FRAGMENTATION OF THE ELECTRON TRANSPORT CHAIN OF ESCHERICHIA COLI. PREPARATION OF A SOLUBLE FORMATE DEHYDROGENASE-CYTOCHROME B1 COMPLEX

Chemiosmotic coupling in oxidative and photosynthetic phosphorylation. 1966

50 years ago Peter Mitchell proposed the chemiosmotic hypothesis for which he was awarded the Nobel Prize for Chemistry in 1978. His comprehensive review on chemiosmotic coupling known as the first "Grey Book", has been reprinted here with permission, to offer an electronic record and easy access to this important contribution to the biochemical literature. This remarkable account of Peter Mitchell's ideas originally published in 1966 is a landmark and must-read publication for any scientist in the field of bioenergetics. As far as was possible, the wording and format of the original publication have been retained. Some changes were required for consistency with BBA formats though these do not affect scientific meaning. A scanned version of the original publication is also provided as a downloadable file in Supplementary Information and can be found online at doi:10.1016/j.bbabio.2011.09.018. See also Editorial in this issue by Peter R. Rich. Original title: CHEMIOSMOTIC COUPLING IN OXIDATIVE AND PHOTOSYNTHETIC PHOSPHORYLATION, by Peter Mitchell, Glynn Research Laboratories, Bodmin, Cornwall, England.

BIOLOGICAL FORMATION OF MOLECULAR HYDROGEN

From a general standpoint, the formation of molecular hydrogen can be considered a device for disposal of electrons released in metabolic oxidations. We presume that this means of performing anaerobic oxidations is of ancient origin and that the hydrogen-evolving system of strict anaerobes represents a primitive form of cytochrome oxidase, which in aerobes effects the terminal step of respiration, namely the disposal of electrons by combination with molecular oxygen. We further assume that the original pattern of reactions leading to H(2) production has become modified in various ways (with respect to both mechanisms and functions) during the course of biochemical evolution, and we believe that this point of view suggests profitable approaches for clarifying a number of problems in the intermediary metabolism of microorganisms which produce or utilize H(2). Of special general importance in this connection is the basic problem of defining more precisely the fundamental elements in the regulatory control of anaerobic energy metabolism. Among the more specific aspects awaiting further elucidation are: the relations between formation of H(2) and use of H(2) as a primary reductant for biosynthetic purposes; the various forms of direct and indirect interactions between hydrogenase and N(2) reduction systems; and the transitional stages between anaerobic and aerobic energy-metabolism patterns of facultative organisms.

Characterization of an Escherichia coli mutant pleiotropically altered in membrane-bound oxidoreductase activities

An Escherichia coli mutant pleiotropically altered in membrane-bound oxidoreductase activities was isolated following nitrosoguanidine treatment. Mutant R23 was able to grow on glucose, but was unable to grow on succinate or other oxidizable substrates as a sole energy source. Isolated membranes prepared from R23 failed to oxidize succinate and formate; while NADH was oxidized at a reduced rate by membranes. The mutant also exhibited markedly reduced cytochrome content, but normal DL-lactate PMS reductase and H(+)-translocating ATPase activities relative to the parent strain. Bacteriophage Plkc was used to transduce R23 to growth on glycerol, DL-lactate or succinate; regardless of the selection procedure, each of the 179 transductants had gained the ability to grow on all three substrates. The suc- mutation in R23 appeared to be responsible for the loss of growth on oxidizable substrates, altered membrane-bound oxidoreductase activities, resistance to neomycin, and reduced levels of cytochrome components. The suc- mutation was localized in the 6 to 6.5 min region of the E. coli chromosome map utilizing episomal transfers.

