Biomedical subjects

M K Johnson

Publications and source records attributed to M K Johnson.

At least 55 records · Page 3Linked to original sources

PubMed1999

Congenital radius deficiency: radiographic outcome and survivorship analysis.

The experience with congenital radius deficiency, or radial hemimelia, at the Shriners' Hospital for Children, Los Angeles Unit, was reviewed. A cohort of 29 limbs in 23 patients was identified with an average follow-up period of 50 months. Radiographic parameters were assessed using the hand-forearm angle, hand-forearm position, and ulnar bow. We compared radialization to modified centralization, assessed the efficacy of ulnar osteotomy, and assessed the effect of age, preoperative deformity, ulnar osteotomy, and Bayne's type on the final result. Revisions were noted and a survivorship analysis performed. The cohort had statistically significant correction of hand-forearm angle and hand-forearm position. Radialization was similar to modified centralization in the final outcome. Ulnar osteotomy was an efficacious way to correct ulnar deformity. Age, preoperative deformity, performance of an ulnar osteotomy, and Bayne's type did not affect the final wrist position. Survivorship analysis was performed using revision as the end point, with a survivorship rate at 5 years of 67%. Significant risk factors for revision included radial or positive hand-forearm angle and young age at the time of the index procedure. There was a suggestion that small postoperative hand-forearm position, or radial translation, increased the risk of revision. Preoperative deformity, performance of an ulnar osteotomy, and Bayne's type did not affect the risk of revision. These data offer support for the hypothesis that a more ulnar translation and an ulnar angulation of the wrist is a means of reducing the radial lever arm and thus the incidence of deformity recurrence and need for revision.

Child↗

PubMed1999

Source memory and eyewitness suggestibility in older adults.

The authors investigated the influence of test format on the source-memory performance of older adults (N = 128). Each participant viewed a picture and wrote a description of the scene. Then half of the participants (control group) read a text that accurately described the scene; the other half (misled group) read a text that contained misinformation. After writing another scene description, the participants were given a surprise memory test. Half were given a yes/no recognition test, and half were given a source-monitoring test. The misled yes/no participants mistakenly indicated more often than the control yes/no participants that misleading-text items were in the picture (suggestibility effect). There was no suggestibility effect for source-monitoring participants. The data are discussed in terms of the source monitoring framework.

Age Factors↗

PubMed1999

The active form of the ferric heme in neutrophil cytochrome b(558) is low-spin in the reconstituted cell-free system in the presence of amphophil.

The spin state of the heme in superoxide (O(2)(.)(-))-producing cytochrome b(558) purified from pig neutrophils was examined by means of room-temperature magnetic circular dichroism (MCD) under physiological conditions. Cytochrome b(558) with varying amounts of low-spin and high-spin heme was prepared by either pH adjustment or heat treatment, and the O(2)(.)(-)-forming activity in a cell-free system was found to correlate with the low-spin heme content. The possibility that the O(2)(.)(-)-forming activity results from a transient high-spin ferric heme form that is induced during activation by anionic amphophils has also been investigated. EPR spectra of cytochrome b(558) activated by either arachidonic acid or myristic acid, showed that a transient high-spin ferric species accounting for approximately 50% of the heme appeared in the presence of arachidonic acid, but not in the presence of myristic acid. Hence the appearance of a transient high-spin ferric heme species on activation with an amphophil does not afford a common activation mechanism in the NADPH oxidase system. The EPR results for cytochrome b(558) activated with arachidonic acid showed that the transient high-spin ferric heme can bind cyanide. However, the high-spin ferric heme does not contribute to the O(2)(.)(-) production of cytochrome b(558) in cell-free assays in the presence of cyanide.

Animals↗

PubMed1999

Facilitation and impairment of event memory produced by photograph review.

Selectively reviewing some items from a larger set of previously learned items increases memory for the items that are reviewed but may also be accompanied by a cost: Memory for the nonreviewed items may be impaired relative to cases where no review occurs at all. This cost to nonreviewed items has primarily been shown in contexts of verbal list learning and in situations where the reviewed and nonreviewed items are categorically or semantically related. Using a more naturalistic impetus to selective review--photographs relating to previously experienced events--we assessed whether the memory of older and younger adults for unrelated complex activities that they themselves had performed was also impaired due to nonreview. Both younger and older adults showed impaired memory for nonreviewed activities when tested with free recall (Experiment 1), but not when tested with recognition or cued recall (Experiment 2). If mitigating retrieval cues are unavailable, selective review may impair memory for nonreviewed everyday events.

