Studies on NO2-Tyr82and NH2-Tyr82 derivatives of adrenodoxin. Effects of chemical modification on electron transferring activity

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

Pseudomonas aeruginosa cytotoxin: the Asp197-Gly-Asp-Tyr-His-Tyr-His-Tyr202 containing loop is critical for plasma membrane binding

The cytotoxin from Pseudomonas aeruginosa is a pore-forming toxin that binds specifically to water channel-related molecules of the erythrocyte membrane. Here, we have defined a domain, Asp197-Gly-Asp-Tyr-His-Tyr202 of the cytotoxin, to be essential for receptor binding. Cytotoxin point mutants from the recombinant gene carrying substitutions in the domain were characterized in terms of inhibiting the binding of radioiodinated natural cytotoxin to rat erythrocyte and producing cytotoxic effects in human granulocytes. A synthetic peptide representing residues 191-211 of the cytotoxin acted as a competitive inhibitor at a concentration of 10(-5) M. In contrast, two other cytotoxin-specific peptides were inactive. Structure prediction of the binding sequence shows a loop structure with similarities to the sequence around His332 in Aeromonas aerolysin essential to receptor binding.

Chemical modification of Tyr34 and Tyr129 in rabbit liver microsomal cytochrome b5 affects interaction with cytochrome P-450 2B4

Rabbit liver microsomal cytochrome b5 was allowed to react with tetranitromethane. Up to three tyrosine residues in each cytochrome b5 molecule were found to be accessible to the nitrating agent. Co-modification of tryptophan and histidine residues could be disregarded. CD-spectral measurements disproved gross changes in cytochrome b5 structure as a consequence of derivatization. Introduction of 1.6 nitro groups/polypeptide chain resulted in a fivefold increase in binding affinity for cytochrome P-450 2B4 (P-450 2B4), whereas spectral interaction with cytochrome c remained unaffected. Furthermore, the capacity of nitrated cytochrome b5 to shift the spin equilibrium to the high-spin conformer of P-4502B4 was diminished by 44% compared with the control. This corresponded with the partial disruption of NADH-dependent electron flow to ferric P-450 2B4. Changes in the redox potential of cytochrome b5 could be discounted as being responsible for this effect. The overall oxidative turnover of 4-nitroanisole did not respond to cytochrome b5 modification. MS analysis and sequencing of peptide fragments produced by tryptic digestion of modified cytochrome b5 permitted the detection of three nitrated tyrosine residues located at positions 11, 34 and 129. Derivatization of cytochrome b5 in the presence of a protective amount of P-450 2B4 provided evidence of the involvement of Tyr34 and Tyr129 in complexation of the two hemoproteins. It is proposed that Tyr129 might control docking of cytochrome b5 to P-450 2B4, whereas Tyr34 could be of functional importance in electron transfer.

Quantitation of interaction between cytochrome P-450scc and adrenodoxin--analysis in the median UV-region by second derivative spectroscopy

Interaction between the essential protein components of the bovine adrenal mitochondrial enzyme system (cytochrome P-450scc, adrenodoxin and adrenodoxin reductase) were studied in the median UV-region utilizing second derivative difference spectroscopy. Complex formation of cytochrome P-450scc with adrenodoxin induces a signal in the second derivative difference spectrum which can be attributed to tyrosine due to its minimum at 283 nm. Based on this signal cytochrome P-450scc was titrated with adrenodoxin in dependence on different effectors (reductase, phospholipid, cholesterol). The dissociation constants (Kd) of the P-450scc/adrenodoxin complexes derived therefrom revealed an increasing affinity between both components starting from titrations in buffer solution without additional components up to the completely reconstituted system. A high affinity between P-450scc and adrenodoxin corresponds to a high turnover rate of cholesterol. Dissociation constants of the P-450scc/adrenodoxin complex were also derived from spectral changes in the Soret region. But these data do not correlate with the substrate turnover.

Chemical modification of cytochrome P-450 LM4. Identification of functionally linked tyrosine residues

Cytochrome P-450 LM4 (RH, reduced flavoprotein:oxygen oxidoreductase (RH-hydroxylating), EC 1.14.14.1) from rabbit liver microsomes was chemically modified with tetranitromethane. Nitration of two tyrosine residues inhibits the p-nitrophenetole O-deethylase activity of the enzyme by about 80%. Sequencing the 3-nitrotyrosine-containing peptides after HPLC tryptic peptide mapping reveals that mainly Tyr-243 and Tyr-271 are nitrated, whereas Tyr-71, Tyr-188 and Tyr-365 are modified to a lower extent. Nitration of tyrosine residues affects the complex formation with p-nitrophenetole, alpha-naphthoflavone and metyrapone as indicated by an increased affinity towards p-nitrophenetole and by a decreased affinity for the latter compounds. Furthermore, nitration interferes with the electron transfer from NADPH-cytochrome P-450-reductase to cytochrome P-450 LM4 resulting in a slowed down reduction reaction. The results suggest that Tyr-243 and Tyr-271 of cytochrome P-450 LM4 are functionally involved in the interaction with NADPH-cytochrome P-450 reductase.

