Reversible resolution of flavoproteins into apoproteins and fee flavins

Proline dehydrogenase from Thermus thermophilus does not discriminate between FAD and FMN as cofactor

Flavoenzymes are versatile biocatalysts containing either FAD or FMN as cofactor. FAD often binds to a Rossmann fold, while FMN prefers a TIM-barrel or flavodoxin-like fold. Proline dehydrogenase is denoted as an exception: it possesses a TIM barrel-like fold while binding FAD. Using a riboflavin auxotrophic Escherichia coli strain and maltose-binding protein as solubility tag, we produced the apoprotein of Thermus thermophilus ProDH (MBP-TtProDH). Remarkably, reconstitution with FAD or FMN revealed that MBP-TtProDH has no preference for either of the two prosthetic groups. Kinetic parameters of both holo forms are similar, as are the dissociation constants for FAD and FMN release. Furthermore, we show that the holo form of MBP-TtProDH, as produced in E. coli TOP10 cells, contains about three times more FMN than FAD. In line with this flavin content, the crystal structure of TtProDH variant ΔABC, which lacks helices αA, αB and αC, shows no electron density for an AMP moiety of the cofactor. To the best of our knowledge, this is the first example of a flavoenzyme that does not discriminate between FAD and FMN as cofactor. Therefore, classification of TtProDH as an FAD-binding enzyme should be reconsidered.

Recognition of enzymes lacking bound cofactor by protein quality control

Protein biogenesis is tightly linked to protein quality control (PQC). The role of PQC machinery in recognizing faulty polypeptides is becoming increasingly understood. Molecular chaperones and cytosolic and vacuolar degradation systems collaborate to detect, repair, or hydrolyze mutant, damaged, and mislocalized proteins. On the other hand, the contribution of PQC to cofactor binding-related enzyme maturation remains largely unexplored, although the loading of a cofactor represents an all-or-nothing transition in regard to the enzymatic function and thus must be surveyed carefully. Combining proteomics and biochemical analysis, we demonstrate here that cells are able to detect functionally immature wild-type enzymes. We show that PQC-dedicated ubiquitin ligase C-terminal Hsp70-interacting protein (CHIP) recognizes and marks for degradation not only a mutant protein but also its wild-type variant as long as the latter remains cofactor free. A distinct structural feature, the protruding C-terminal tail, which appears in both the mutant and wild-type polypeptides, contributes to recognition by CHIP. Our data suggest that relative insufficiency of apoprotein degradation caused by cofactor shortage can increase amyloidogenesis and aggravate protein aggregation disorders.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of a type II NADH:quinone oxidoreductase from the human pathogen Staphylococcus aureus

In recent years, type II NADH dehydrogenases (NDH-IIs) have emerged as potential drug targets for a wide range of human disease causative agents. In this work, the NDH-II enzyme from the Gram-positive human pathogen Staphylococcus aureus was recombinantly expressed in Escherichia coli, purified, crystallized and a crystallographic data set was collected at a wavelength of 0.873 Å. The crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 81.8, b = 86.0, c = 269.9 Å, contained four monomers per asymmetric unit and diffracted to a resolution of 3.32 Å. A molecular-replacement solution was obtained and model building and refinement are currently under way.

NMR spectroscopy on flavins and flavoproteins

(1)H-, (11)B-, (13)C-, (15)N-, (17)O-, (19)F-, and (31)P-NMR chemical shifts of flavocoenzymes and derivatives of it, as well as of alloxazines and isoalloxazinium salts, from NMR experiments performed under various experimental conditions (e.g., dependence of the chemical shifts on temperature, concentration, solvent polarity, and pH) are reported. Also solid-state (13)C- and (15)N-NMR experiments are described revealing the anisotropic values of corresponding chemical shifts. These data, in combination with a number of coupling constants, led to a detailed description of the electronic structure of oxidized and reduced flavins. The data also demonstrate that the structure of oxidized flavin can assume a configuration deviating from coplanarity, depending on substitutions in the isoalloxazine ring, while that of reduced flavin exhibits several configurations, from almost planar to quite bended. The complexes formed between oxidized flavin and metal ions or organic molecules revealed three coordination sites with metal ions (depending on the chemical nature of the ion), and specific interactions between the pyrimidine moiety of flavin and organic molecules, mimicking specific interactions between apoflavoproteins and their coenzymes. Most NMR studies on flavoproteins were performed using (13)C- and (15)N-substituted coenzymes, either specifically enriched in the pterin moiety of flavin or uniformly labeled flavins. The chemical shifts of free flavins are used as a guide in the interpretation of the chemical shifts observed in flavoproteins. Although the hydrogen-bonding pattern in oxidized and reduced flavoproteins varies considerably, no correlation is obvious between these patterns and the corresponding redox potentials. In all reduced flavoproteins the N(1)H group of the flavocoenzyme is deprotonated, an exception is thioredoxin reductase. Three-dimensional structures of only a few flavoproteins, mostly belonging to the family of flavodoxins, have been solved. Also the kinetics of unfolding and refolding of flavodoxins has been investigated by NMR techniques. In addition, (31)P-NMR data of all so far studied flavoproteins and some (19)F-NMR spectra are discussed.

