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C C Moser

Publications and source records attributed to C C Moser.

11 recordsLinked to original sources
PubMed1998

A reductant-induced oxidation mechanism for complex I.

A model for energy conversion in Complex I is proposed that is a conservative expansion of Mitchell's Q-cycle using a simple mechanistic variation of that already established experimentally for Complex III. The model accommodates the following proposals. (1) The large number of flavin and iron-sulfur redox cofactors integral to Complex I form a simple but long electron transfer chain guiding submillisecond electron transfer from substrate NADH in the matrix to the [4Fe-4S] cluster N2 close to the matrix-membrane interface. (2) The reduced N2 cluster injects a single electron into a ubiquinone (Q) drawn from the membrane pool into a nearby Qnz site, generating an unstable transition state semiquinone (SQ). The generation of a SQ species is the primary step in the energy conversion process in Complex I, as in Complex III. In Complex III, the SQ at the Qo site near the cytosolic side acts as a strong reductant to drive electronic charge across the membrane profile via two hemes B to a Qi site near the matrix side. We propose that in Complex I, the SQ at the Qnz site near the matrix side acts as a strong oxidant to pull electronic charge across the membrane profile via a quinone (Qny site) from a Qnx site near the cytosolic side. The opposing locations of matrix side Qnz and cytosolic side Qo, together with the opposite action of Qnz as an oxidant rather than a reductant, renders the Complex I and III processes vectorially and energetically complementary. The redox properties of the Qnz and Qo site occupants can be identical. (3) The intervening Qny site of Complex I acts as a proton pumping element (akin to the proton pump of Complex IV), rather than the simple electron guiding hemes B of Complex III. Thus the transmembrane action of Complex I doubles to four (or more) the number of protons and charges translocated per NADH oxidized and Q reduced. The Qny site does not exchange with the pool and may even be covalently bound. (4) The Qnx site on the cytosol side of Complex I is complementary to the Qi site on the matrix side of Complex III and can have the same redox properties. The Qnx site draws QH2 from the membrane pool to be oxidized in two single electron steps. Besides explaining earlier observations and making testable predictions, this Complex I model re-establishes a uniformity in the mechanisms of respiratory energy conversion by using engineering principles common to Complexes III and IV: (1) all the primary energy coupling reactions in the different complexes use oxygen chemistry in the guise of dioxygen or ubiquinone, (2) these reactions are highly localized structurally, utilizing closely placed catalytic redox cofactors, (3) these reactions are also highly localized energetically, since virtually all the free energy defined by substrates is conserved in the form of transition state that initiates the transmembrane action and (4) all complexes possess apparently supernumerary oxidation-reduction cofactors which form classical electron transfer chains that operate with high directional specificity to guide electron at near zero free energies to and from the sites of localized coupling.

Electron Transport↗
PubMed1995

Ubiquinone pair in the Qo site central to the primary energy conversion reactions of cytochrome bc1 complex.

The mechanistic heart of the ubihydroquinone-cytochrome c oxidoreductase (cyt bc1 complex) is the catalytic oxidation of ubihydroquinone (QH2) at the Qo site. QH2 oxidation is initiated by ferri-cyt c, mediated by the cyt c1 and [2Fe-2S] cluster of the cytochrome bc1 complex. QH2 oxidation in turn drives transmembrane electronic charge separation through two b-type hemes to another ubiquinone (Q) at the Qi site. In earlier studies, residues F144 and G158 of the b-heme containing polypeptide of the Rhodobacter capsulatus cyt bc1 complex were shown to be influential in Qo site function. In the present study, F144 and G158 have each been singly substituted by neutral residues and the dissociation constants measured for both Q and QH2 at each of the strong and weak binding Qo site domains (Qos and Qow). Various substitutions at F144 or G158 were found to weaken the affinities for Q and QH2 at both the Qos and Qow domains variably from zero to beyond 10(3)-fold. This produced a family of Qo sites with Qos and Qow domain occupancies ranging from nearly full to nearly empty at the prevailing approximately 3 x 10(-2) M concentration of the membrane ubiquinone pool (Qpool). In each mutant, the affinity of the Qos domain remained typically 10-20-fold higher than that of the Qow domain, as is found for wild type, thereby indicating that the single mutations caused comparable extents of the weakening at each domain. Moreover, the substitutions were found to cause similar decreases of the affinities of both Q and QH2 in each domain, thereby maintaining the Q/QH2 redox midpoint potentials (Em7) of the Qo site at values similar to that of the wild type. Measurement of the yield and rate of QH2 oxidation generated by single turnover flashes in the family of mutants suggests that the Qos and Qow domains serve different roles for the catalytic process. The yield of the QH2 oxidation correlates linearly with Qos domain occupancy (QH2 or Q), suggesting that the Qos domain exchanges Q or QH2 with the Qpool at a rate which is much slower than the time scale of turnover.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1995

