The possible role of aromatic residues of Clostridium acidi-urici ferredoxin in electron transport

On internal electron transfer in xanthine oxidase

The structural basis for intramolecular electron transfer in xanthine oxidase (EC 1.2.3.2) has been probed using temperature-jump perturbation and optical spectroscopic methods. Redox equilibria were found to be temperature-insensitive; hence it is argued that electron transfer is not accompanied by any extensive macromolecular conformational changes. No evidence for absorption phenomena ascribable to optical electron transfer could be found throughout the course of reductive titration of the biological particle. The combined results suggest that long-range electron transfer in the xanthine oxidase can best be described as occurring between only weakly interacting redox sites embedded in a rigid protein matrix.

Side-chain mobility and the calculation of tyrosyl circular dichroism of proteins. Implications of a test with insulin and des-B1-phenylalanine insulin

Previous calculations using crystal structure coordinates (Strickland and Mercola [1976], Biochemistry. 15: 3857) have predicted that about 40 percent of the calculated tyrosyl circular dichroism of hexameric insulin is due to one of the four tyrosine residues: viz. the A14-tyrosine interacting with the nearby B1-phenylalanine ring group. We have tested this prediction by measuring the tyrosyl circular dichroism of an isomorphous analogue of insulin, des-B1-phenylalanine-insulin. Contrary to expectation, the resulting circular dichroism was the same as that of insulin. It is concluded that the B1-phenylalanine residue does not in fact make a large contribution to the circular dichroism of A14-tyrosine. This result is probably due to the thermal motion of the B1 and A14 ring groups not taken into account by the calculations. An example of the effects of thermal motion on the calculated circular dichroism is given and improvements that do take into account thermal motion are discussed.

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.

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

Bovine apoadrenodoxin was treated with tetranitromethane to introduce a nitro group into the tyrosyl residue at position 82 of this protein. The degrees of nitration under the best conditions were estimated to be 90% and nearly 100% on the basis of amino acid analysis and the spectrophotometric method, respectively. An amino derivative was prepared by reducing the nitro group with sodium dithionite. The apoadrenodoxin derivatives could be reconstituted to have an iron-sulfur chromophore similar to the native adrenodoxin which contains a 1:1 molar ratio of labile sulfur to iron content and displays absorption peaks at 414 and 450 nm. The enzymatic acitivies of these reconstituted nitro and amino derivatives toward cytochrome c reduction in the presence of adrenodoxin reductase and NADPH were 19 and 7% of native adrenodoxin, respectively. We studied the kinetics of the direct reduction of the reconstituted amino derivative in the presence of NADPH and adrenodoxin reductase under anaerobic conditons. The initial rate of reduction for the amino derivative was 7% of the native adrenodoxin, which is in good agreement with its activity toward cytochrome c reduction. From these results, it is concluded that by modifying the tyrosyl residue at position 82 of the adrenodoxin polypeptide, the electron-transferring activity of the molecule is largely diminished.

Carbon-13 nuclear-magnetic-resonance spectroscopy of whole cells and of cytochrome C from Neurospora crass grown with (S-Me-13C)methionine

Neurospora crassa cytochrome C biosynthetically labelled with [S-Me-13C]methionine was prepared and analysed by 13C nuclear-magnetic-resonance spectroscopy. The methyl group of methionine is extensively incorporated into an N-trimethyl-lysine-72 residue arise from S-adenosylmethionine transmethylation, and that the methyl carbons of methionine residues are sufficiently close to the haem centre to experience chemical shifts from the ring currents of the tetrapyrrole pi electrons and broadening due to binding of methionine-80 with the haem, as well as interaction of the S-E113C]methyl groups with the paramagnetic iron centre. Although whole cells of the labelled Neurospora produced a 13C resonance at the expected position for the methionyl methyl group most of the methyl label was diverted into N-tetra-alkyl ammonium compounds. After an active state of growth these labelled N-methyl compounds appear, in the main, to be low-molecular-weight derivatives of choline which, if associated with membrane, are in a sufficiently fluid environment to have short rotational correlation times. During a subsequent dormant growth period these compounds become associated to some extent with relatively more immobile phases as a result of membrane binding or an increase in membrane rigidity.

Proton magnetic resonance studies of the tyrosine residues of hen lysozyme-assignment and detection of conformational mobility

This paper reports the detection of all the proton magnetic resonances from the aromatic rings of the three tyrosine residues of hen egg white lysozyme. Procedures, including spin decoupling, for the full assignment of these residues are described. To explain the appearance and behaviour of these resonances it is postulated that the tyrosine rings are able to flip between equivalent conformations at rates exceeding 10 4 s -1 .

Synthesis and properties of Clostridium acidi-urici (Leu2)-ferredoxin: a function of the peptide chain and evidence against the direct role of the aromatic residues in electron transfer

Tyrosyl or other aromatic residues generally occur in two conserved positions in the peptide chain of clostridial-type ferredoxins and have been implicated in the electron transfer function of these iron-sulfur proteins. We have prepared and determined some of the properties of a derivative of Clostridium acidi-urici ferredoxin, [Leu(2)]-ferredoxin, in which a leucyl residue has been substituted for the tyrosyl residue in position 2 from the amino terminus. [Leu(2)]-ferredoxin is fully active as an electron carrier in two biological assays, the phosphoroclastic enzyme system and the ferredoxin-dependent reduction of cytochrome c in the presence of ferredoxin-TPN reductase and TPNH. Quantitative electron paramagnetic resonance experiments indicate that [Leu(2)]-ferredoxin accepts nearly two electrons upon enzymatic reduction by pyruvate-ferredoxin oxidoreductase and an excess of pyruvate. If electron transfer to an iron-sulfur cluster is the rate-limiting step in the assays used, and if the rate of electron transfer through Tyr(30) is not much faster than through Tyr(2), these results indicate that the primary pathway of electron transfer in clostridial-type ferredoxins is not via Tyr or other aromatic amino-acid residues. The syntheses of other ferredoxin derivatives with amino-acid substitutions or deletions in positions 1 and 2 indicate that a large bulky residue, but not necessarily an aromatic residue, is needed in position 2 for the stability of this ferredoxin. The residue in position 2, therefore, appears to act as a hydrophobic shield for an iron-sulfur cluster.

Proton magnetic resonance and magnetic susceptibility characterization of ferredoxin I from Bacillus polymyxa

Magnetic susceptibility and proton magnetic resonance spectra are reported for the oxidized and reduced forms of the iron-sulfur protein Bacillus polymyxa ferredoxin I. The magnetic susceptibility of the oxidized form indicates antiferromagnetic exchange coupling between component iron atoms that is quantitatively similar to that observed for the clostridial ferredoxins and for the [(C(2)H(5))(4)N](2) [Fe(4)S(4)(SCH(2)C(6)H(5))(4)] analog. Contact-shifted resonances observed in the proton-magnetic-resonance spectra of oxidized and reduced forms of the B. polymyxa protein can be correlated with the contact-shifted resonances of corresponding redox forms of the clostridial ferredoxins. Characteristics of the contact-shifted resonances observed in partially reduced B. polymyxa ferredoxin I are compatible with a "slow" rate of electron exchange between redox forms, which suggests that the "fast" electron exchange earlier observed in the eight-iron clostridial ferredoxins may derive from an intramolecular component.