LOV takes a pick: thermodynamic and structural aspects of the flavin-LOV-interaction of the blue-light sensitive photoreceptor YtvA from Bacillus subtilis

LOV domains act as versatile photochromic switches servicing multiple effector domains in a variety of blue light sensing photoreceptors abundant in a multitude of organisms from all kingdoms of life. The perception of light is realized by a flavin chromophore that upon illumination reversibly switches from the non-covalently bound dark-state to a covalently linked flavin-LOV adduct. It is usually assumed that most LOV domains preferably bind FMN, but heterologous expression frequently results in the incorporation of all natural occurring flavins, i.e. riboflavin, FMN and FAD. Over recent years, the structures, photochemical properties, activation mechanisms and physiological functions of a multitude of LOV proteins have been studied intensively, but little is known about its affinities to physiologically relevant flavins or the thermodynamics of the flavin-LOV interaction. We have investigated the interaction of the LOV domain of the well characterized bacterial photoreceptor YtvA with riboflavin, FMN and FAD by ITC experiments providing binding constants and thermodynamic profiles of these interactions. For this purpose, we have developed a protocol for the production of the apo forms of YtvA and its isolated LOV domain and we demonstrate that the latter can be used as a molecular probe for free flavins in cell lysates. Furthermore, we show here using NMR spectroscopic techniques and Analytical Ultracentrifugation that the flavin moiety stabilizes the conformation of the LOV domain and that dimerization of YtvA is caused not only by intermolecular LOV-LOV but also by STAS-STAS contacts.

A set of engineered Escherichia coli expression strains for selective isotope and reactivity labeling of amino acid side chains and flavin cofactors

Biological reactions are facilitated by delicate molecular interactions between proteins, cofactors and substrates. To study and understand their dynamic interactions researchers have to take great care not to influence or distort the object of study. As a non-invasive alternative to a site-directed mutagenesis approach, selective isotope labeling in combination with vibrational spectroscopy may be employed to directly identify structural transitions in wild type proteins. Here we present a set of customized Escherichia coli expression strains, suitable for replacing both the flavin cofactor and/or selective amino acids with isotope enriched or chemically modified substrates. For flavin labeling we report optimized auxotrophic strains with significantly enhanced flavin uptake properties. Labeled protein biosynthesis using these strains was achieved in optimized cultivation procedures using high cell density fermentation. Finally, we demonstrate how this approach is used for a clear assignment of vibrational spectroscopic difference signals of apoprotein and cofactor of a flavin containing photoreceptor of the BLUF (Blue Light receptors Using FAD) family.

Flavin adenine dinucleotide content of quinone reductase 2: analysis and optimization for structure-function studies

Quinone reductase 2 (NQO2) is a broadly expressed enzyme implicated in responses to a number of compounds, including protein kinase inhibitors, resveratrol, and antimalarial drugs. NQO2 includes a flavin adenine dinucleotide (FAD) cofactor, but X-ray crystallographic analysis of human NQO2 expressed in Escherichia coli showed that electron density for the isoalloxazine ring of FAD was weak and there was no electron density for the adenine mononucleotide moiety. Reversed-phase high-performance liquid chromatography (HPLC) of the NQO2 preparation indicated that FAD was not present and only 38% of the protomers contained flavin mononucleotide (FMN), explaining the weak electron density for FAD in the crystallographic analysis. A method for purifying NQO2 and reconstituting with FAD such that the final content approaches 100% occupancy with FAD is presented here. The enzyme prepared in this manner has a high specific activity, and there is strong electron density for the FAD cofactor in the crystal structure. Analysis of NQO2 crystal structures present in the Protein Data Bank indicates that many may have sub-stoichiometric cofactor content and/or contain FMN rather than FAD. This method of purification and reconstitution will help to optimize structural and functional studies of NQO2 and possibly other flavoproteins.

