying from 3.0 x 10(6) to 2.5 x 10(8) M(-1) s(-1). In extremely basic solution (pH > 12) the mechanism switches to electron transfer.

Advances in flavin and flavoprotein optical spectroscopy

Flavins and flavoproteins are versatile redox cofactors that can perform both one- and two-electron transfer. Because they are highly colored in all three oxidation states, optical spectroscopy has been exploited for decades to study these redox changes. This review summarizes the application of optical spectroscopies to flavins and flavoproteins since 1990. Special emphasis is placed on new techniques, such as Stark spectroscopy, as well as significant refinements in more well known techniques, such as resonance Raman spectroscopy and ultrafast spectroscopy.

A trail of research on cofactors: an odyssey with friends

Over the span of 40 y and with the participation of over 60 students and postdoctoral colleagues, my laboratory has been able to elucidate numerous aspects of cofactor metabolism and function. Findings have been on the absorption, transport, utilization and excretion of vitamin B-6, riboflavin, biotin, lipoate and ascorbate. Specificity studies on those trace but essential enzymes that catalyze conversion of such vitamins as B-6 and riboflavin to their functional coenzymes led to our development of "biochemically specific absorbents" that prototypically exemplified what later was called "affinity chromatography." Characterization of the purified kinases for B-6 and riboflavin revealed preference for Zn2+ with the eucaryotic enzymes and delimited effects of inhibitors that relate to drug action. Flavin adenine dinucleotide synthetase, separable from flavokinase in mammals, prefers Mg2+. Specifics for binding and function of flavocoenzymes were delineated for several flavoproteins. The flavin mononucleotide-dependent oxidase that converts the 5'-phosphates of pyridoxine and of pyridoxamine to pyridoxal phosphate is a connection between riboflavin and B-6 that we characterized in mechanistic detail and found to be the primary control point for conversion of B-6 to its coenzyme. Sequencing and cloning of a side-chain oxidase for riboflavin was achieved. Isolation and identification of metabolites of biotin and of lipoic acid, first from bacteria obtained by enrichment culture and then from mammals, provided seminal information on catabolic pathways involved, as have our other studies with flavin catabolites isolated from milk and urine.

Visible radiation effects on flavocytochrome b2 in dilute aqueous solution: a steady-state and laser flash photolysis study

Irradiation of flavocytochrome b2 by visible radiation at 450 nm in dilute aqueous solution is found to have a devastating effect not only on its activity but also on the important flavin mononucleotide (FMN) constituents. The active site and the substrate binding site are also found to be largely modified on exposure to visible radiation. This has a telling effect on the constituent aromatic amino acids, tryptophan and tyrosine, and therefore justifies the role of FMN as a very potent photosensitizer. Partial unfolding of the irradiated enzyme molecule is also observed. Damage is much greater in deaerated conditions, which indicates that molecular oxygen plays a protecting role in this particular system. The inactivation is mediated through rapid electron transfer from tryptophan and tyrosine to excited flavin, forming flavin semiquinone and tryptophanyl and tyrosinyl radicals, which in turn cause permanent damage at the molecular level.

Rat lens glycolysis after in vivo exposure to narrow band UV or blue light radiation

UV radiation and short wavelength visible light are known to damage various tissues in the eye. This paper investigates the effect on rat lens glycolysis after in vivo exposure with 90 kJ m-2 narrow band UV radiation (UVB, 300 nm) and 90 kJ m-2 blue light (435 nm) radiation. After exposure, all lenses were incubated in Medium 199. Samples of culture medium were withdrawn after 2, 4, 6 h and 5, 10, 20 h in two UVB studies and after 5, 10 and 20 h in a blue light study. Lactate is the major end product of lens glycolysis. Lactate was determined with a modified enzymatic-photometric method. Intralenticular lactate was determined in one UVB experiment. In the UVB experiments we found a lower lactate production in the exposed lenses 2-6 h after exposure. There was an accumulation of lactate inside UVB-exposed lenses after 6 h incubation compared with their contralateral lenses. No significant effect on lactate production was observed in the blue light experiment. CONCLUSIONS. UVB induced a reversible inhibition of glycolysis. UVB also induced an accumulation of lactate inside the lens. Blue light tended to increase glycolysis.