Adolescent↗

PubMed1998

Heterologous biosynthesis and characterization of the [2Fe-2S]-containing N-terminal domain of Clostridium pasteurianum hydrogenase.

The primary structure of Clostridium pasteurianum hydrogenase I appears to be composed of modules suggesting that the various iron-sulfur clusters present in this enzyme might be segregated in structurally distinct domains. On the basis of this observation, a gene fragment encoding the 76 N-terminal residues of this enzyme has been expressed in Escherichia coli. The polypeptide thus produced contains a [2Fe-2S]n+ cluster of which the oxidized level (n = 2) has been monitored by UV-visible absorption, circular dichroism, and resonance Raman spectroscopy. This cluster can be reduced by dithionite or electrochemically to the n = 1 level which has been investigated by EPR and by low-temperature magnetic circular dichroism. The redox potential of the +2 to +1 transition is -400 mV (vs the normal hydrogen electrode). The spectroscopic and redox results indicate a [2Fe-2S]2+/+ chromophore coordinated by four cysteine ligands in a protein fold similar to that found in plant- and mammalian-type ferredoxins. Among the five cysteines present in the N-terminal hydrogenase fragment, four (in positions 34, 46, 49, and 62) are conserved in other sequences and are therefore the most likely ligands of the [2Fe-2S] site. The fifth cysteine, in position 39, can be dismissed on the grounds that the Cys39Ala mutation does not alter any of the properties of the iron-sulfur cluster. The spectroscopic signatures of this chromophore are practically identical with some of those reported for full-size hydrogenase. This confirms that C. pasteurianum hydrogenase I contains a [2Fe-2S] cluster and indicates that the polypeptide fold around the metal site of the N-terminal fragment is very similar, if not identical, to that occurring in the full-size protein. The N-terminal sequence of this hydrogenase is homologous to sequences of a number of proteins or protein domains, including a subunit of NADH-ubiquinone oxidoreductase of respiratory chains. From that, it can be anticipated that the structural domain isolated and described here is a building block of electron transfer complexes involved in various bioenergetic processes.

Amino Acid Sequence↗

PubMed1998

Left prefrontal activation during episodic remembering: an event-related fMRI study.

The major current theory relating brain mechanisms in prefrontal cortex (PFC) to memory for discrete events (episodic memory) emphasizes the role of right PFC during retrieval. Using event-related fMRI, we found both right and left PFC activity during episodic remembering, but only the left PFC activity was related to the amount of episodic detail required at test. We suggest that right PFC subserves relatively simple, heuristic, cognitive processes and that left PFC is recruited for more reflectively demanding, systematic, processes. Episodic remembering often requires such systematic processes and, under those circumstances, recruits left, as well as right, PFC.

Acoustic Stimulation↗

PubMed1998

Evidence that the fourth ligand to the [2Fe-2S] cluster in animal ferrochelatase is a cysteine. Characterization of the enzyme from Drosophila melanogaster.

In a previous study, site-directed mutagenesis experiments identified three of the four ligands to the [2Fe-2S] cluster in animal ferrochelatase as conserved cysteines in the COOH-terminal extension, Cys-403, Cys-406, and Cys-411 in human ferrochelatase (Crouse, B. R., Sellers, V. M., Finnegan, M. G., Dailey, H. A. & Johnson, M. K. (1996) Biochemistry 35, 16222-16229). The nature of the fourth ligand was left unresolved, and spectroscopic studies raised the possibility of one noncysteinyl, oxygenic ligand. In this work, we report two lines of evidence that strongly suggest the fourth ligand is a cysteine residue. Cysteine at position 196 in human recombinant ferrochelatase when changed to a serine results in an inactive enzyme that is lacking the [2Fe-2S] cluster. Furthermore, whole cell EPR studies demonstrate that in the C196S mutant the cluster fails to assemble. Additionally, the cloning and expression of Drosophila melanogaster ferrochelatase has allowed the identification, by EPR and UV-visible spectroscopy, of a [2Fe-2S]2+ cluster with properties analogous to those of animal ferrochelatases. The observation that Drosophila ferrochelatase contains only four conserved cysteines at positions 196, 403, 406, and 411, is in accord with the proposal that these residues function as cluster ligands. In the case of the ferrochelatase iron-sulfur cluster ligands, NH2-Cys-X206-Cys-X2-Cys-X4-Cys-COOH, the position distant from other ligands may lead to a spatial positioning of the cluster near the enzyme active site or at the interface of two domains, thereby explaining the loss of enzyme activity that accompanies cluster degradation and reinforcing the idea that the cluster functions as a regulatory switch.

