Purification and characterization of a 7Fe ferredoxin from Streptomyces griseus

Cytochrome P450 systems—biological variations of electron transport chains

Cytochromes P450 (P450) are hemoproteins encoded by a superfamily of genes nearly ubiquitously distributed in different organisms from all biological kingdoms. The reactions carried out by P450s are extremely diverse and contribute to the biotransformation of drugs, the bioconversion of xenobiotics, the bioactivation of chemical carcinogens, the biosynthesis of physiologically important compounds such as steroids, fatty acids, eicosanoids, fat-soluble vitamins and bile acids, the conversion of alkanes, terpenes and aromatic compounds as well as the degradation of herbicides and insecticides. Cytochromes P450 belong to the group of external monooxygenases and thus receive the necessary electrons for oxygen cleavage and substrate hydroxylation from different redox partners. The classical as well as the recently discovered P450 redox systems are compiled in this paper and classified according to their composition.

Microbial transformations of steroids--XII. Progesterone hydroxylation profiles are modulated by post-translational modification of an electron transfer protein in Streptomyces roseochromogenes

When Streptomyces roseochromogenes strain 10984 was incubated with exogenous progesterone for 25 h the major monohydroxylated metabolite, 16alpha-hydroxyprogesterone was produced in 3.6 fold excess to the minor metabolite 2beta,16alpha-dihydroxyprogesterone. In a reconstituted system containing highly purified progesterone 16alpha-hydroxylase cytochrome P-450, and electron transfer proteins ferredoxin-like redoxin (roseoredoxin) and redoxin reductase (roseoredoxin reductase), both metabolites were produced but in a 10:1 ratio. When S. roseochromogenes was pre-incubated for 8 h with 0.32 mM progesterone and the purified components of the hydroxylase system incubated as before, the ratio of 16alpha-hydroxyprogesterone to 2beta,16alpha-dihydroxyprogesterone produced decreased to 2.8:1, virtually identical to the ratio in whole cell transformations. Reconstitution assays containing all combinations of hydroxylase proteins purified from progesterone pre-incubated and control cells showed that the roseoredoxin was solely responsible for the observed changes in in vitro metabolite ratios. The fact that the lower 16alpha-hydroxyprogesterone to 2beta,16alpha-dihydroxyprogesterone ratio was also obtained when S. roseochromogenes was exposed to 0.335 mM cycloheximide for 8 h prior to the progesterone pre-incubation, pointed to post-translation modification of the roseoredoxin. Separation of two isoforms of roseoredoxin by isoelectric focusing supported this proposition.

Complex formation between Azotobacter vinelandii ferredoxin I and its physiological electron donor NADPH-ferredoxin reductase

In Azotobacter vinelandii, deletion of the fdxA gene, which encodes ferredoxin I (FdI), leads to activation of the expression of the fpr gene, which encodes NADPH-ferredoxin reductase (FPR). In order to investigate the relationship of these two proteins further, the interactions of the two purified proteins have been examined. AvFdI forms a specific 1:1 cross-linked complex with AvFPR through ionic interactions formed between the Lys residues of FPR and Asp/Glu residues of FdI. The Lys in FPR has been identified as Lys258, a residue that forms a salt bridge with one of the phosphate oxygens of FAD in the absence of FdI. UV-Vis and circular dichroism data show that on binding FdI, the spectrum of the FPR flavin is hyperchromatic and red-shifted, confirming the interaction region close to the FAD. Cytochrome c reductase assays and electron paramagnetic resonance data show that electron transfer between the two proteins is pH-dependent and that the [3Fe-4S]+ cluster of FdI is specifically reduced by NADPH via FPR, suggesting that the [3Fe-4S] cluster is near FAD in the complex. To further investigate the FPR:FdI interaction, the electrostatic potentials for each protein were calculated. Strongly negative regions around the [3Fe-4S] cluster of FdI are electrostatically complementary with a strongly positive region overlaying the FAD of FPR, centered on Lys258. These proposed interactions of FdI with FPR are consistent with cross-linking, peptide mapping, spectroscopic, and electron transfer data and strongly support the suggestion that the two proteins are physiological redox partners.

Characterization of a 2[4Fe-4S] ferredoxin obtained by chemical insertion of the Fe-S clusters into the apoferredoxin II from Rhodobacter capsulatus

The Rhodobacter capsulatus ferredoxin II (FdII) belongs to a family of 7Fe ferredoxins containing one [3Fe-4S] cluster and one [4Fe-4S] cluster. This protein, encoded by the fdxA gene, has been overproduced in Escherichia coli as a soluble apoferredoxin. The purified recombinant protein was subjected to reconstitution experiments by chemical incorporation of the Fe-S clusters under anaerobic conditions. A brown protein was obtained, the formation of which was dependent upon the complete unfolding of the polypeptide prior to incorporation of iron and sulfur atoms. The yield of the reconstituted product was higher when the reaction was carried out at slightly basic pH. The reconstituted ferredoxin was purified and shown to be distinct from the native [7Fe-8S] ferredoxin, based on several biochemical and spectroscopic criteria. In the oxidized state, EPR revealed the quasi-absence of [3Fe-4S] cluster. 1H-NMR spectroscopic analyses provided evidence that the protein was reconstituted as a 2[4Fe-4S] ferredoxin. This conclusion was further supported by the determination by electrospray mass spectrometry of the molecular mass of the reconstituted protein, which matched within 2 Da to the mass of the FdII polypeptide incremented of eight atoms each of iron and sulfur. Exposure of the reconstituted protein to air resulted in a fast and irreversible oxidative denaturation of the Fe-S clusters, without formation of [7Fe-8S] form. Unlike the natural 7Fe ferredoxin, the reconstituted ferredoxin appeared incompetent in an electron-transfer assay coupled to nitrogenase activity. The fact that the apoFdII was reconstituted as a highly unstable 8Fe ferredoxin instead of the 7Fe naturally occurring FdII is discussed in relation to the results obtained with other types of ferredoxins.