Biological electron transfer.

Many oxidoreductases are constructed from (a) local sites of strongly coupled substrate-redox cofactor partners participating in exchange of electron pairs, (b) electron pair/single electron transducing redox centers, and (c) nonadiabatic, long-distance, single-electron tunneling between weakly coupled redox centers. The latter is the subject of an expanding experimental program that seeks to manipulate, test, and apply the parameters of theory. New results from the photosynthetic reaction center protein confirm that the electronic-tunneling medium appears relatively homogeneous, with any variances evident having no impact on function, and that control of intraprotein rates and directional specificity rests on a combination of distance, free energy, and reorganization energy. Interprotein electron transfer between cytochrome c and the reaction center and in lactate dehydrogenase, a typical oxidoreductase from yeast, are examined. Rates of interprotein electron transfer appear to follow intraprotein guidelines with the added essential provision of binding forces to bring the cofactors of the reacting proteins into proximity.

Electron Transport↗
PubMed1994

Femtosecond coherent transient infrared spectroscopy of reaction centers from Rhodobacter sphaeroides.

Protein and cofactor vibrational dynamics associated with photoexcitation and charge separation in the photosynthetic reaction center were investigated with femto-second (300-400 fs) time-resolved infrared (1560-1960 cm-1) spectroscopy. The experiments are in the coherent transient limit where the quantum uncertainty principle governs the evolution of the protein vibrational changes. No significant protein relaxation accompanies charge separation, although the electric field resulting from charge separation modifies the polypeptide carbonyl spectra. The potential energy surfaces of the "special pair" P and the photoexcited singlet state P* and environmental perturbations on them are similar as judged from coherence transfer measurements. The vibrational dephasing time of P* modes in this region is 600 fs. A subpicosecond transient at 1665 cm-1 was found to have the kinetics expected for a sequential electron transfer process. Kinetic signatures of all other transient intermediates, P, P*, and P+, participating in the primary steps of photosynthesis were identified in the difference infrared spectra.

Kinetics↗
PubMed1994

Design and synthesis of multi-haem proteins.

A water-soluble, 62-residue, di-alpha-helical peptide has been synthesized which accommodates two bis-histidyl haem groups. The peptide assembles into a four-helix dimer with 2-fold symmetry and four parallel haems that closely resemble native haems in their spectral and electrochemical properties, including haem-haem redox interaction. This protein is an essential intermediate in the synthesis of molecular 'maquettes', a novel class of simplified versions of the metalloproteins involved in redox catalysis and in energy conversion in respiratory and photosynthetic electron transfer.

Amino Acid Sequence↗
PubMed1993

Picosecond infrared studies of the dynamics of the photosynthetic reaction center.

The changes in the vibrational transitions of the protein and redox cofactors of the photosynthetic reaction center were examined by picosecond infrared spectroscopy. The spectra in the vibrational mid-infrared region (1800-1550 cm-1) of hydrated and partially dehydrated reaction centers were investigated from 50 ps to 4 ns after photoinitiation of the electron transfer. Features in the infrared difference spectra were identified with both protein and redox cofactor vibrational modes and correlated with electron transfer events whose kinetics were measured in the infrared and visible regions. The observed protein response is confined to a few amide I transitions (1644 cm-1, 1661 cm-1, 1665 cm-1) and carboxylic residues (1727 cm-1). About 85% of the observed signal corresponded to alterations in the cofactor-associated ester and keto carbonyls. The amide I and carboxylic transitions appeared prior to 50 ps, suggesting that the primary electron transfer event is coupled with a specific piece of the protein backbone and to glutamic or aspartic residues nearby the special pair. Infrared absorption changes accompanying bacteriochlorophyll-dimer cation formation dominated the signal at all times investigated. Infrared spectral changes observed in hydrated and partially dehydrated reaction centers were distinctly different; a band at 1665 cm-1 with a spectral width of 6 cm-1 in the hydrated protein, corresponding to a protein amide I bleach, was not present in the dehydrated film. These differences are discussed in terms of the markedly different electron transfer kinetics observed in the presence of water.