An Escherichia coli mutant conditionally altered in respiratory chain components

Nitrosoguanidine mutagenesis was employed to isolate an Escherichia coli mutant conditionally altered in respiratory chain components. Mutant R25 was able to grow on glucose, fructose, and glycerol but failed to grow on succinate and acetate (suc-). Also, R25 exhibited leaky growth on DL-lactate, fumarate, and malate (lct*). The lct* mutation pleiotropically affected a number of respiratory chain components and its expression was conditional with the growth substrate. Glucose-grown R25 resting cell suspensions oxidized DL-lactate and formate; however, these two substrates were not oxidized by fructose- or glycerol-grown cell suspensions. The same conditional pattern was observed for the concentration of cytochrome components, the membrane-associated oxidation of NADH and formate, and formate phenazine methosulfate (PMS) reductase activity; succinate oxidase and PMS reductase activities were not exhibited by membranes under any growth condition due to the suc- mutation. R25 membrane-associated H(+)-translocating ATPase activity was not conditional with the growth substrate. R25PC, a spontaneous lct+ suc- partial revertant of R25, did not exhibit the conditional pattern of R25. The lct* mutation was found to map in the 27-30-min region and the suc- mutation in the 15-17-min region of the E. coli genome. Two distinct classes of R25 P1kc transductants were isolated that differed in both their growth response on succinate and DL-lactate and their oxidase activities.

The formate dehydrogenase involved in electron transport from formate to fumarate in Vibrio succinogenes

1. The formate dehydrogenase of Vibrio succinogenes, which is involved in electron transport with fumarate as terminal acceptor, was solubilized with Triton X-100 and purified some 200-fold by means of chromatography on hydroxyapatite, sucrose-density-gradient centrifugation and chromatography on DEAE-Sephadex. Gel filtration failed to increase the specific acitivity of the enzyme while gel electrophoresis in the presence of dodecylsulfate revealed that 73% of the protein of the preparation consisted of a polypeptide of Mr 110 000. The Mr of the functional enzyme was found to be 263 000 on the basis of the Stokes radius (5.8 nm) and the sedimentation coefficient (11.3 S). 2. The preparation contained 9 micronmol molybdenum/g protein and about 170 mumol iron-sulfur/g protein. The contents of b and c cytochromes varied and were lower than that of molybdenum. The low-potential cytochrome b [Kröger, A. and Innerhofer, A. (1976) Eur. J. Biochem. 69, 497-506] present in the preparation was reduced by formate. 3. The preparation catalyzed the reduction of a variety of dyes by formate, but not of NAD, FMN, ferredoxin or oxygen. The reduction of CO2 or bicarbonate by reduced methyl viologen was not catalyzed. The reaction with benzyl viologen obeyed the rate law consistent with a ping-pong mechanism. The Km for formate was 1.5 mM at infinite concentration of benzyl viologen while that for benzyl viologen was 0.53 mM at infinite formate concentration. Enzymic activity was inhibited by azide, KCN and HgCl2, but not by 4-chloromercuriphenylsulfonate or 2-(n-nonyl)-4-hydroxyquinoline-N-oxide, both of which inhibit overall electron transport. The inhibition by azide was competitive with formate; the Ki was 45 micron. 4. The midpoint potential of the low-potential cytochrome b of the membrane fraction was shifted -40 mV by the presence of 2-(n-nonyl)-4-hydroxyquinoline-N-oxide. 5. It is concluded that the formate dehydrogenase of V. succinogenes is isolated as a dimer consisting of two identical subunits of Mr 110,000, each of which carries one atom of molybdenum and iron-sulfur groups. The low-potential cytochrome b is the direct acceptor for the electrons of formate dehydrogenase in the electron transport of formate-fumarate reduction of V. succinogenes. Inhibition of electron transport of the membrane fraction between formate dehydrogenase and menaquinone by 2-(n-nonyl)-4-hydroxyquinoline-N-oxide [Kröger, A. and Innerhofer, A. (1976) Eur. J. Biochem. 69, 487-495] is caused by the inhibitor binding to the low-potential cytochrome b.

Formation of the formate-nitrate electron transport pathway from inactive components in Escherichia coli