Characterization of the [3Fe-4S] and [4Fe-4S] clusters in unbound PsaC mutants C14D and C51D. Midpoint potentials of the single [4Fe-4S] clusters are identical to FA and FB in bound PsaC of photosystem I

In a previous paper we showed that the C51D mutant of PsaC contains a [3Fe-4S] cluster in the FA site and a [4Fe-4S] cluster in the FB site and that the C14D mutant contains an uncharacterized cluster in the FB site and a [4Fe-4S] cluster in the FA site [Zhao, J. D., Li, N., Warren, P. V., Golbeck, J. H., & Bryant, D. A. (1992) Biochemistry 31, 5093-5099]. In this paper we describe the electrochemical and electron spin resonance properties of the recombinant C14D and C51D holoproteins after in vitro reinsertion of the iron-sulfur clusters. Unbound PsaC shows no significant resonances in the oxidized state, but the unbound C14D and C51D mutant proteins show an intense set of resonances at g approximately 2.02 and 1.99 characteristic of an oxidized [3Fe-4S]1+/0 cluster. The Em' values for the [3Fe-4S]1+/0 clusters in C14D (FB*) and C51D (FA*) are -98 mV, and both represent one-electron transfers. After reduction with dithionite at pH 10.0, wild-type PsaC shows a broad set of resonances resulting from the superposition of FA- and FB- characterized by a low-field peak at an apparent g value of 2.051 and a high-field trough at an apparent g value of 1.898. The FB resonances in C51D were slightly narrower, with a low-field peak at an apparent g value of 2.039 and high-field trough at an apparent g value of 1.908.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression in Escherichia coli and characterization of a recombinant 7Fe ferredoxin of Rhodobacter capsulatus

The 7Fe ferredoxin of Rhodobacter capsulatus (FdII) could be expressed in Escherichia coli by cloning the fdxA gene coding for FdII downstream from the lac promoter. The expressed recombinant ferredoxin appeared as a brown protein which was specifically recognized in E. coli cell-free extracts by anti-FdII serum. The purified recombinant ferredoxin was indistinguishable from R. capsulatus FdII on the basis of its molecular, redox and spectroscopic properties. These results indicate that the [3Fe-4S] and [4Fe-4S] clusters were correctly inserted into the recombinant ferredoxin.

Cloning, nucleotide sequence determination and expression of the genes encoding cytochrome P-450soy (soyC) and ferredoxinsoy (soyB) from Streptomyces griseus

Xenobiotic transformation by Streptomyces griseus (ATCC13273) is catalysed by a cytochrome P-450, designated cytochrome P-450soy. A DNA segment carrying the structural gene encoding P-450soy (soyC) was cloned using an oligonucleotide probe constructed from the protein sequence of a tryptic peptide. Following DNA sequencing the deduced amino acid sequence of P-450soy was compared with that for P-450cam, revealing conservation of important structural components including the haem pocket. Expression of the cloned soyC gene product was demonstrated in Streptomyces lividans by reduced CO:difference spectral analysis and Western blotting. Downstream of soyC, a gene encoding a putative [3Fe-4S] ferredoxin (soyB), named ferredoxinsoy, was identified.

Purification and characterization of a soybean flour-inducible ferredoxin reductase of Streptomyces griseus

We have purified an NADH-dependent ferredoxin reductase from crude extracts of Streptomyces griseus cells grown in soybean flour-enriched medium. The purified protein has a molecular weight of 60,000 as determined by sodium dodecyl sulfate gel electrophoresis. The enzyme requires Mg2+ ion for catalytic activity in reconstituted assays, and its spectral properties resemble those of many other flavin adenine dinucleotide-containing flavoproteins. A relatively large number of hydrophobic amino acid residues are found by amino acid analysis, and beginning with residue 7, a consensus flavin adenine dinucleotide binding sequence, GXGXXGXXXA, is revealed in this protein. In the presence of NADH, the ferredoxin reductase reduces various electron acceptors such as cytochrome c, potassium ferricyanide, dichlorophenolindophenol, and nitroblue tetrazolium. However, only cytochrome c reduction by the ferredoxin reductase is enhanced by the addition of ferredoxin. In the presence of NADH, S. griseus ferredoxin and cytochrome P-450soy, the ferredoxin reductase mediates O dealkylation of 7-ethoxycoumarin.

Primary structure of a 7Fe ferredoxin from Streptomyces griseus

The complete primary structure of a Streptomyces griseus (ATCC 13273) 7Fe ferredoxin, which can couple electron transfer between spinach ferredoxin reductase and S. griseus cytochrome P-450soy for NADPH-dependent substrate oxidation, has been determined by Edman degradation of the whole protein and peptides derived by Staphylococcus aureus V8 proteinase and trypsin digestion. The protein consists of 105 amino acids and has a calculated molecular weight, including seven irons and eight sulfurs, of 12,291. The ferredoxin sequence is highly homologous (73%) to that of the 7Fe ferredoxin from Mycobacterium smegmatis. The N-terminal half of the sequence, which is the Fe-S clusters binding domain, has more than 50% homology with other 7Fe ferredoxins. In particular, the seven cysteines known from the crystal structure of Azotobacter vinelandii ferredoxin I to be involved in binding the two Fe-S clusters are conserved.