Expression and purification of untagged and histidine-tagged folate-dependent tRNA:m5U54 methyltransferase from Bacillus subtilis

Folate-dependent tRNA m(5)U methyltransferase TrmFO is a flavoprotein that catalyzes the C(5)-methylation of uridine at position 54 in the TPsiC loop of tRNA in several bacteria. Here we report the cloning and optimization of expression in Escherichia coli BL21 (DE3) of untagged, N-terminus, C-terminus (His)(6)-tagged TrmFO from Bacillus subtilis. Tagged and untagged TrmFO were purified to homogeneity by metal affinity or ion exchange and heparin affinity, respectively, followed by size-exclusion chromatography. The tag did not significantly alter the expression level, flavin content, activity and secondary structure of the protein.

The lycopene cyclase CrtY from Pantoea ananatis (formerly Erwinia uredovora) catalyzes an FADred-dependent non-redox reaction

The cyclization of lycopene generates provitamin A carotenoids such as beta-carotene and paves the way toward the formation of cyclic xanthophylls playing distinct roles in photosynthesis and as precursors for regulatory molecules in plants and animals. The biochemistry of lycopene cyclization has been enigmatic, as the previously proposed acid-base catalysis conflicted with the possibility of redox catalysis as predicted by the presence of a dinucleotide binding site. We show that reduced FAD is the essential lycopene cyclase (CrtY) cofactor. Using flavin analogs, mass spectrometry, and mutagenesis, evidence was obtained based on which a catalytic mechanism relying on cryptic (net) electron transfer can be refuted. The role of reduced FAD is proposed to reside in the stabilization of a transition state carrying a (partial) positive charge or of a positively charged intermediate via a charge transfer interaction, acid-base catalysis serving as the underlying catalytic principle. Lycopene cyclase, thus, ranks among the novel class of non-redox flavoproteins, such as isopentenyl diphosphate:dimethylallyl diphosphate isomerase type 2 (IDI-2) that requires the reduced form of the cofactor.

Cholesterol oxidase: biochemistry and structural features

Cholesterol oxidases are bifunctional flavoenzymes that catalyze the oxidation of steroid substrates which have a hydroxyl group at the 3beta position of the steroid ring system. The enzyme is found, in a wide range of bacterial species, in two forms: one with the FAD cofactor bound noncovalently to the enzyme; and one with the cofactor linked covalently to the protein. Here we discuss, compare and contrast the salient biochemical properties of the two forms of the enzyme. Specifically, the structural features are discussed that affect the redox potentials of the flavin cofactor, the chemical mechanism of substrate dehydrogenation by active-center amino acid residues, the kinetic parameters of both types of enzymes and the reactivity of reduced enzymes with molecular dioxygen. The presence of a molecular tunnel that is proposed to serve in the access of dioxygen to the active site and mechanisms of its control by a 'gate' formed by amino acid residues are highlighted.

What's in a covalent bond? On the role and formation of covalently bound flavin cofactors

Many enzymes use one or more cofactors, such as biotin, heme, or flavin. These cofactors may be bound to the enzyme in a noncovalent or covalent manner. Although most flavoproteins contain a noncovalently bound flavin cofactor (FMN or FAD), a large number have these cofactors covalently linked to the polypeptide chain. Most covalent flavin-protein linkages involve a single cofactor attachment via a histidyl, tyrosyl, cysteinyl or threonyl linkage. However, some flavoproteins contain a flavin that is tethered to two amino acids. In the last decade, many studies have focused on elucidating the mechanism(s) of covalent flavin incorporation (flavinylation) and the possible role(s) of covalent protein-flavin bonds. These endeavors have revealed that covalent flavinylation is a post-translational and self-catalytic process. This review presents an overview of the known types of covalent flavin bonds and the proposed mechanisms and roles of covalent flavinylation.