When Escherichia coli was grown on medium containing 10 mM tungstate the formation of active formate dehydrogenase, nitrate reductase, and the complete formate-nitrate electron transport pathway was inhibited. Incubation of the tungstate-grown cells with 1 mM molybdate in the presence of chloramphenicol led to the rapid activation of both formate dehydrogenase and nitrate reductase, and, after a considerable lag, the complete electron transport pathway. Protein bands which corresponded to formate dehydrogenase and nitrate reductase were identified on polyacrylamide gels containing Triton X-100 after the activities were released from the membrane fraction and partially purified Cytochrome b1 was associated with the protein band corresponding to formate dehydrogenase but was not found elsewhere on the gels. When a similar fraction was prepared from cells grown on 10 mM tungstate, an inactive band corresponding to formate dehydrogenase was not observed on polyacrylamide gels; rather, a new faster migrating band was present. Cytochrome b1 was not associated with this band nor was it found anywhere else on the gels. This new band disappeared when the tungstate-grown cells were incubated with molybdate in the presence of chloramphenicol. The formate dehydrogenase activity which was formed, as well as a corresponding protein band, appeared at the original position on the gels. Cytochrome b1 was again associated with this band. The protein band which corresponded to nitrate reductase also was severely depressed in the tungstate-grown cells and a new faster migrating band appeared on the polyacrylamide gels. Upon activation of the nitrate reductase by incubation of the cells with molybdate, the new band diminished and protein reappeared at the original position. Most of the nitrate reductase activity which was formed appeared at the original position of nitrate reductase on gels although some was present at the position of the inactive band formed by tungstate-grown cells. Apparently, inactive forms of both formate dehydrogenase and nitrate reductase accumulate during growth on tungstate which are electrophoretically distinct from the active enzymes. Activation by molybdate results in molecular changes which include the reassociation of cytochrome b1 with formate dehydrogenase and restoration of both enzymes to their original electrophoretic mobilities.

Anaerobic cytochrome b1 in Escherichia coli: association with and regulation of nitrate reductase

Nitrate reductase solubilized from the membrane of Escherichia coli by alkaline heat treatment was purified to homogeneity and used to prepare specific antibody. Nitrate reductase, precipitated by this antibody from Triton extracts of the membrane, contained a third subunit, not present in the purified enzyme used to prepare the antibody. This third subunit was identified as the cytochrome b1 apoprotein. This cytochrome is bound to nitrate reductase from wild-type E. coli in a ratio of 2 mol of cytochrome per mol of enzyme complex. In mutants unable to synthesize heme, this cytochrome b1 apoprotein is not bound to nitrate reductase. In these same mutants, the enzyme is overproduced and accumulates in the cytoplasm. The absence of cytochrome also affects the stability of the membrane-bound form of the enzyme.

Factor 420-dependent pyridine nucleotide-linked formate metabolism of Methanobacterium ruminantium

Methanobacterium ruminantium was shown to possess a formate dehydrogenase which is linked to factor 420 (F420) as the first low-molecular-weight or anionic electron transfer coenzyme. Reduced F420 obtained from the formate dehydrogenase can be further linked to the formation of hydrogen via the previously described F420-dependent hydrogenase reaction, thus constituting an apparently simple formate hydrogenlyase system, or to the reduction of nicotinamide adenine dinucleotide phosphate via F420:nicotinamide adenine dinucleotide phosphate oxidoreductase. The results indicate that hydrogen and formate, the only known energy sources for M. ruminantium and many other methanogenic bacteria, should be essentially equivalent as sources of electrons in the metabolism of this organism.

Respiration and protein synthesis in Escherichia coli membrane-envelope fragments. VI. Solubilization and characterization of the electron transport chain

Membranes obtained from Escherichia coli have been solubilized with deoxycholate. The solubilized dehydrogenases and cytochromes are not sedimented at 105,000 g. These components readily penetrate the "included space" of Sepharose 4B (Pharmacia Fine Chemicals Inc., Uppsala, Sweden) and polyacrylamide gels and have been fractionated on the basis of molecular size. Solubilization destroys nicotinamide adenine dinucleotide, reduced form (NADH) oxidase and D-lactate oxidase activities, but leaves an appreciable part of the original succinoxidase activity intact. Evidence for a succinate dehydrogenase-cytochrome b(1) complex is given. Menadione added to the solubilized preparation does not elicit NADH oxidase activity nor stimulate the existing succinoxidase activity, but does provoke an active D-lactate oxidase activity. This D-lactate oxidase activity, however, does not use cytochromes and is not sensitive to cyanide.

Effects of selenium compounds on formate metabolism and coincidence of selenium-75 incorporation and formic dehydrogenase activity in cell-free preparations of Escherichia coli

The effects of selenite, selenocystine, and selenomethionine in a defined growth medium on formic dehydrogenase biosynthesis in aerobically and anaerobically grown Escherichia coli have been studied. Sucrose gradient centrifugation of a partially purified enzyme demonstrated a coincidence of (75)Se incorporation and formic dehydrogenase activity.