Amino Acid Sequence↗

PubMed1998

The Azotobacter vinelandii NifEN complex contains two identical [4Fe-4S] clusters.

The nifE and nifN gene products from Azotobacter vinelandii form an alpha2beta2 tetramer (NifEN complex) that is required for the biosynthesis of the nitrogenase FeMo cofactor. In the current model for NifEN complex organization and function, the complex is structurally analogous to the nitrogenase MoFe protein and provides an assembly site for a portion of FeMo cofactor biosynthesis. In this work, gene fusion and immobilized metal-affinity chromatography strategies were used to elevate the in vivo production of the NifEN complex and to facilitate its rapid and efficient purification. The NifEN complex produced and purified in this way exhibits an FeMo cofactor biosynthetic activity similar to that previously described for the NifEN complex purified by traditional chromatography methods. UV-visible, EPR, variable-temperature magnetic circular dichroism, and resonance Raman spectroscopies were used to show that the NifEN complex contains two identical [4Fe-4S]2+ clusters. These clusters have a predominantly S = 1/2 ground state in the reduced form, exhibit a reduction potential of -350 mV, and are likely to be coordinated entirely by cysteinyl residues on the basis of spectroscopic properties and sequence comparisons. A model is proposed where each NifEN complex [4Fe-4S] cluster is bridged between a NifE-NifN subunit interface at a position analogous to that occupied by the P clusters in the nitrogenase MoFe protein. In contrast to the MoFe protein P clusters, the NifEN complex [4Fe-4S] clusters are proposed to be asymmetrically coordinated to the NifEN complex where NifE cysteines-37, -62, and -124 and NifN cysteine-44 are the coordinating ligands. On the basis of a homology model of the three-dimensional structure of the NifEN complex, the [4Fe-4S] cluster sites are likely to be remote from the proposed FeMo cofactor assembly site and are unlikely to become incorporated into the FeMo cofactor during its assembly.

Artificial Gene Fusion↗

PubMed1998

Extensive ligand rearrangements around the [2Fe-2S] cluster of Clostridium pasteurianum ferredoxin.

The [2Fe-2S] cluster of the ferredoxin from Clostridium pasteurianum is coordinated by cysteines 11, 56, and 60 and by a fourth cysteine, residue 24 in the wild-type protein, located on a flexible and deletable loop around residues 14-30. New mutated forms of this ferredoxin show that the fourth cysteine ligand can be located in any one of positions 14, 16, 21, 24, or 26. Another set of molecular variants has unveiled a new case of ligand swapping on the cysteine 60 ligand site. Replacement of cysteine 60 by alanine and introduction of a cysteine in position 21 yielded a ferredoxin that assembles a [2Fe-2S] cluster of which the ligands are cysteines 11, 21, 24, and 56. This cysteine ligand pattern is similar to that occurring in plant-type or mammalian-type ferredoxins, although the overall sequence similarities are below detection. Moreover, the vibrational and electronic properties of the resulting [2Fe-2S]2+/+ center, as revealed by resonance Raman and EPR studies, are strikingly similar to those of mammalian-type ferredoxins. The extensive set of mutated forms of the C. pasteurianum ferredoxin now available indicates that cysteine ligand exchange may occur on residues 24 and 60, but not on residues 11 and 56. It is thus suggested that cysteines 24 and 60 are part of a solvent accessible aspect of the Fe-S cluster, whereas cysteines 11 and 56 are buried and form the more rigid part of the polypeptide ligand framework. In view of the unprecedented versatility of this [2Fe-2S] cluster and of its polypeptidic environment, the introduction of ligands other than cysteine in various positions has been attempted. These experiments have remained unsuccessful, and even including previous studies, noncysteinyl ligation has been obtained with this protein in only very few cases. The data provide an extensive confirmation that Fe-S clusters have a strong preference for thiolate ligation and rationalize the relatively rare occurrence of noncysteinyl ligation in native Fe-S proteins.

Amino Acid Substitution↗

PubMed1998

Role of the [Fe4S4] cluster in mediating disulfide reduction in spinach ferredoxin:thioredoxin reductase.