Apoenzyme reconstitution as a chemical tool for structural enzymology and biotechnology

Many enzymes contain a nondiffusible organic cofactor, often termed a prosthetic group, which is located in the active site and essential for the catalytic activity of the enzyme. These cofactors can often be extracted from the protein to yield the respective apoenzyme, which can subsequently be reconstituted with an artificial analogue of the native cofactor. Nowadays a large variety of synthetic cofactors can be used for the reconstitution of apoenzymes and, thus, generate novel semisynthetic enzymes. This approach has been refined over the past decades to become a versatile tool of structural enzymology to elucidate structure-function relationships of enzymes. Moreover, the reconstitution of apoenzymes can also be used to generate enzymes possessing enhanced or even entirely new functionality. This Review gives an overview on historical developments and the current state-of-the-art on apoenzyme reconstitution.

Characterization and mechanistic studies of type II isopentenyl diphosphate:dimethylallyl diphosphate isomerase from Staphylococcus aureus

The recently identified type II isopentenyl diphosphate (IPP):dimethylallyl diphosphate (DMAPP) isomerase (IDI-2) is a flavoenzyme that requires FMN and NAD(P)H for activity. IDI-2 is an essential enzyme for the biosynthesis of isoprenoids in several pathogenic bacteria including Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis, and thus is considered as a potential new drug target to battle bacterial infections. One notable feature of the IDI-2 reaction is that there is no net change in redox state between the substrate (IPP) and product (DMAPP), indicating that the FMN cofactor must start and finish each catalytic cycle in the same redox state. Here, we report the characterization and initial mechanistic studies of the S. aureus IDI-2. The steady-state kinetic analyses under aerobic and anaerobic conditions show that FMN must be reduced to be catalytically active and the overall IDI-2 reaction is O2-sensitive. Interestingly, our results demonstrate that NADPH is needed only in catalytic amounts to activate the enzyme for multiple turnovers of IPP to DMAPP. The hydride transfer from NAD(P)H to reduce FMN is determined to be pro-S stereospecific. Photoreduction and oxidation-reduction potential studies reveal that the S. aureus IDI-2 can stabilize significant amounts of the neutral FMN semiquinone. In addition, reconstitution of apo-IDI-2 with 5-deazaFMN resulted in a dead enzyme, whereas reconstitution with 1-deazaFMN led to the full recovery of enzyme activity. Taken together, these studies appear to support a catalytic mechanism in which the reduced flavin coenzyme mediates a single electron transfer to and from the IPP substrate during catalysis.

Deflavination and reconstitution of flavoproteins

Flavoproteins are ubiquitous redox proteins that are involved in many biological processes. In the majority of flavoproteins, the flavin cofactor is tightly but noncovalently bound. Reversible dissociation of flavoproteins into apoprotein and flavin prosthetic group yields valuable insights in flavoprotein folding, function and mechanism. Replacement of the natural cofactor with artificial flavins has proved to be especially useful for the determination of the solvent accessibility, polarity, reaction stereochemistry and dynamic behaviour of flavoprotein active sites. In this review we summarize the advances made in the field of flavoprotein deflavination and reconstitution. Several sophisticated chromatographic procedures to either deflavinate or reconstitute the flavoprotein on a large scale are discussed. In a subset of flavoproteins, the flavin cofactor is covalently attached to the polypeptide chain. Studies from riboflavin-deficient expression systems and site-directed mutagenesis suggest that the flavinylation reaction is a post-translational, rather than a cotranslational, process. These genetic approaches have also provided insight into the mechanism of covalent flavinylation and the rationale for this atypical protein modification.

A His-tag based immobilization method for the preparation and reconstitution of apoflavoproteins

The NifL PAS domain from Azotobacter vinelandii is a flavoprotein with FAD as the prosthetic group. Here we describe a novel immobilization procedure for the large-scale preparation of apo NifL PAS domain and its efficient reconstitution with either 2,4a-13C-FAD or 2,4a-13C-FMN. In this procedure, the His-tagged holoprotein is bound to an immobilized metal affinity column and the flavin is released by washing the column with buffer containing 2 M KBr and 2 M urea. The apoprotein is reconstituted on-column with the (artificial) flavin cofactor, and then eluted with buffer containing 250 mM imidazole. Alternatively, the immobilized apoprotein can be released from the column matrix before reconstitution. The His-tag based immobilization method of preparing reconstituted (or apo) NifL PAS domain protein has the advantage that it combines a protein affinity chromatography technique with limited protein loss, resulting in a high protein yield with extremely efficient flavin reconstitution. This on-column reconstitution method can also be used in cases where the apoprotein is unstable. Therefore, it may develop as a universal method for replacement of flavin or other cofactors.