Formate dehydrogenase from Clostridium acidiurici

Partial purification of formate dehydrogenase from Clostridium acidiurici has been accomplished, and some properties of the enzyme have been determined. The molecular weight of the protein is at least 200,000 daltons. The enzyme showed marked instability to freezing and thawing and was inhibited strongly by oxygen and by light. Such inhibition was not reversed by incubation in the presence of thiol compounds. Cyanide inhibited the enzyme 90% at 0.1 mm concentrations, but ethylenediaminetetraacetate produced only slight inhibition at concentrations as high as 50 mm. The purified enzyme showed no ferredoxin activity in the Clostridium pasteurianum clastic system during pyruvate oxidation. Crude preparations of the enzyme could be coupled through ferredoxin to the reduction of nicotinamide adenine dinucleotide during formate oxidation, but the purified enzyme could not catalyze the reduction of pyridine nucleotides by formate in the presence of ferredoxin. Formate oxidation with the purified enzyme was readily coupled to benzyl viologen reduction, in which case ferredoxin was not required. An exchange between formate and bicarbonate was catalyzed by both crude and purified preparations of the enzyme, but the net synthesis of formate from CO(2) was not accomplished.

Effects of molybdate and selenite on formate and nitrate metabolism in Escherichia coli

The effects of adding molybdate and selenite to a glucose-minimal salts medium on the formation of enzymes involved in the anaerobic metabolism of formate and nitrate in Escherichia coli have been studied. When cells were grown anaerobically in the presence of nitrate, molybdate stimulated the formation of nitrate reductase and a b-type cytochrome, resulting in cells that had the capacity for active nitrate reduction in the absence of formate dehydrogenase. Under the same conditions, selenite in addition to molybdate was required for forming the enzyme system which permits formate to serve as an effective electron donor for nitrate reduction. When cells were grown anaerobically on a glucose-minimal salts medium without nitrate, active hydrogen production from formate as well as formate dehydrogenase activity depended on the presence of both selenite and molybdate. The effects of these metals on the formation of formate dehydrogenase was blocked by chloramphenicol, suggesting that protein synthesis is required for the increases observed. It is proposed that the same formate dehydrogenase is involved in nitrate reduction, hydrogen production, and in aerobic formate oxidation.

Nitrate reductase complex of Escherichia coli K-12: participation of specific formate dehydrogenase and cytochrome b1 components in nitrate reduction

The participation of distinct formate dehydrogenases and cytochrome components in nitrate reduction by Escherichia coli was studied. The formate dehydrogenase activity present in extracts prepared from nitrate-induced cells of strain HfrH was active with various electron acceptors, including methylene blue, phenazine methosulfate, and benzyl viologen. Certain mutants which are unable to reduce nitrate had low or undetectable levels of formate dehydrogenase activity assayed with methylene blue or phenazine methosulfate as electron acceptor. Of nine such mutants, five produced gas when grown anaerobically without nitrate and possessed a benzyl viologen-linked formate dehydrogenase activity, suggesting that distinct formate dehydrogenases participate in the nitrate reductase and formic hydrogenlyase systems. The other four mutants formed little gas when grown anaerobically in the absence of nitrate and lacked the benzyl viologen-linked formate dehydrogenase as well as the methylene blue or phenazine methosulfate-linked activity. The cytochrome b(1) present in nitrate-induced cells was distinguished by its spectral properties and its genetic control from the major cytochrome b(1) components of aerobic cells and of cells grown anaerobically in the absence of nitrate. The nitrate-specific cytochrome b(1) was completely and rapidly reduced by 1 mm formate but was not reduced by 1 mm reduced nicotinamide adenine dinucleotide; ascorbate reduced only part of the cytochrome b(1) which was reduced by formate. When nitrate was added, the formate-reduced cytochrome b(1) was oxidized with biphasic kinetics, but the ascorbate-reduced cytochrome b(1) was oxidized with monophasic kinetics. The inhibitory effects of n-heptyl hydroxyquinoline-N-oxide on the oxidation of cytochrome b(1) by nitrate provided evidence that the nitrate-specific cytochrome is composed of two components which have different redox potentials but identical spectral properties. We conclude from these studies that nitrate reduction in E. coli is mediated by the sequential operation of a specific formate dehydrogenase, two specific cytochrome b(1) components, and nitrate reductase.