Catalyzing prokaryotic bacteria

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

This family includes the sulfite oxidases and dehydrogenases of prokaryotes Thiobacilli sp.), plants, birds, and animals, and the assimilatory nitrate reductases from bacteria, algae, fungi, and plants. The sulfite oxidases of higher eukaryotes are 100-110kDa homodimers (Table 1) they are located in the mitochrondrial intermembrane space and catalyze the oxidation of toxic sulfite to innocuous sulfate (equation 7). Human sulfite oxidase deficiency leads to major neurological abnormalities, mental retardation, dislocation of the ocular lenses, and early death. ... 

Superoxide dismutase (SOD) is a widely distributed enzyme that exists in a variety of forms. The copper-zinc enzyme (Cu,ZnSOD) is primarily located in the cytosol of eukaryotic cells. Mitochondria contain, in the matrix space, a distinctive cyanide-insensitive manganese-containing enzyme (MnSOD) similar to that found in prokaryotes. In addition, a ferrienzyme (FeSOD) has been identified in bacteria that is also insensitive to cyanide. Amino acid sequence homologies indicate two families of superoxide dismutases. One of these is composed of the Cu,ZnSODs and the other of MnSODs and FeSODs. All these superoxide dismutases catalyze the same reaction (2H -H O2 -h OJ H2O2 -t- O2) and with comparable efficiency. 

In prokaryotic cells such as bacteria the initial step of the metabolism of aromatic compounds is the cycloaddition of oxygen catalyzed by a dioxygenase. In living microbial cells, the resulting e t/o-peroxide (dioxetane) is then enzymatically reduced to yield synthetically useful cis-glycols (Eq. 2-9). 

These are the eukaryotic assimilatory nitrate reductases and three distinct bacterial enzymes, comprising the cytoplasmic assimilatory (Nas), the membrane-bound (Nar), and the periplasmic dissimilatory (Nap) nitrate reductases [11], Nitrite oxidoreductase, a membrane-bound enzyme from nitrifying bacteria also exhibits nitrate reductase activity. This enzyme shows high sequence similarity to the membrane-bound Nar, and catalyzes the nitrite oxidation to nitrate, to allow chemoautotrophic growth [75]. Many bacteria have more than one of the four types of nitrate reductases [9]. The functional, biochemical, and structural properties of prokaryotic and eukaryotic nitrate reductases have been recently reviewed [3,4,76,77]. Protein sequence data have been used to determine phylogenetic relationships and to examine similarities in structure and function of nitrate reductases. Three distinct clades of nitrate reductase evolved the eukaryotic assimilatory Nas, the membrane-associated prokaryotic Nar, and a clade that included both the periplasmatic Nap and prokaryotic assimilatory Nas [78]. 

It is well established that most of the known anaerobic prokaryotes perform oxidative phosphorylation without O2. Depending on the species and the metabolic conditions, these bacteria may use a large variety of inorganic (e.g., nitrate, nitrite, sulfate, thiosulfate, elemental sulfm, polysulfide sulfur) or organic compounds (e.g., fumarate, dimethylsulfoxide, trimethylamine-A -oxide, vinyl- or arylchlorides) as terminal electron acceptor instead of oxygen. The redox reactions with these acceptors are catalyzed by membrane-integrated electron transport chains and are coupled to the generation of an electrochemical proton potential (Ap) across the membrane. Oxidative phosphorylation in the absence of O2 is also termed anaerobic respiration . Oxidative phosphorylation with elemental sulfur is called sulfm respiration . Oxidative phosphorylation with polysulfide sulfur is called polysulfide respiration . 

Catalase, the enzyme which catalyzes the dismutation of hydrogen peroxide to molecular oxygen and water, is contained in both eukaryotes and prokaryotes. Most catalases isolated so far have common properties with respect to subunit composition, absorption spectra, pH dependence of the catalatic activity and inhibition by inhibitors. Recently catalases from a few bacteria, such as Rhodobacter capsulatus [1], Escherichia coli [2] and Comamomas compransoris [5], have shown to have different properties from those of typical catalases. 

Fatty acids in plants are synthesized by a series of reactions similar to those for fatty acid biosynthesis in animals, yeasts, bacteria, and other organisms. The synthetases of prokaryotic cytoplasmic systems are freely soluble and readily separable as discrete proteins those in mammalian systems are localized in the cytoplasm as large soluble synthetase complexes. The exact nature of association of the enzymes in plants is unknown (Ohlrogge et al., 1993). The reactions of plant fatty acid biosynthesis are catalyzed by individual polypeptides (Somerville and Browse, 1991) once organelles are disrupted, the enzymes also are soluble and may be isolated (Somerville and Browse, 1991 Stumpf, 1976). In contrast to mammalian systems, plant fatty acid synthetases are plastid localized. 

Poly(A) polymerases are found in a wide variety of organisms from higher animals and plants to yeast, bacteria, and certain viruses (1—3). In eukaryotes, the enzymes are presumed to play a key role in the polyadenylation of pre-mRNAs in the nucleus. However, in prokaryotes such as E. coli where polyadenylated mRNAs are minor species, the function of poly(A) polymerases remains a matter of speculation. In most species, poly(A) polymerases are single polypeptide ( 60 kDa) enzymes and catalyze a linear polymerization reaction as follows (Scheme 7.1). 

---