Gene cluster on pAO1 of Arthrobacter nicotinovorans involved in degradation of the plant alkaloid nicotine: cloning, purification, and characterization of 2,6-dihydroxypyridine 3-hydroxylase

A 27,690-bp gene cluster involved in the degradation of the plant alkaloid nicotine was characterized from the plasmid pAO1 of Arthrobacter nicotinovorans. The genes of the heterotrimeric, molybdopterin cofactor (MoCo)-, flavin adenine dinucleotide (FAD)-, and [Fe-S] cluster-dependent 6-hydroxypseudooxynicotine (ketone) dehydrogenase (KDH) were identified within this cluster. The gene of the large MoCo subunit of KDH was located 4,266 bp from the FAD and [Fe-S] cluster subunit genes. Deduced functions of proteins encoded by open reading frames (ORFs) of the cluster were correlated to individual steps in nicotine degradation. The gene for 2,6-dihydroxypyridine 3-hydroxylase was cloned and expressed in Escherichia coli. The purified homodimeric enzyme of 90 kDa contained 2 mol of tightly bound FAD per mol of dimer. Enzyme activity was strictly NADH-dependent and specific for 2,6-dihydroxypyridine. 2,3-Dihydroxypyridine and 2,6-dimethoxypyridine acted as irreversible inhibitors. Additional ORFs were shown to encode hypothetical proteins presumably required for holoenzyme assembly, interaction with the cell membrane, and transcriptional regulation, including a MobA homologue predicted to be specific for the synthesis of the molybdopterin cytidine dinucleotide cofactor.

Purification and characterization of morphinone reductase from Pseudomonas putida M10

The NADH-dependent morphinone reductase from Pseudomonas putida M10 catalyses the reduction of morphinone and codeinone to hydromorphone and hydrocodone respectively. Morphinone reductase was purified from crude cell extracts to apparent homogeneity in a single affinity-chromatography step using Mimetic Yellow 2. The purified enzyme was a dimeric flavoprotein with two identical subunits of M(r) 41,100, binding non-covalently one molecule of FMN per subunit. The N-terminal sequence was PDTSFSNPGLFTPLQ. Morphinone reductase was active against morphinone, codeinone, neopinone and 2-cyclohexen-1-one, but not against morphine, codeine or isocodeine. The apparent Km values for codeinone and 2-cyclohexen-1-one were 0.26 mM and 5.5 mM respectively. The steroids progesterone and cortisone were potent competitive inhibitors; the apparent K1 for cortisone was 35 microM. The pH optimum for codeinone reduction was 8.0 in phosphate buffer. No reverse reaction could be detected, and NADPH could not be used as a reducing substrate in place of NADH. Morphinone reductase activity was strongly inhibited by 0.01 mM CuSO4 and p-hydroxymercuribenzoate, suggesting the presence of a vital thiol group. Steady-state kinetic studies suggested a Ping Pong (substituted enzyme) kinetic mechanism; however, product-inhibition patterns were inconsistent with a classical Ping Pong mechanism. Morphinone reductase may, like several other flavoprotein dehydrogenases, operate by a hybrid two-site Ping Pong mechanism.

Reductive inactivation of the mitochondrial three subunit NADH dehydrogenase

The 3-subunit iron-sulfur flavoprotein (NADH-artificial electron acceptor oxidoreductase) derived from complex I (EC 1.6.5.3) is rapidly and irreversibly inactivated in the presence of NADH. The rate of inactivation increases with a decrease of the enzyme concentration. The activities with ferricyanide, menadione and cytochrome c were lost synchronously during preincubation of the enzyme in the presence of NADH or dithionite under either aerobic or anaerobic conditions. The titration of the inactivation rate with the NADH/NAD+ pair suggests that reduction of a component with Em' = -325 mV (n = 2) is a prerequisite for a loss of the enzyme activity. Among the compounds tested only FMN and NAD+ were able to protect the enzyme against the reductive inactivation. NADH-induced loss of the enzyme activity in diluted solutions is accompanied with the synchronous appearance of a fluorescence characteristic for free FMN. It is concluded that the reduction of flavin leads to a strong decrease of FMN affinity to its specific binding site, and possible implications of the redox-dependent affinity changes in operation of NADH-ubiquinone reductase are discussed.