Properties and significance of a riboflavin-binding hexamerin in the hemolymph of Hyalophora cecropia

A riboflavin-binding hexamerin isolated from pupal hemolymph of Hyalophora cecropia has a native M(r) of 510,000, subunit M(r) of 85,000, and a 5% carbohydrate content. An intrachain cross-link was confirmed in protease limit digests. Ellman titration confirmed the presence of a sulfhydryl group, which is needed for this linkage. Though Cu2+ is known to promote the linkage, heavy metals were not detected in the isolate. Heat denaturation released ligand with the absorbency, fluorescence spectra, and chromatographic behavior of riboflavin. Binding resulted in substantial quenching of the fluorescence of both the isoalloxazine in riboflavin and of aromatic groups in the apoprotein. Kinetic analysis indicated a KD of 2.5 x 10(-7) M for riboflavin, 1.3 x 10(-7) M for lumiflavin, and greater than 1 x 10(-6) M for FMN and FAD. Over four moles of flavin were bound per mole of hexamerin. The amount of riboflavin in pupal hemolymph is sufficient to occupy only 2-3 of these sites. Riboflavin is also associated with lipophorin and vitellogenin, but the molar ratios after protein isolation were low. On a standard laboratory diet, riboflavin is in great excess, but most of it is apparently excreted before the apoprotein first appears in the hemolymph, just before wandering. The concentration of riboflavin-binding hexamerin rises to 15-30 mg/ml in pupae; relative to other hexamerins, very little is stored in the fat body. All of the apoprotein and 75% of riboflavin disappear from the hemolymph during adult development. An amount of flavin at least equal to that stored in pupal hemolymph is transferred to the eggs formed during this period.

Interactions of flavins with melanin. Studies on equilibrium binding of riboflavin to dopa-melanin and some spectroscopic characteristics of flavin-melanin complex

Natural melanins are photoprotective pigments that in mammals are principally found in the skin, hair, and eyes. Although the molecular mechanism of photoprotection of pigmented cells has not yet been established, several hypotheses have been proposed with melanin acting as a light filter, free radical scavenger, and quencher of electronically excited states of reactive intermediates. It can be expected that the detoxicating efficiency of melanin should be enhanced if the melanin and potentially cytotoxic species are brought close together. Such a situation may occur for a number of photosensitizing dyes that have the ability to bind to melanin. The interaction of melanin with flavins has been studied under strictly controlled experimental conditions. The equilibrium dialysis method has been employed to determine dissociation constants and the number of binding sites in melanin at pH 5-9. The data reveal that synthetic DOPA-melanin has two different classes of binding sites with dissociation constants of 10(-6) and 10(-5) M, respectively. The overall binding capacity of melanin, at pH 7, is 250 nmol RF/mg melanin. The amount of bound-to-melanin RF increases with pH. The absorption spectra of melanin complexes with RF and lumiflavin indicate that hydrophobic interaction may be involved in the binding of these flavins by melanin. No changes in flavin fluorescence have been detected after binding of flavin to melanin. It appears that, contrary to cationic photosensitizing dyes, the singlet excited state of flavin molecules is not quenched by melanin.

Investigations on the fluorescence concentration quenching of flavomononucleotide in glycerine-water solutions

We measured concentration changes of the fluorescence quantum yield eta/eta 0 of flavomononucleotide (FMN) in glycerine-water solutions of various viscosities. The results obtained show that FMN dimers are traps for the exciting light energy as well as the energy transferred non-radiatively. Very good agreement between the experimental and theoretical eta/eta 0 results was obtained. It has been shown that in the investigated solutions non-radiative energy transfer from monomers to dimers takes place, preceded by the migration of energy. It has been stated that no monomer quenching occurs in these solutions. The contribution of the individual fluorescence concentration quenching mechanisms has been determined.

Reactivities of tyrosine, histidine, tryptophan, and methionine in radical pair formation in flavin triplet induced protein nuclear magnetic polarization

In order to check the validity of several basic assumptions of protein photochemically induced nuclear polarization (protein photo-CIDNP), we have investigated the quenching processes of the dye triplets by the side chains of tyrosine, histidine, and tryptophan in a variety of molecular systems and environments. The quenching (H atom or electron transfer) is the generating process of the triplet electron-spin-correlated radical pair, the evolution of which gives rise to nuclear polarization. At pH 7 the quenching of 10-(carboxyethyl)flavin triplets by tyrosine and tryptophan is almost diffusion controlled. Quenching by histidine is slower. We have also investigated the slow quenching (by electron transfer) by the side chains of methionine and could show that quenching by cysteine S derivatives is negligible. Quenching by tyrosine and histidine peptides and by the tyrosines of the pancreatic trypsin inhibitor protein is slightly slower than by free side chains. Quenching is strongly viscosity controlled, to be expected of a process requiring bimolecular contact. Reactivity trends at high viscosities resemble those observed in fluid aqueous solutions. Activation energies of quenching by tyrosine, tryptophan, and histidine are similar. No difference could be detected in the mechanism of quenching by these side chains. No fast static quenching was observed that could compete with the diffusional process.

