Abstract
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 methylotroph Methylophilus 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 A resolution), models for the reduced state (2.1 A resolution), the oxidized molecule liganded with cyanide (1. 90 A resolution), and the reduced molecule liganded with nitric oxide (2.20 A 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, Asn(88), 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 Asn(88) stabilizes solvent-shielded ligands through a hydrogen bond.
Highlights
The photosynthetic bacterium Rhodobacter sphaeroides produces a heme protein (SHP), which is an unusual c-type cytochrome capable of transiently binding oxygen during autooxidation
Besides a model for the oxidized state, 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 absorption spectrum of reduced SHP is perturbed by Tris, Hepes, Bis-Tris-Propane, and other nitrogenous bases [11], suggesting that they are bound to the protein but not as heme ligands
Summary
The photosynthetic bacterium Rhodobacter sphaeroides produces a heme protein (SHP), which is an unusual c-type cytochrome capable of transiently binding oxygen during autooxidation. SHP has a high spin heme, indicating lack of a strong field sixth ligand, which confers to SHP spectral properties similar to those of the globins and of cytochromes cЈ. Structure Refinement of Reduced SHP and Liganded States—A difference Fo-Fc Fourier map, calculated with phases obtained from the oxidized SHP structure showed several portions of positive electron density in the heme vicinity.
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