A study on apoenzyme from Rhodotorula gracilis D-amino acid oxidase

The apoenzyme of D-amino acid oxidase from Rhodotorula gracilis was obtained at pH 7.5 by dialyzing the holoenzyme against 2 M KBr in 0.25 M potassium phosphate, 0.3 mM EDTA, 5 mM 2-mercaptoethanol and 20% glycerol. To recover a reconstitutable and highly stable apoprotein, it is essential that phosphate ions and glycerol be present at high concentrations. Apo-D-amino acid oxidase is entirely present as a monomeric protein, while the reconstituted holoenzyme is a dimer of 79 kDa. The equilibrium binding of FAD to apoprotein was measured from the quenching of flavin fluorescence and by differential spectroscopy: a Kd of 2.0 x 10(-8) M was calculated. The kinetics of formation of the apoprotein-FAD complex were studied by the quenching of protein and flavin fluorescence, by differential spectroscopy and by activity measurements. In all cases a two-stage process was shown to be present with a fairly rapid first phase, followed by a slow secondary change which represents only 4-6% of the total recombination process. In no conditions was a lag in the recovery of maximum catalytic activity observed. The process of FAD binding to yeast D-amino acid oxidase appears to be of the type Apo + FAD in equilibrium holoenzyme, even though the existence of a transient intermediate not detectable under our conditions cannot be ruled out.

Reconstitution of Escherichia coli photolyase with flavins and flavin analogues

Escherichia coli DNA photolyase contains two chromophore cofactors, 1,5-dihydroflavin adenine dinucleotide (FADH2) and (5,10-methenyltetrahydrofolyl)polyglutamate (5,10-MTHF). A procedure was developed for reversible resolution of apophotolyase and its chromophores. To investigate the structures important for the binding of FAD to apophotolyase and of photolyase to DNA, reconstitution experiments with FAD, FMN, riboflavin, 1-deazaFAD, 5-deazaFAD, and F420 were attempted. Only FAD and 5-deazaFAD showed high-affinity binding to apophotolyase. The apoenzyme had no affinity to DNA but did regain its specific binding to thymine dimer containing DNA upon binding stoichiometrically to FAD or 5-deazaFAD. Successful reduction of enzyme-bound FAD with dithionite resulted in complete recovery of photocatalytic activity.

Large-scale preparation and reconstitution of apo-flavoproteins with special reference to butyryl-CoA dehydrogenase from Megasphaera elsdenii. Hydrophobic-interaction chromatography

A new method is described for the large-scale reversible dissociation of flavoproteins into apoprotein and prosthetic group using hydrophobic-interaction chromatography. Lipoamide dehydrogenase from Azotobacter vinelandii and butyryl-CoA dehydrogenase from Megasphaera elsdenii are selected to demonstrate the usefulness of the method. In contrast to conventional methods, homogeneous preparations of apoproteins in high yields are obtained. The apoproteins show high reconstitutability. The holoenzymes are bound to phenyl-Sepharose CL-4B at neutral pH in the presence of ammonium sulfate. FAD is subsequently removed at pH 3.5-4.0 by addition of high concentrations of KBr. Large amounts of apoenzymes (200-500 mg), showing negligible residual activity, are eluted at neutral pH in the presence of 50% ethylene glycol. The holoenzyme of lipoamide dehydrogenase can be reconstituted while the apoprotein is still bound to the column or the apoenzyme can be isolated in the free state. In both cases the yield and degree of reconstitution of holoenzyme is more than 90% of starting material. Apo-lipoamide-dehydrogenase exists mainly as a monomer in solution and reassociates to the native dimeric structure in the presence of FAD. The apoenzyme is stable for a long period of time when kept in 50% ethylene glycol at -18 degrees C. Steady-state fluorescence-polarization measurements of protein-bound FAD indicate that reconstituted lipoamide dehydrogenase possesses a high stability which is governed by the low dissociation rate constant of the apoenzyme-FAD complex. The holoenzyme of butyryl-CoA dehydrogenase cannot be reconstituted when the apoenzyme is bound to the column. However, stable apoprotein can be isolated in the free state yielding 50-80% of starting material, depending on the immobilization conditions. The coenzyme A ligand present in native holoenzyme is removed during apoprotein preparation. The apoenzyme is relatively stable when kept in 50% ethylene glycol at -18 degrees C. From kinetic and gel filtration experiments it is concluded that the reconstitution reaction of butyryl-CoA dehydrogenase is governed by both the pH-dependent hydrodynamic properties of apoenzyme and the pH-dependent stability of reconstituted enzyme. At pH 7, the apoenzyme is in equilibrium between dimeric and tetrameric forms and reassociates to a native-like tetrameric structure in the presence of FAD. The stability of reconstituted enzyme is strongly influenced by the presence of CoA ligands as shown by fluorescence-polarization measurements. The degree of reconstitution of butyryl-CoA dehydrogenase is more than 80% of the original specific activity under certain conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Flavin radicals: chemistry and biochemistry

The redox properties of free and protein-bound flavin are discussed extensively. The interaction of one and two-electron reduced flavin with oxygen is emphasized.