Thioredoxin reduction in plant chloroplasts is catalyzed by a unique class of disulfide reductases which use a one-electron donor, [Fe2S2]2+,+ ferredoxin, and has an active site involving a disulfide in close proximity to a [Fe4S4]2+ cluster. In this study, spinach ferredoxin:thioredoxin reductase (FTR) reduced with stoichiometric amounts of reduced benzyl viologen or frozen under turnover conditions in the presence of thioredoxin is shown to exhibit a slowly relaxing S = 1/2 resonance (g = 2.11, 2.00, 1.98) identical to that of a modified form of the enzyme in which one of the cysteines of the active-site disulfide is alkylated with N-ethylmaleimide (NEM-FTR). Hence, in accord with the previous proposal [Staples, C.R., Ameyibor, E., Fu, W., Gardet-Salvi, L., Stritt-Etter, A.-L., Schürmann, P., Knaff, D.B., and Johnson, M.K. (1996) Biochemistry 35, 11425-11434], NEM-FTR is shown to be a stable analogue of a one-electron-reduced enzymatic intermediate. The properties of the Fe-S cluster in NEM-FTR have been further investigated by resonance Raman and electron nuclear double resonance spectroscopies; the results, taken together with the previous UV-visible absorption, variable temperature magnetic circular dichroism, and resonance Raman data, indicate the presence of a novel type of [Fe4S4]3+ cluster that is coordinated by five cysteinates with little unpaired spin density delocalized onto the cluster-associated cysteine of the active-site disulfide. While the ligation site of the fifth cysteine remains undefined, the best candidate is a cluster bridging sulfide. On the basis of the spectroscopic and redox results, mechanistic schemes are proposed for the benzyl viologen-mediated two-electron-reduction of FTR and the catalytic mechanism of FTR. The catalytic mechanism involves novel S-based cluster chemistry to facilitate electron transfer to the active-site disulfide resulting in covalent attachment of the electron-transfer cysteine and generation of the free interchange cysteine that is required for the thiol-disulfide interchange reaction with thioredoxin.

Benzyl Viologen↗

PubMed1998

Insight into externally bound 5,10,15,20-tetrakis(2-N-methylpyridyl) porphyrinatopalladium(II), PdP(2), with B-form DNA duplexes poly(G-c)2, poly(A-t)2, and CT DNA by using combined MCD, CD, and optical data.

The Soret (B0) region of free and externally DNA-bound 5,10,15,20-tetrakis(2-N-methylpyridyl) porphyrinatopalladium(II), PdP(2), was investigated by electronic magnetic circular dichroism (MCD), natural circular dichroism (CD), and optical (UV-visible) absorption spectroscopies. We conclude that four-coordinate, "thick" PdP(2) binds to the exterior of each of poly(A-T)2 and calf thymus DNA (CT DNA) by two distinctly different AT-specific minor and major groove modes, with site 5'TA3' being favored for both modes. The minor groove mode involves an edge-on orientation of PdP(2), for which porphyrin electric dipole transition moments (edtms) mu x (most perturbed direction of the bound porphyrin) and mu y (least perturbed direction) have approximate orientation angles of alpha/beta/beta' = approximately 90 degrees/0 degree/0 degree and approximately 45 degrees/0 degree/90 degrees, respectively. Major groove binding is by a face-on mode, which results in the porphyrin plane being approximately parallel to the helix axis, such that mu x (most perturbed direction) and mu y (least perturbed direction) have approximate orientation angles of alpha/beta/beta' = approximately 45 degrees/180 degrees/90 degrees and approximately 45 degrees/180 degrees/270 degrees, respectively. The Soret MCD and optical band alterations upon binding (i.e., sign retention of the tetragonal, genuine MCD (+) A-term on becoming the (+) pseudo-A-term of similar amplitude and small DNA-induced optical red (delta lambda) and hypochromic (H) shifts) are all consistent with exterior binding perturbations of the porphyrin's p pi MOs (1a1u 3a2u 4eg) by the A and T bases of each polymer being weaker than caused by intercalation. Furthermore, that the (+) A-term of PdP(2) retains the (+) sign upon binding informs that the 4eg splitting, or delta LUMO, is less than the energy separation magnitude of 1a1u-3a2u, or delta HOMO. For the third system, PdP(2)/poly (G-C)2, the B0 CD spectrum has two extremely weak (+) and (-) CD bands at higher and lower energy, respectively, indicating that weak outside binding (wob) interactions are taking place between the cationic porphyrin and the electron-rich phosphate backbone of this rigid polymer. The composite of our CD, MCD, and optical data are suggestive of a face-on mode at the GC major groove. Band parameter extraction is performed on the Soret CD and MCD bands of each of the three bound systems, and it is determined that (1) very little spatial rotation of molecular charge is induced during CD excitation and (2) the excited state angular momentum, 'Lj', changes very little upon binding of PdP(2) to each duplex. These findings are also consistent with each PdP(2)/B-DNA interaction not being very strong.