Electron Transport↗
PubMed1992

Engineering protein structure for electron transfer function in photosynthetic reaction centers.

A basic relationship is defined that incorporates the three parameters that effectively modulate the rate of intraprotein electron transfer, namely distance, free energy and reorganization energy. This empirically validated relationship is used to explore the minimal requirements for protein-catalyzed conversion of excited electronic states into stable charge separated states, the essence of photosynthesis.

Electron Transport↗
PubMed1992

Nature of biological electron transfer.

Powerful first-order analysis of intraprotein electron transfer is developed from electron-transfer measurements both in biological and in chemical systems. A variation of 20 A in the distance between donors and acceptors in protein changes the electron-transfer rate by 10(12)-fold. Protein presents a uniform electronic barrier to electron tunnelling and a uniform nuclear characteristic frequency, properties similar to an organic glass. Selection of distance, free energy and reorganization energy are sufficient to define rate and directional specificity of biological electron transfer, meeting physiological requirements in diverse systems.

Animals↗
PubMed1988

Cytochrome c and c2 binding dynamics and electron transfer with photosynthetic reaction center protein and other integral membrane redox proteins.

To further the understanding of the details of c-type cytochrome action as a redox carrier between major electron-transfer proteins, the single-turnover kinetics time course of cytochrome c and cytochrome c2 oxidation by flash-activated photosynthetic reaction center (purified from the bacterium Rhodobacter sphaeroides) has been examined under a wide variety of conditions of concentration, ionic strength, and viscosity with reaction center present in detergent dispersion and phosphatidylcholine proteoliposomes. We find that the three-state model proposed by Overfield and Wraight [Overfield, R. E., & Wraight, C. A. (1980) Biochemistry 19, 3322-3327] is generally sufficient to model the kinetics time course; many similarities are found with the cytochrome c-cytochrome c oxidase interaction in mitochondria. Further, we find the following: (1) Significant "product inhibition" by oxidized cytochrome c (c2) bound to the reaction center is apparent. (2) The viscosity sensitivity of the electron transfer into the reaction center from bound cytochrome c (c2) suggests a physical interpretation of the distal state. (3) The exchange dynamics of oxidized and reduced cytochrome c (c2) are similar regardless of the state of activation of the reaction center. (4) Preferential binding of the oxidized form of cytochrome c is revealed upon reconstitution of the reaction center into neutral lipid vesicles, permitting an independent confirmation of the binding suggested by the kinetics. (5) Flash-activated electron-transfer kinetics in reaction center hybrid protein systems have shown that diffusion and competitive binding characterize the behavior of cytochrome c as a redox carrier between the reaction center protein and either the cytochrome bc1 complex or the cytochrome c oxidase.

Bacterial Proteins↗
PubMed1984

Two distinct quinone-modulated modes of antimycin-sensitive cytochrome b reduction in the cytochrome bc1 complex.

Reduction of cytochrome b-560 (analogous to cyt b-562 of mitochondria) via an antimycin-sensitive route has been revealed in chromatophores of the photosynthetic bacterium, Rhodopseudomonas sphaeroides Ga. Indeed, the results suggest that two reductive mechanisms can be operative. One is consistent with the idea that the quinol generated at the reaction center QB site enters the Q pool and, via the Qc site, equilibrates with cytochrome b-560. The other reductive mode circumvents redox equilibrium with the pool; we consider that this could result from a direct encounter of the reaction center with the bc1 complex perhaps involving a direct QB-Qc site interaction. This latter reaction is suppressed by occupancy of the Qc site, not only by antimycin but by ubiquinol and ubiquinone.

Antimycin A↗