The influence of oxidation-reduction state on the kinetic stability of pig kidney general acyl-CoA dehydrogenase and other flavoproteins

Pig kidney general acyl-CoA dehydrogenase is markedly stabilized against loss of flavin and activity in 7.3 M-urea or at 60 degrees C upon reduction with sodium dithionite or octanoyl-CoA. Electron transferring flavoprotein is similarly stabilized, whereas egg white riboflavin-binding protein loses flavin more readily on reduction. These and other data support the anticipated correlation between the kinetic stability of the holoproteins and the oxidation-reduction potential of their bound flavins.

Preparation of apoprotein from spinach ferredoxin-NADP+ reductase. Studies on the resolution process and characterization of the FAD reconstituted holoenzyme

1. Ferredoxin-NADP+ reductase resolved into apoprotein and flavin by incubation with 2.5 M CaCl2 at pH 7.5 and 2 degrees C. Essential factors to recover a reconstitutable apoprotein are dithiothreitol, glycerol and guanidine/HCl. The apoprotein is stable for at least a week at -20 degrees C. 2. The release of the prosthetic group from the protein by the Ca2+ ions is a multi-step process. Three different effects of these ions are identifiable: (a) a rapid 20-25% inhibition of catalytic activity, probably caused by an increase in the ionic strength of the medium; other cations can produce it as well; (b) a slower induction of a conformational change in the protein which causes complete loss of activity and exposure to solvent of the flavin moiety; the FAD is finally released from the protein; (c) complete conversion of FAD to FMN, which blocks reconstitution to holoenzyme, caused by the well-known hydrolytic action of Ca2+ ions on the pyrophosphate bridge of FAD. 3. Binding of FAD by the apoferredoxin-NADP+ reductase is very rapid and it is complete in a few minutes even at 0 degrees C. A Kd of 3.4 X 10(-9) M has been determined by fluorescence titration. The reconstituted holoenzyme has catalytic activity, spectral and fluorescence properties nearly identical to the native enzyme. The gel electrophoresis and isoelectrofocusing patterns of the two enzymes are very similar. Removal of factors from the apoprotein solution such as dithiothreitol and glycerol promotes the appearance of protein aggregates.

The purification and properties of 2,4-dichlorophenol hydroxylase from a strain of Acinetobacter species

1. 2,4-Dichlorophenol hydroxylase has been purified 13-fold from Acinetobacter grown on 2,4-dichlorophenoxyacetic acid as sole carbon source. The enzyme was estimated to be 80-90% pure by electrophoresis. 2. The enzyme has a relative molecular mass of about 240 000 and consists of four subunits of identical size. 3. The enzyme contains FAD as the prosthetic group. FAD could not be replaced by riboflavin or FMN in reconstituting active enzyme from apoenzyme. 4. The reaction catalysed is an NADPH-dependent hydroxylation of 2,4-dichlorophenol with the formation of 3,5-dichlorocatechol as product. The reaction stoichiometry is typical of a monooxygenase with an external electron donor. NADPH is the preferred reduced pyridine nucleotide substrate but the enzyme can function with NADH. 5. The enzyme possesses broad effector specificity. In addition to 2,4-dichlorophenol, 4-chlorophenol and 4-chloro-2-methylphenol are true substrates for the enzyme. A number of 'non-substrate effectors' has been found. 6. The enzyme is sensitive to thiol-inhibiting reagents.

Apoflavodoxin aggregation following dissociation of flavin

Flavodoxins were isolated for the cyanobacteria Anacystis nidulans and Nostoc strain MAC, and from the red alga Chondrus crispus, and apoflavodoxins prepared by five methods. Gel electrophoretic studies showed that whereas the apoproteins of A. nudulans and Nostoc strain MAC were recovered in monomeric form, the removal of riboflavin 5'-phosphate from C. crispus flavodoxin resulted in extensive aggregation of the apoprotein. In extent and nature this aggregation differed with the dissociating agent used.