M.A. Cusanovich

Light-induced electron transport in Chromatium strain D II. Light-induced absorbance changes in Chromatium chromatophores

Abstract The light-particle preparations, described previously as derived from chromatophore fractions of Chromatium, exhibit absorbance changes on steady-state illumination with actinic light. Conditions for optimal absorbance changes were established and used to study the effects of variation in redox potential effective at the light-activated electron transport system. The results indicate that at least six components participate in photo-induced changes in oxidation states; two of these can be correlated with the known cytochrome components—cytochrome c−552 and cytochrome c−555. The natures of the remaining components remain to be established. A model is proposed for the electron transport mechanisms activated by light, the salient feature of which is the existence of two functionally different pathways of photo-activated electron movement. One of these involves a pigment-heme protein complex consisting of the 890 mμ active center bacteriochlorophyll coupled to cytochrome c−555, cytochrome cc′ and a third cytochrome (component No. 3); the other consists of a 905 mμ active center bacteriochlorophyll coupled to cytochrome c−552 and a low potential component (P−135). A rationalization of all observations on photometabolism of Chromatium on the basis of this model is presented.

The cytochromes of Chlorobium thiosulfatophilum

Three c-type cytochromes (c-551, c-553, c-555) have been isolated and characterized from a strain of the green photosynthetic bacterium Chlorobium thiosulfatophilum. These cytochromes are atypical when compared to horse heart cytochrome c in many properties, among them: oxidation-reduction potential at pH 7.0 (c-551, 135 mV; c-553, 98 mV; c-555, 145 mV), molecular weight (c-551, 45000–60000; c-553, 50000; c-555, 10000) and isolelectric point (c-551, 6.0; c-553, 6.7). No protoheme was detected in whole cells or cell-free extracts.

Light-induced electron transport in Chromatium strain D. I. Isolation and characterization of Chromatium chromatophores.

Abstract Fractionation of Chromatium Strain D chromatophores by centrifugation in sucrose density gradients results in two fractions, designated ‘light’ and ‘heavy’. The light particles are enriched in photosynthetic pigments, are relatively homogeneous and can be prepared reproducibly. They do not scatter light appreciably, contain 41% of the bacteriochlorophyll, 26% of the carotenoid, 25% of the heme and 4% of the protein found in intact cells, and are approx. 75% lipid, and 25% protein, 30% of which can be ascribed to cytochromes. The molecular weight is 12.9.10 6 . In contrast, the heavy particles are heterogeneous, can not be prepared reproducibly, scatter light and contain approx. 14% of the bacteriochlorophyll, 9% of the carotenoid, 8% of the heme and 6% of the protein present in intact cells. Three meso-heme proteins were identified in the light particles: cytochrome c −552, cytochrome cc′ , and cytochrome c −555. Both cytochrome c −552 and cytochrome cc′ bound CO when solubilized but not when bound in the light particles. No protoheme ( b - type cytochrome) was found, even in whole cells. These results permit characterization of the chemical and physical properties of a subcellular fraction of Chromatium Strain D, which does not appear to be an artifact of preparation.

A high potential cytochrome c from Chromatium chromatophores

Abstract A new cytochrome, C-553 (550), with properties like those of several isolated algal cytochromes f has been isolated from membrane fractions of the photosynthetic bacterium, Chromatium Strain D. The minimum molecular weight of the cytochrome is approx. 13000, but the isolated protein is heterogeneous in size and appears to polymerize in solution. This cytochrome may be related to the membrane-bound cytochrome c−555 which is implicated as a primary reductant in light-induced reactions of the Chromatium chromatophore reaction center.

Pseudomonas denitrificans cytochrome cc

Abstract The denitrifying bacterium, Pseudomonas denitrificans, yields a c-type cytochrome with many properties similar to those of hemoglobin and myoglobin. Presented is a detailed study of the chemical and physical properties of the Pseudomonas cytochrome including the amino acid sequence of the heme peptide. This cytochrome, initially isolated by Iwasaki and co-workers and termed “cryptocytochrome c,” can be classified as a cytochrome cc′, examples of which have been found previously only in the photosynthetic bacteria.

Light-induced electron transfer in Chromatium strain D III. Photophosphorylation by Chromatium chromatophores

Abstract Photophosphorylation at relatively high rates occurs in light-particle preparations described previously as derived from Chromatium chromatophores. It is shown that the optimal conditions for photophosphyorylation correlate with changes in absorbance established in previous researches as the result of ambient redox potentials between 50 and 100 mV. Effects of various dyes, phenazine methosulfate, oxygen and ascorbate are presented and discussed. Attempts to demonstrate by direct measurement the effective redox potentials for photophosphorylation are described. The evidence accumulated favors occurrence of a cyclic phosphorylation mechanism associated with one of the electron transport pathways (the ‘P-890 pathway’) postulated in a model presented in a previous paper.

Crystal structures of an oxygen-binding cytochrome c from Rhodobacter sphaeroides.

The photosynthetic bacterium Rhodobacter sphaeroides produces a heme protein (SHP), which is an unusual c-type cytochrome capable of transiently binding oxygen during autooxidation. Similar proteins have not only been observed in other photosynthetic bacteria but also in the obligate methylotrophMethylophilus methylotrophus and the metal reducing bacterium Shewanella putrefaciens. A three-dimensional structure of SHP was derived using the multiple isomorphous replacement phasing method. Besides a model for the oxidized state (to 1.82 Å resolution), models for the reduced state (2.1 Å resolution), the oxidized molecule liganded with cyanide (1.90 Å resolution), and the reduced molecule liganded with nitric oxide (2.20 Å resolution) could be derived. The SHP structure represents a new variation of the class I cytochrome c fold. The oxidized state reveals a novel sixth heme ligand, Asn88, which moves away from the iron upon reduction or when small molecules bind. The distal side of the heme has a striking resemblance to other heme proteins that bind gaseous compounds. In SHP the liberated amide group of Asn88 stabilizes solvent-shielded ligands through a hydrogen bond.

Euglena gracilis cytochrome 558

Abstract Euglena gracilis contains both the classical cytochrome f (cytochrome c 552 ) found in algae and plants, and an atypical cytochrome 558, which has covalently bound heme. We report here some of the chemical and physical properties of cytochrome 558, including amino acid composition, molecular size, and spectral properties. Further, the atypical nature of the pyridine ferrohemochrome of this cytochrome is considered and it is suggested that the heme is bound to the peptide chain via a single thioether link.