Base Pairing↗

PubMed1998

Iron-sulfur proteins: new roles for old clusters.

Several major advances in our understanding of the structure, function and properties of biological iron-sulfur clusters have occurred in the past year. These include a new structural type of cluster in the inappropriately named prismane protein, the establishment of redox-mediated [Fe2S2]2+ <--> [Fe4S4]2+ cluster conversions, and the characterization of valence-delocalized [Fe2S2]+ and all ferrous clusters with [Fe2S2]0, [Fe3S4]2- and [Fe4S4]0 cores. The emergence of novel types of redox, regulatory and enzymatic roles have also raised the possibility of iron-sulfur clusters mediating two electron redox processes, coupling proton and electron transfer, and catalyzing disulfide reduction and reductive cleavage of S-adenosylmethionine via sulfur-based cluster chemistry.

Bacterial Proteins↗

PubMed1998

Using social science to inform the law of intestacy: the case of unmarried committed partners.

Laws of intestate succession determine how the estate of a person who dies without a will is distributed. Researchers have struggled with the question of how to infer the donative intent of persons who die intestate. Based on an empirical study of unmarried committed partners, we compare the usefulness of two methods of social research for informing intestacy law: will studies and interviews with living persons about their preferences for estate distribution. The results indicate that for some groups of unmarried committed partners, will studies may not adequately reflect the extent to which intestate decedents wish their partner to share in their estate. In addition, the results demonstrate a close correspondence between respondents' actual wills, when they had them, and their distributive preferences on hypothetical scenarios. These findings are discussed as they relate to an examination of which sources of social scientific evidence are most useful in informing the law of intestacy.

Adult↗

PubMed1998

Aging and source monitoring: cognitive processes and neuropsychological correlates.

This study shows that relative to younger adults, older adults are more adversely influenced by similar items when judging a memory's source, and the phenomenal features of their correctly and incorrectly attributed memories have greater overlap. The authors argue in accordance with the source monitoring framework that this age-related impairment in source accuracy is related to processes involved in binding features into complex memories and those involved in accessing and evaluating contextual features of memories. These processes are linked to medial temporal and frontal brain regions, respectively, as evidenced by correlations in older adults between source accuracy and neuropsychological tests often used to assess medial temporal and frontal function. The results suggest that adequate feature binding is particularly important when items from different source share similar features and access-evaluation processes are particularly important after a delay.

Adult↗

PubMed1998

Post-event review in older and younger adults: improving memory accessibility of complex everyday events.

Recalling an event at 1 time often increases the likelihood that it will be remembered at a still later time. The authors examined the degree to which older and younger adults' memory for everyday events that they watched on a videotape was improved by later seeing photographs or reading brief verbal descriptions of those events. Both older and younger adults recalled more events, in greater detail, with than without review. Verbal descriptions enhanced later recall to the same degree as reviewing photographs. Younger adults generally gained more from review than older adults on measures of the absolute number of details recalled and when facilitation was assessed relative to a no-review control condition, but not when memory for reviewed events was expressed as a proportion of each individual's total recall. Post-event review has clear potential practical benefits for improving memory of older adults.

Adult↗

PubMed1998

Effect of exercise induced hypoxaemia on myocardial repolarisation in severe chronic obstructive pulmonary disease.