Cross-linking studies of steroidogenic electron transfer: covalent complex of adrenodoxin reductase with adrenodoxin

Bifunctional reagents 3,3'-dithiobis(succinimidyl propionate), 1-ethyl 3-(3-dimethylaminopropyl)carbodiimide and N-succinimidyl 3-(2-pyridyldithio)propionate have been used in an attempt to study molecular organization and covalent cross-linking of adrenodoxin reductase with adrenodoxin, the components of steroidogenic electron transfer system in bovine adrenocortical mitochondria. There was no cross-linking of individual proteins by the bifunctional reagents used, except for adrenodoxin cross-linking with water-soluble carbodiimide. Substantial cross-linking of adrenodoxin reductase with adrenodoxin was observed when water-soluble carbodiimide was used as cross-linking reagent. However, the cross-linked complex failed to transfer electrons. Significant amounts of the functional cross-linked complex (up to 42%) were observed when the proteins were cross-linked with N-succinimidyl 3-(2-pyridyldithio)propionate. Using gel filtration, ion-exchange chromatography and affinity chromatography on adrenodoxin-Sepharose, the complex was obtained in a highly purified form. In the presence of cytochrome P-450scc or cytochrome c, the cross-linked complex of adrenodoxin reductase with adrenodoxin was active in electron transfer from NADPH to heme proteins. The data obtained indicate that there are distinct binding sites on the adrenodoxin molecule responsible for the adrenodoxin reductase and cytochrome P-450scc binding, which suggests that steroidogenic electron transfer may be realized in an organized complex.

ACTH stimulation on cholesterol side chain cleavage activity of adrenocortical mitochondria. Transfer of the stimulus from plasma membrane to mitochondria

Adrenocortical mitochondrial cholesterol side chain cleavage reactions are regulated by the influence of pituitary ACTH. The mechanism of the stimulation involves adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase, and ribosomal labile protein synthesis. Through these reactions the stimulus reaches the mitochondrial side chain cleavage enzyme system. In this review article, the current implications on the stimulus transfer from the plasma membrane to the mitochondrial inner membrane are summarized. In particular the availability of cholesterol to P-450scc was discussed in terms of the distribution of cholesterol molecules in the membranes.

Studies on nitrotyrosine-82 and aminotyrosine-82 derivatives of adrenodoxin. Effects of chemical modification on the complex formation with adrenodoxin reductase

The coordination structure of the iron-sulfur center of the nitrotyrosine and the aminotyrosine derivates of bovine adrenodoxin was investigated by electron paramagnetic resonance spectroscopy. The reduced form of both modified samples exhibited signals identical with those for the native protein at g= 1.94 and g=2.01. From these results together with optical absorption and chemical analyses, it was concluded that the coordination structure of the iron-sulfur chromophore for both the derivatives was identical with the binuclear tetrahedral structure of native adrenodoxin. The configuration of the iron-binding area in nitro- and amino-adrenodoxin was studied by ovserving the circular dichroism spectra between 350 and 600 nm. The maxima for the nitro or amino derivatives were all identical with those for the native protein but different in the magnitude of their molar ellipticity. The molar ellipticities at 440 nm were 45.8 X 10(3), 14.5 X 10(3), and 9.5 X 10(6) deg cm2 per mol of iron for native adrenodoxin, nitro or amino derivative, respectively. These results suggest that the chemical modification of the tyrosine residue causes a conformational change in the iron-binding area. We have previously reported that the enzymatic activities of these reconstituted nitro and amino derivatives toware cytochrome c reduction in the presence of adrenodoxin reductase and reduced nicotinamide adenine dinucleotide phosphate were 19 and 7% of native adrenodoxin, respectively. The cytochrome c reductase activities of nitro- and aminoadrenodixin were drastically affected by the ionic strength of the assay medium, as found in native adrenodoxin. Fluorometric titration of the reductase with aminoadrenodoxin revealed that aminoadrenodoxin forms a 1:1 molar complex with the reductase. These results suggest that both the nitro and amino derivatives form a complex with the reductase. The dissociation constants of nitro- and aminoadrenodoxin for the reductase were 6.1 X 10(-7)M and 3.3 X 10(-7) M at mu = 0.04 and 1.9 X 10(-6) M and 2.0 X 10(-6) M at mu = 0.20, respectively. Comparison of these values with those of native adrenodoxin (approximately 10(-9) M at mu = 0.04 and 2.2 X 10(-7) M at mu = 0.20) suggests that an increase in the dissociation constant for the reductase is responsible for the decreased electron transferring activity of the modified adrenodoxins.