On the Nature of the Spin Coupling between the Iron-Sulfur Clusters in the Eight-Iron Ferredoxins

Mathews, Rob; Charlton, S C; Sands, Richard H.; Palmer, Graham

doi:10.1016/s0021-9258(19)42521-0

Cited by 158 publications

(33 citation statements)

References 8 publications

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“…A very complex spectrum was observed at this potential, in agreement with those reported previously, , which can be simulated as a series of six spin systems, including the well-characterized H ox and H ox -CO ( g = 2.083, 2.020, 2.014) states, although the H ox -CO represents a relatively minor spin contribution to the signal (5% at −402 mV). At more negative potentials (−469 and −475 mV), the spectra broaden, presumably from weak spin–spin exchange among individual F-clusters, such as that reported in 2x[4Fe-4S] containing ferredoxins . We cannot exclude the possibility of additional spin contribution from EPR active, two electron reduced H-cluster states such as H sred , which contains a [4Fe-4S] H 1+ cluster. , It has been previously observed that the CrHydA PDT has residual levels of activity, , which is consistent with our observation of drift in potential of the CpI PDT .…”

Section: Resultssupporting

confidence: 91%

“…At more negative potentials (−469 and −475 mV), the spectra broaden, presumably from weak spin−spin exchange among individual F-clusters, such as that reported in 2x[4Fe-4S] containing ferredoxins. 32 We cannot exclude the possibility of additional spin contribution from EPR active, two electron reduced H-cluster states such as H sred , which contains a [4Fe-4S] H 1+ cluster. 26,27 It has been previously observed that the CrHydA PDT has residual levels of activity, 24,33 which is consistent with our observation of drift in potential of the CpI PDT .…”

Section: Journal Of the American Chemical Societymentioning

confidence: 97%

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Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron–Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis

et al. 2017

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An [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, is a model system for biological H activation. In addition to the catalytic H-cluster, CpI contains four accessory iron-sulfur [FeS] clusters in a branched series that transfer electrons to and from the active site. In this work, potentiometric titrations have been employed in combination with electron paramagnetic resonance (EPR) spectroscopy at defined electrochemical potentials to gain insights into the role of the accessory clusters in catalysis. EPR spectra collected over a range of potentials were deconvoluted into individual components attributable to the accessory [FeS] clusters and the active site H-cluster, and reduction potentials for each cluster were determined. The data suggest a large degree of magnetic coupling between the clusters. The distal [4Fe-4S] cluster is shown to have a lower reduction potential (∼ < -450 mV) than the other clusters, and molecular docking experiments indicate that the physiological electron donor, ferredoxin (Fd), most favorably interacts with this cluster. The low reduction potential of the distal [4Fe-4S] cluster thermodynamically restricts the Fd/Fd ratio at which CpI can operate, consistent with the role of CpI in recycling Fd that accumulates during fermentation. Subsequent electron transfer through the additional accessory [FeS] clusters to the H-cluster is thermodynamically favorable.

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Section: Resultssupporting

confidence: 91%

Section: Journal Of the American Chemical Societymentioning

confidence: 97%

Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron–Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis

et al. 2017

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“… 61 Triplet states resulting from the spin–spin interaction between two S = 1/2 species have also been observed in reduced ferredoxins containing two [4Fe–4S] clusters and in di- and trimethylamine hydrogenases containing a flavin radical and a Fe–S cluster. 62 , 63 …”

Section: Discussionmentioning

confidence: 99%

“…More recently, a biradical redox state that produced a triplet EPR signal was identified in the pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenase, which was attributed to a cysteine disulfide radical dipolarly coupled to a PQQ radical . Triplet states resulting from the spin–spin interaction between two S = 1/2 species have also been observed in reduced ferredoxins containing two [4Fe–4S] clusters and in di- and trimethylamine hydrogenases containing a flavin radical and a Fe–S cluster. , …”

