RESUMO
The structure of cytochrome b562 from Escherichia coli has been refined at 1.4 A resolution against X-ray data collected on a Picker four-circle diffractometer. The triclinic unit cell parameters are a = 33.68 A, b = 50.48 A, c = 32.67 A, alpha = 102.51 degrees, beta = 86.56 degrees and gamma = 107.01 degrees and there are two molecules in the asymmetric unit. A total of 138 cycles of restrained crystallographic refinement using the program PROLSQ were augmented at intermediate stages by two cycles of simulated annealing refinement using X-PLOR. The final crystallographic R-factor is 16.4% for data in the resolution range 6.0 A to 1.4 A for a model containing 1650 protein atoms, 86 heme atoms, 165 water molecules and four sulfate anions. The root-mean-square deviations from ideal bond lengths and angles are 0.012 A and 2.0 degrees, respectively. Each molecule consists of a bundle of four alpha-helices arranged in a simple up-down-up-down manner with a non-covalently bound heme group inserted between the first and fourth helices. In addition, there is a very short 3(10) helix in the 15-residue loop connecting the first and second pairs of helices. The two independent molecules show r.m.s. differences of 0.30 A for main-chain atoms and 0.88 A for all atoms. A detailed comparison with the structurally similar cytochrome c' from Rhodospirulum molishianum is presented. In addition, the titration behavior of cytochrome b562 in solution is discussed in terms of its molecular structure.
Assuntos
Grupo dos Citocromos b/química , Proteínas de Escherichia coli , Escherichia coli/química , Evolução Biológica , Simulação por Computador , Cristalografia por Raios X/métodos , Grupo dos Citocromos c/química , Transporte de Elétrons , Heme/química , Ligação de Hidrogênio , Análise dos Mínimos Quadrados , Modelos Moleculares , Dados de Sequência Molecular , Oxirredução , Conformação Proteica , Estrutura Secundária de Proteína , Sulfatos/química , Água/químicaRESUMO
The structure of flavocytochrome b2 from baker's yeast was solved at 3.0-A resolution by the multiple isomorphous replacement method combined with solvent leveling procedures, using data collected from an area detector. The tetramer of Mr 230,000 has 4-fold symmetry. Each subunit contains a cytochrome domain consisting of the first 100 residues, a flavin-binding domain containing the next 386 residues, and an extended C-terminal tail of 25 residues. The cytochrome domain closely resembles microsomal cytochrome b5, whereas the flavin-binding domain contains a parallel beta 8/alpha 8 barrel motif similar to glycolate oxidase and trimethylamine dehydrogenase. Two of the four cytochrome domains are disordered in the crystals. The flavin ring and heme group are separated by about 16 A between their centers, and their planes are inclined by about 17 degrees to each other.
Assuntos
L-Lactato Desidrogenase , Saccharomyces cerevisiae/enzimologia , L-Lactato Desidrogenase (Citocromo) , Substâncias Macromoleculares , Modelos Moleculares , Peso Molecular , Conformação Proteica , Difração de Raios XRESUMO
The atomic models of the cytochrome b562 and cytochrome c' monomers have been compared. When the respective heme groups are superimposed, the four alpha-helices of each nearly coincide. Four aromatic side chains, including the heme ligands, and a methionine occur in spatially equivalent positions in contact with the heme groups. This structural evidence suggests that the two cytochrome families may have diverged from a common molecular ancestor.
Assuntos
Evolução Biológica , Grupo dos Citocromos b , Grupo dos Citocromos c , Citocromos , Proteínas de Escherichia coli , Hemeproteínas , Escherichia coli , Modelos Moleculares , Peso Molecular , Conformação Proteica , RhodospirillumRESUMO
The structure of cytochrome b562 from Escherichia coli has been determined at 2.5 A resolution by x-ray diffraction methods. Protein phases were computed by the single isomorphous replacement method with anomalous scattering measurements from the native and uranyl acetate-substituted crystals. The electron density was averaged about the noncrystallographic 2-fold axis relating 2 molecules in the triclinic unit cell. The protein consists of four nearly parallel alpha helices and represents a new class of cytochrome structure. The heme group is inserted between the helices near one end of the molecule with one heme face partially exposed to solvent. The two heme ligands are histidine and methionine. The 2 phenylalanines are packed internally near the heme group, and the 2 tyrosines are on the surface, also near the heme group. The folding of the protein resembles that of hemerythrin and tobacco mosaic virus protein and shows a different topology from that of cytochrome b5, cytochrome c, or the globins.
Assuntos
Citocromos , Escherichia coli/enzimologia , Sequência de Aminoácidos , Modelos Moleculares , Conformação Proteica , Difração de Raios XRESUMO
A three-dimensional x-ray diffraction study of aspartate transcarbamylase to 5.5-angstrom resolution, with the aid of four isomorphous heavy atom derivatives, indicates the presence of a central aqueous cavity approximating an oblate spheroid about 25 by 50 by 50 angstroms in dimension, within a molecule about 90 by 110 by 110 angstroms in largest dimensions.
