Location of heme axial ligands in the cytochrome d terminal oxidase complex of Escherichia coli determined by site-directed mutagenesis

The cytochrome d terminal oxidase complex is one of two terminal oxidases which are components of the aerobic respiratory chain of Escherichia coli. This membrane-bound enzyme catalyzes the two-electron oxidation of ubiquinol and the four-electron reduction of oxygen to water. Enzyme turnover generates proton and voltage gradients across the bilayer. The oxidase is a heterodimer containing 2 mol of protoheme IX and 1 or 2 mol of heme d per mol of complex. To explain the functional properties of the enzyme, a simple model has been proposed in which it is speculated that the heme prosthetic groups define two separate active sites on opposite sides of the membrane at which the oxidation of quinol and the reduction of water, respectively, are catalyzed. This paper represents an initial effort to define the axial ligands of each of the three or four hemes within the amino acid sequence of the oxidase subunits. Each of the 10 histidine residues has been altered by site-directed mutagenesis with the expectation that histidine residues are likely candidates for heme ligands. Eight of the 10 histidine residues are not essential for enzyme activity, and 2 appear to function as heme axial ligands. Histidine 186 in subunit I is required for the cytochrome b558 component of the enzyme. This residue is likely to be located near the periplasmic surface of the membrane. Histidine 19, near the amino terminus of subunit I also appears to be a heme ligand. It is concluded that two of the four or five expected heme axial ligands have been tentatively identified, although further work is required to confirm these conclusions. A minimum of two additional axial ligands must be residues other than histidine.

The nucleotide sequence of the cyd locus encoding the two subunits of the cytochrome d terminal oxidase complex of Escherichia coli

The cytochrome d terminal oxidase complex is one of two terminal oxidases in the aerobic respiratory chain of Escherichia coli. The enzyme is located in the cytoplasmic membrane where it oxidizes ubiquinol-8 in the bilayer and reduces oxygen to water. Enzyme turnover is coupled to the generation of a proton-motive force, resulting in electrogenic translocation across the membrane of one proton per electron passing through the system. The enzyme is an alpha beta heterodimer containing four hemes. The cyd locus, encoding both subunits, has previously been genetically mapped and cloned. This work describes an insertion and deletion analysis of cyd which indicates the direction of transcription, defines the coding regions, and suggests that cyd is an operon. In addition, the complete DNA sequence of the cyd gene is reported. Two open reading frames, separated by 18 base pairs, encode the two subunits of the oxidase complex. Hydropathy profiles of the deduced protein sequence indicate that subunits I and II are each likely to have multiple transmembrane elements. There are only 10 histidines in both subunits, several of which are likely to serve as heme axial ligands.

Olfactory tests as possible probes for detecting and monitoring Alzheimer's disease

One of the characteristics of Alzheimer's disease is the early loss of neurons in pathways involved in processing olfactory information. Olfactory function was assessed in subjects with Alzheimer's disease using a conventional Smell Identification Test and a simple three odor match-to-sample problem. The patients exhibited a diminished capacity to identify common odors but were severely impaired in their ability to use novel odors in a match-to-sample task. Subjects with Parkinson's disease had a severe deficit for identifying common odors with the majority scoring as anosmic. Multiple sclerosis was not accompanied by detectable changes in olfactory functioning. The results of the Alzheimer's group are similar to recent animal studies that have shown lesions of the piriform-entorhinal cortex produce a variety of memory deficits that are particularly acute in tasks involving novel odors.

Identification of the cydC locus required for expression of the functional form of the cytochrome d terminal oxidase complex in Escherichia coli

The aerobic respiratory chain of Escherichia coli contains two terminal oxidases which are differentially regulated. The cytochrome o complex predominates under growth conditions of high aeration, whereas the cytochrome d complex predominates when the oxygen tension is low. Either terminal oxidase will support aerobic growth. The goal of the work presented in this paper was to identify genes required for the expression of the functional form of the cytochrome d complex, other than the genes encoding the polypeptide components of the oxidase complex (cyd locus). A strain lacking the cytochrome o complex (cyo mutant strain) was mutagenized by using a lambda-Mu hybrid hopper bacteriophage, lambda placMu53, which inserts randomly into the chromosome and carries a kanamycin resistance marker. Strains were isolated and examined which were unable to grow aerobically, i.e., which lacked functional cytochrome d complex, and which could not be complemented by introduction of the cyd gene on F-prime episomes. One strain was selected for characterization. The phage insert was mapped to min 18.9 on the genetic linkage map, defining a new genetic locus, cydC. Evidence described in the text suggests that the gene product is probably required for the synthesis of the unique heme d component of the cytochrome d complex.