Section: Discussionmentioning

confidence: 99%

Unique Biradical Intermediate in the Mechanism of the Heme Enzyme Chlorite Dismutase

et al. 2021

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The heme enzyme chlorite dismutase (Cld) catalyzes O–O bond formation as part of the conversion of the toxic chlorite (ClO 2 – ) to chloride (Cl – ) and molecular oxygen (O 2 ). Enzymatic O–O bond formation is rare in nature, and therefore, the reaction mechanism of Cld is of great interest. Microsecond timescale pre-steady-state kinetic experiments employing Cld from Azospira oryzae ( Ao Cld), the natural substrate chlorite, and the model substrate peracetic acid (PAA) reveal the formation of distinct intermediates. Ao Cld forms a complex with PAA rapidly, which is cleaved heterolytically to yield Compound I, which is sequentially converted to Compound II. In the presence of chlorite, Ao Cld forms an initial intermediate with spectroscopic characteristics of a 6-coordinate high-spin ferric substrate adduct, which subsequently transforms at k obs = 2–5 × 10 4 s –1 to an intermediate 5-coordinated high-spin ferric species. Microsecond-timescale freeze-hyperquench experiments uncovered the presence of a transient low-spin ferric species and a triplet species attributed to two weakly coupled amino acid cation radicals. The intermediates of the chlorite reaction were not observed with the model substrate PAA. These findings demonstrate the nature of physiologically relevant catalytic intermediates and show that the commonly used model substrate may not behave as expected, which demands a revision of the currently proposed mechanism of Clds. The transient triplet-state biradical species that we designate as Compound T is, to the best of our knowledge, unique in heme enzymology. The results highlight electron paramagnetic resonance spectroscopic evidence for transient intermediate formation during the reaction of Ao Cld with its natural substrate chlorite. In the proposed mechanism, the heme iron remains ferric throughout the catalytic cycle, which may minimize the heme moiety’s reorganization and thereby maximize the enzyme’s catalytic efficiency.

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“…This gives rise to the lag period and, when the enzyme concentration increases, this process takes less time, resulting in a shorter lag period, and when the monophenol concentration increases, more diphenol is accumulated, the lag period becoming longer. amino acid sequences (George et al, 1985), a three-dimensional structure (Adman et al, 1976;Backes et al, 1991) and a host of data on the electromagnetic properties and topology of the [4Fe-4S] clusters (Mathews et al, 1974;Moulis et al, 1984).…”

mentioning

confidence: 99%

Laser flash photolysis studies of electron transfer to the cytochrome b5-cytochrome c complex

Rivera²,

Fa³

et al. 1993

Biochemistry

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Rate constants for electron transfer in the complex between recombinant rat mitochondrial outer membrane cytochrome b5 or the tryptic fragment of bovine liver cytochrome b5 and horse mitochondrial cytochrome c were measured by laser flash photolysis of 5-deazariboflavin-EDTA solutions. When an excess of cytochrome b5 was titrated with increasing amounts of cytochrome c at low ionic strength and electron transfer was initiated by a laser flash, both proteins were rapidly reduced by deazariboflavin semiquinone. The initial photoreduction was followed by a slower second-order reduction of b5 complexed oxidized cytochrome c by free reduced cytochrome b5. At an 8:1 ratio of cytochromes b5 to c, the pseudo-first-order rate constant for reduction of complexed cytochrome c increased 3-5-fold between ionic strengths of 5 and 40 mM, and then dropped precipitously at higher ionic strengths. The ionic strength dependent increase in rate constant is likely to be due to relief of steric hindrance via rearrangement of cytochrome c in the complex. The reaction rate showed no sign of saturation at any ionic strength, indicating a first-order rate constant greater than 10(4) s-1 within a transient ternary protein complex; i.e., interprotein electron transfer approaches the largest values previously reported for the stable binary protein complex (approximately 4 x 10(5) s-1). Our results emphasize the flexibility of electron-transfer protein complexes, which had previously been modeled in a single conformation with specific salt bridges. It appears that a variety of orientations can exist within such protein-protein complexes and that the population of conformations changes with ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)

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On the Nature of the Spin Coupling between the Iron-Sulfur Clusters in the Eight-Iron Ferredoxins

Cited by 158 publications

References 8 publications

Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron–Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis

Reduction Potentials of [FeFe]-Hydrogenase Accessory Iron–Sulfur Clusters Provide Insights into the Energetics of Proton Reduction Catalysis

Unique Biradical Intermediate in the Mechanism of the Heme Enzyme Chlorite Dismutase

Laser flash photolysis studies of electron transfer to the cytochrome b5-cytochrome c complex

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