Membrane in bacteria

The determination of the structure of the iron transporter, ferric-binding, protein (hFBP)t from Haemophilus influenzae (Bruns et ah, 1997) at 0.16 nm resolution shows that it is a member of the transferrin superfamily, which includes both the transferrins and a number of periplasmic binding proteins (PBP). The PBPs transport a wide variety of nutrients, including sugars, amino acids and ions, across the periplasm from the outer to the inner (plasma) membrane in bacteria (see Chapter 3). Iron binding by transferrins (see below) requires concomitant binding of a carbonate anion, which is located at the N-terminus of a helix. This corresponds to the site at which the anions are specifically bound in the bacterial periplasmic sulfate- and... 

The oxidative processes of cells have been hard to study, largely because the enzymes responsible are located in or on cell membranes. In bacteria the sites of electron transport and oxidative phosphorylation are on the inside of the plasma membrane or on membranes of mesosomes. In eukaryotes they are found in the inner membranes of the mitochondria and, to a lesser extent, in the endoplasmic reticulum. For this reason we should probably start with a closer look at mitochondria, the "power plants of the cell."... 

Photosynthesis, the conversion of sunlight into chemical energy, occurs in algae and some bacteria, in addition to higher plants. In plants, the photochemical reactions of photosynthesis take place in the chloro-plast thylakoid membrane. In bacteria, they occur in the plasma membrane. 

The charge inside the cell is negative relative to the outside. The actual membrane potential A W is somewhat smaller than AE because of nonspecific cation leakage across membranes. In bacteria, an ATPase generates a potent proton gradient across plasma membranes, the pH outside being lower than that inside the cell. In some bacterial cultures, pH difference may be as high as 2 (alkaline inside). 

The most well understood pathway is the one that delivers secretory and membrane proteins to the endoplasmic reticulum (ER) membrane in eukaryotic cells and to the inner membrane in bacteria. In both kinds of cells, the pivotal role is played by the so-called Sec61 (in eukaryotes) or SecYEG (in prokaryotes) translocon, a multisubunit translocation channel that provides a conduit for soluble proteins to cross the membrane. The same translocon also serves to integrate membrane proteins into the lipid bilayer. 

Cytochrome oxidases are transmembrane protein complexes, which are localized at the inner mitochondrial membrane in eukaryotes or at the plasma membrane in bacteria. In addition to the reduction of oxygen, all cytochrome oxidases studied so far function as proton pumps as well, maintaining the proton gradient for the production of ATP [283 - 286]. While all cytochrome oxidases oxidize oxygen, they vary in their electron donors. Those receiving electrons from cytochrome c are called cytochrome c oxidases and those from ubiquinone ubiquinone oxidases [287]. 

The multiprotein FqFi complex catalyzes ATP synthesis as protons flow back through the Inner mitochondrial membrane (plasma membrane In bacteria) down their electrochemical proton gradient. 

Cytochrome c oxidase is the terminal member of the respiratory chain in all animals and plants, aerobic yeasts, and some bacteria." " This enzyme is always found associated with a membrane the inner mitochondrial membrane in higher organisms or the cell membrane in bacteria. It is a large, complex, multisubunit enzyme whose characterization has been complicated by its size, by the fact that it is membrane-bound, and by the diversity of the four redox metal sites, i.e., two copper ions and two heme iron units, each of which is found in a different type of environment within the protein. Because of the complexity of this system and the absence of detailed structural information, spectroscopic studies of this enzyme and comparisons of spectral properties with 02-binding proteins (see Chapter 4) and with model iron-porphyrin and copper complexes have been invaluable in its characterization. 

Oxidative phosphorylation occurs on membranes. In bacteria, chemiosmotic ATP synthesis occurs at the cytoplasmic membrane. In plant and animal cells, these reactions occur in the mitochondrion, a double-membraned organelle (Figure 11-1). The ancestor of mitochondria was a bacterial cell incorporated into a nucleated cell, which subsequently lost much (although not all) of its DNA. Most mitochondrial proteins are encoded by nuclear DNA. Some respiratory proteins, along with mitochondrial ribosomal RNA and transfer RNAs, are encoded by mitochondrial DNA. 

Yuan, J. et al (2010) Protein transport across and into cell membranes in bacteria and archaea. Cell Mol. Life Scl,... 

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

Chemically, the membrane is known to consist of phospholipids and proteins, many of which have enzymic properties. The phospholipid molecules are arranged in a bimolecular layer with the polar groups directed outwards on both sides. The structures of some phospholipids found in bacteria are shown in Fig. 1.6. Earlier views held that the protein part ofthe membrane was spread as a continuous sheet on either side ofthe... 

AGAC-modifying enzymes are active outside the cytoplasmic membrane, in the periplasmic space in Gram-negative bacteria and extracellularly in Gram-positives. Table 9.4 summarizes some of the enzymes involved in AGAC resistance and their spectrum of activity. 

When the new term permease was coined to designate bacterial membrane proteins specialized in the transport of specific metabolites [1,2], it covered a concept which was not quite new. The existence of membrane transport systems had been demonstrated in animal tissues by Cori as early as 1925 (see [3]). However, the discovery and characterization of permeases in bacteria revolutionized prospects for studying the properties of transport systems, opening the way to a new field and a very fruitful methodology. 

In bacteria, accumulation of substrates against a concentration gradient can occur through two main classes of transport systems (see [30] for a summary). The prototype of the first class of transporters is the /3-galactoside permease of Escherichia coli (see [31]). It is a relatively simple system involving only a single membrane-bound protein. It catalyzes a lactose-H symport. Other transporters... 

The operation of cytochrome P450 in alkane oxidation has been reported both in bacteria and in yeasts. It has been shown that alkane hydroxylases of CHYP 153 are widespread both in Gram-negative and Gram-positive bacteria that lack the integral membrane alkane hydroxylase (van Beilin et al. 2006). 

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