Bacterial 3-barrel

Back Propagation Bacterial Toxins Bacteriophage Barbiturates Baroreceptor Reflex (3 Barrel... 

One paradigm for membrane transport of iron is the binding of the receptor protein to an iron-free siderophore molecule, followed by exchange of iron from an external ferri-siderophore to the receptor bound iron-free siderophore, and subsequent transfer across the cellular membrane. This shuttle mechanism has been explored in the transport system of ferric pyoverdine in P. aeruginosa (215,216). It is unclear why the bacterial system behaves in this manner, but mutagenesis studies of the protein suggest that residues involved in the closure of the P-barrel will not interact in the same way with the iron-free siderophore as they do with the ferri-siderophore. A similar mechanism has been suggested for A hydrophila and E. coli (182). 

Typically, functional porins are homotrimers, which assemble from monomers and then integrate into the outer membrane. The general porins, water-filled diffusion pores, allow the passage of hydrophilic molecules up to a size of approximately 600 Daltons. They do not show particular substrate specificity, but display some selectivity for either anions or cations, and some discrimination with respect to the size of the solutes. The first published crystal structure of a bacterial porin was that of R. capsulatus [48]. Together with the atomic structures of two proteins from E. coli, the phosphate limitation-induced anion-selective PhoE porin and the osmotically regulated cation-selective OmpF porin, a common scheme was found [49]. Each monomer consists of 16 (3-strands spanning the outer membrane and forming a barrel-like structure. 

Buchanan, S. K. (1999). Beta-barrel proteins from bacterial outer membranes structure, function and refolding, Curr. Opin. Struct. Biol., 9, 455-461. 

Fig. 6. Distribution of the most common folds in selected bacterial, archaeal, and eukaryotic proteomes. The vertical axis shows the fraction of all predicted folds in the respective proteome. Fold name abbreviations FAD/NAD, FAD/NAD(P)-binding Rossman-like domains TIM, TIM-barrel domains SAM-MTR, S-adenosylmethionine-dependent methyltransferases PK, serine-threonine protein kinases PP-Loop, ATP pyrophosphatases. mge, Mycoplasma genitalium rpr, Rickettsiaprowazekii hh x, Borrelia burgdorferi ctr, Chlamydia trachomatis hpy, Helicobacter pylori tma, Thermotoga maritima ssp, Synechocystis sp. mtu, Mycobacterium tuberculosis eco, Escherichia coli mja, Methanococcus jannaschii pho, Pyrococcus horikoshii see, Saccharomyces cerevisiae, cel, Caenorhabditis elegans.

Striking is the resemblance between our model structure and the multi-stranded -barrels known for various membrane proteins [42] and poreforming toxins [43]. The formation of an aqueous pore in the lipid bilayer would indeed offer an explanation for the observed bilayer conductivity induced by gramicidin S upon membrane binding [6]. The peptidedipid ratio of 1 40 at which this structure could be trapped for NMR analysis appears to be biologically relevant, as the minimum inhibitory concentration of gramicidin S corresponds to far more than an equimolar ratio of peptides per lipid molecule on the bacterial surface [34,35]. 

The consequences of contamination are largely irreversible, contagious, and calamitous. Unchecked bacterial spoilage in barrels can rapidly change the flavor of the whole wine inventory. Usually, because of the insulating nature of wood fiber, re-sterilization of badly infected barrels is impossible. The barrels should be burned and replaced without further delay. It is important to detect barrel spoilage early in the cycle. 

The eight-stranded P cylinder of plastocyanin (Fig. 2-16A) is somewhat flattened and can also be regarded as a P sandwich.116118 However, the P barrel of triose phosphate isomerase (see Fig. 2-28) is surrounded by eight a helices which provide additional stability and a high symmetry. Bacterial outer membranes contain pores created by very large P cylinders within proteins called porins.119120 Tire one shown in Fig. 8-20 has 16 strands. 

Membranes contain many largely a-helical proteins. Cell surface receptors often appear to have one, two, or several membrane-spanning helices (see Chapter 8). The single peptide chain of the bacterial light-operated ion pump bacteriorhodopsin (Fig. 23-45) folds back upon itself to form seven helical rods just long enough to span the bacterial membrane in which it functions.189 Photosynthetic reaction centers contain an a helix bundle which is formed from two different protein subunits (Fig. 23-31).190 A recently discovered a,a barrel contains 12 helices. Six parallel helices form an inner barrel and 6 helices antiparallel to the first 6 form an outer layer (see Fig. 2-29).191-193... 

In contrast to the flavin oxidases, flavin dehydrogenases pass electrons to carriers within electron transport chains and the flavin does not react with 02. Examples include a bacterial trimethylamine dehydrogenase (Fig. 15-9) which contains an iron-sulfur duster that serves as the immediate electron acceptor167 169 and yeast flavocytochrome b2, a lactate dehydrogenase that passes electrons to a built-in heme group which can then pass the electrons to an external acceptor, another heme in cytochrome c.170-173 Like glycolate oxidase, these enzymes bind their flavin coenzyme at the ends of 8-stranded a(i barrels similar... 

In green plants, which contain little or no cholesterol, cydoartenol is the key intermediate in sterol biosynthesis.161-1623 As indicated in Fig. 22-6, step c, cydoartenol can be formed if the proton at C-9 is shifted (as a hydride ion) to displace the methyl group from C-8. A proton is lost from the adjacent methyl group to close the cyclopropane ring. There are still other ways in which squalene is cyclized,162/163/1633 including some that incorporate nitrogen atoms and form alkaloids.1631 One pathway leads to the hop-anoids. These triterpene derivatives function in bacterial membranes, probably much as cholesterol does in our membranes. The three-dimensional structure of a bacterial hopene synthase is known.164 1643 Like glucoamylase (Fig. 2-29) and farnesyl transferase, the enzyme has an (a,a)6-barrel structure in one domain and a somewhat similar barrel in a second domain. 

Bishop, R.E. Structural biology of membrane-intrinsic beta-barrel enzymes sentinels of the bacterial outer membrane. Biochim Biophys Acta 1778 (2008) 1881-1896. 

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