On the interaction between flavin-adenine rings and between flavin-indole rings by X-ray structural studies

Two crystal structures of 7,8-dimethylisoalloxazine-10-acetic acid:adenine-9-ylethylamine(1:1)hepatahydrate and 7,8-dimethylisoalloxazine-10-acetic acid:L-tryptophan methylester(1:1)heptahydrate complexes were determined as models for the flavin-adenine and flavin-indole interactions, respectively. In the former complex, both molecules were connected by Hoogsteen-type hydrogen bonds between the pyrimidinoid portion of flavin and the adenine, in addition to the normal stacking of both aromatic rings. On the other hand, parallel stackings and intermolecular vertical spacings less than the normal van der Waals separation distance were observed between the flavin and indole rings of the latter complex, indicative of the pi D-pi A charge-transfer interaction in their ground states. Comparing with the X-ray findings of related complexes, we discussed the interaction modes between flavin and adenine rings and between flavin and indole rings.

Chemical and catalytic properties of the peroxisomal acyl-coenzyme A oxidase from Candida tropicalis

The peroxisomal acyl-CoA oxidase has been purified from extracts of the yeast Candida tropicalis grown with alkanes as the principal energy source. The enzyme has a molecular weight of 552,000 and a subunit molecular weight of 72,100. Using an experimentally determined molar extinction coefficient for the enzyme-bound flavin, a minimum molecular weight of 146,700 was determined. Based on these data, the oxidase contains eight perhaps identical subunits and four equivalents of FAD. No other beta-oxidation enzyme activities are detected in purified preparations of the oxidase. The oxidase flavin does not react with sulfite to form an N(5) flavin-sulfite complex. Photochemical reduction of the oxidase flavin yields a red semiquinone; however, the yield of semiquinone is strongly pH dependent. The yield of semiquinone is significantly reduced below pH 7.5. The flavin semiquinone can be further reduced to the hydroquinone. The behavior of the oxidase flavin during photoreduction and its reactivity toward sulfite are interpreted to reflect the interaction in the N(1)-C(2)O region of the flavin with a group on the protein which acts as a hydrogen-bond acceptor. Like the acyl-CoA dehydrogenases which catalyze the same transformation of acyl-CoA substrates, the oxidase is inactivated by the acetylenic substrate analog, 3-octynoyl-CoA, which acts as an active site-directed inhibitor.

Solution properties of synthetic chlorophyllide--and bacteriochlorophyllide--apomyoglobin complexes

Well-defined 1:1 complexes have been formed between apomyoglobin (apoMb) and a number of chlorophyllide derivatives. The chlorophyllides substitute for heme in the pocket of myoglobin. These include magnesium chlorophyllide a, magnesium and zinc pyrochlorophyllide a, zinc pyrochlorophyllide b, zinc pyrochlorophyllide d, zinc pyromesochlorophyllide a, zinc 2-acetyl-2-devinylpyrochlorophyllide a, zinc protopyrochlorophyllide a, and zinc bacteriopyrochlorophyllide a. The effects of the protein on the electronic absorption, circular dichroism (CD), magnetic circular dichroism, and triplet state electron spin resonance spectra and fluorescence lifetimes in solution are compared with appropriate models in organic solvents. With the exception of the CD spectra, the protein causes shifts and intensity changes which are within the range observed for solvent effects. The CD spectra change substantially: the signs of several transitions are entirely reversed in the chlorins, and 3-6-fold intensity increases are observed with zinc bacteriochlorophyllide a. High-field 1H NMR spectra of ring current shifted Val-E11 methyl protons for the series porphyrin-, chlorin-, and bacteriochlorin-apoMb are used to establish the probable absolute orientation of the chromophore in the heme pocket. Doubled peaks in the NMR spectra of certain complexes are shown to arise from interconvertible species. The temperature dependence of the peak intensities and saturation transfer studies show that the species giving rise to the doubled peaks exchange on the time scale of about 1-60 s. Arguments are presented against inversion of the macrocycle in the heme pocket by either an inter- or an intramolecular mechanism as the origin of doubled peaks, and simple two-site exchange is ruled out by the NMR data. We suggest that the data are consistent with the idea that at least two slowly interconverting conformational substrates of the protein are populated, depending sensitively on small changes in rings I and II of the macrocycle and temperature.