BACKGROUND: Exercise training is being promoted increasingly for patients with chronic obstructive pulmonary disease (COPD). Many of these patients experience exercise related arterial desaturation but the clinical importance of these hypoxaemic episodes is not known. QTc dispersion is a marker of myocardial repolarisation abnormalities and there has been much interest in its role as a non-invasive predictor of cardiac arrhythmias and sudden death. However, little is known about the dynamic effects that exercise and hypoxaemia have on QTc dispersion in patients with COPD. METHODS: 20 patients with severe COPD (FEV1 < 40% predicted) undertook two 15 minute treadmill tests at a speed calculated to produce a constant workload of 50% maximum oxygen consumption (VO2max) during which they were blindly given either air or 35% oxygen in random order. Physiological measurements taken throughout exercise included 12 lead electrocardiograms from which QTc dispersion values were calculated according to standard criteria. Nine of the patients who desaturated with exercise were studied further. A similar degree of hypoxaemia was induced at rest by giving them a titrated mixture of air and oxygen and the changes in QTc dispersion were recorded. RESULTS: 11 of the 20 patients developed significant hypoxaemia (desaturation by > or = 5% to < 90%) with exercise breathing air. There were no significant changes in QTc dispersion with either exercise or hypoxaemia. There were no significant changes in QTc dispersion when comparing those who did and did not desaturate, and those with and without a high baseline QTc dispersion values (60 ms). Induced hypoxaemia without exercise also failed to worsen QTc dispersion. CONCLUSIONS: No evidence was found to suggest that exercise, even when associated with hypoxaemia, causes myocardial repolarisation abnormalities in patients with COPD.

Aged↗

PubMed1997

Source monitoring and memory distortion.

Memory distortion reflects failures to identify the sources of mental experience (reality monitoring failures or source misattributions). For example, people sometimes confuse what they inferred or imagined and what actually happened, what they saw and what was suggested to them, one person's actions and another's what they heard and what they previously knew, and fiction and fact. Source confusions arise because activated information is incomplete or ambiguous and the evaluative processes responsible for attributing information to sources are imperfect. Both accurate and inaccurate source attributions result from heuristic processes and more reflectively complex processes that evaluate a mental experience for various qualities such as amount and type of perceptual, contextual, affective, semantic and cognitive detail, that retrieve additional supporting or disconfirming evidence, and that evaluate plausibility and consistency given general knowledge, schemes, biases and goals. Experimental and clinical evidence regarding cognitive mechanisms and underlying brain structures of source monitoring are discussed.

Humans↗

PubMed1997

[2Fe-2S] to [4Fe-4S] cluster conversion in Escherichia coli biotin synthase.

The type and properties of the Fe-S cluster in recombinant Escherichia coli biotin synthase have been investigated in as-prepared and dithionite-reduced samples using the combination of UV-visible absorption and variable-temperature magnetic circular dichroism (VTMCD), EPR, and resonance Raman spectroscopies. The results confirm the presence of one S = 0 [2Fe-2S]2+ cluster in each subunit of the homodimer in aerobically purified samples, and the Fe-S stretching frequencies suggest incomplete cysteinyl-S coordination. However, absorption and resonance Raman studies show that anaerobic reduction with dithionite in the presence of 60% (v/v) ethylene glycol or glycerol results in near-stoichiometric conversion of two [2Fe-2S]2+ clusters to form one S = 0 [4Fe-4S]2+ cluster with complete cysteinyl-S coordination. The stoichiometry and ability to effect reductive cluster conversion without the addition of iron or sulfide suggest that the [4Fe-4S]2+ cluster is formed at the subunit interface via reductive dimerization of [2Fe-2S]2+ clusters. EPR and VTMCD studies indicate that more than 50% of the Fe is present as [4Fe-4S]+ clusters in samples treated with 60% (v/v) glycerol after prolonged dithionite reduction. The [4Fe-4S]+ cluster exists as a mixed spin system with S = 1/2 (g = 2. 044, 1.944, 1.914) and S = 3/2 (g = 5.6 resonance) ground states. Subunit-bridging [4Fe-4S]2+,+ clusters, that can undergo oxidative degradation to [2Fe-2S]2+ clusters during purification, are proposed to be a common feature of Fe-S enzymes that require S-adenosylmethionine and function by radical mechanisms involving the homolytic cleavage of C-H or C-C bonds, i.e., biotin synthase, anaerobic ribonucleotide reductase, pyruvate formate lyase, lysine 2, 3-aminomutase, and lipoic acid synthase. The most likely role for the [4Fe-4S]2+,+ cluster lies in initiating the radical mechanism by directly or indirectly facilitating reductive one-electron cleavage of S-adenosylmethionine to form methionine and the 5'-deoxyadenosyl radical. It is further suggested that oxidative cluster conversion to [2Fe-2S]2+ clusters may play a physiological role in these radical enzymes, by providing a method of regulating enzyme activity in response to oxidative stress, without irreversible cluster degradation.

Dithionite↗