Glucose -6- Phosphate Dehydrogenase Deficiency Is Controlling By

FIGURE 11-12 Tyr and Trp residues of membrane proteins clustering at the water-lipid interface. The detailed structures of these five integral membrane proteins are known from crystallographic studies. The K+ channel (PDB ID 1 BL8) is from the bacterium Streptomyces lividans (see Fig. 11-48); maltoporin (PDB ID 1AF6), outer membrane phospholipase A (PDB ID 1QD5), OmpX (PDB ID 1QJ9), and phos-

phoporin E (PDB ID 1PHO) are proteins of the outer membrane of E. coli. Residues of Tyr (orange) and Trp (red) are found predominantly where the nonpolar region of acyl chains meets the polar head group region. Charged residues (Lys, Arg, Glu, Asp) are shown in blue; they are found almost exclusively in the aqueous phases.

FIGURE 11-12 Tyr and Trp residues of membrane proteins clustering at the water-lipid interface. The detailed structures of these five integral membrane proteins are known from crystallographic studies. The K+ channel (PDB ID 1 BL8) is from the bacterium Streptomyces lividans (see Fig. 11-48); maltoporin (PDB ID 1AF6), outer membrane phospholipase A (PDB ID 1QD5), OmpX (PDB ID 1QJ9), and phos-

phoporin E (PDB ID 1PHO) are proteins of the outer membrane of E. coli. Residues of Tyr (orange) and Trp (red) are found predominantly where the nonpolar region of acyl chains meets the polar head group region. Charged residues (Lys, Arg, Glu, Asp) are shown in blue; they are found almost exclusively in the aqueous phases.

OmpLA

Maltoporin a-Hemolysin toxin

FIGURE 11-13 Membrane proteins with P-barrel structure. Five examples are shown, viewed in the plane of the membrane; The first four are from the E. coli outer membrane. FepA (PDB ID 1FEP), involved in iron uptake, has 22 membrane-spanning 3 strands. OmpLA (derived from PDB ID 1QD5), a phospholipase, is a 12-stranded 3 barrel that exists as a dimer in the membrane. Maltoporin (derived from PDB ID 1MAL), a maltose transporter, is a trimer, each monomer constructed of 16 3 strands. TolC (PDB ID 1EK9), another transporter, has three separate subunits, each contributing four 3 strands in this 12-stranded barrel. The Staphylococcus aureus «-hemolysin toxin (PDB ID 7AHL; top view below) is composed of seven identical sub-units, each contributing one hairpin-shaped pair of 3 strands to the 14-stranded barrel.

a-Hemolysin toxin

Maltoporin

OmpLA

FIGURE 11-13 Membrane proteins with P-barrel structure. Five examples are shown, viewed in the plane of the membrane; The first four are from the E. coli outer membrane. FepA (PDB ID 1FEP), involved in iron uptake, has 22 membrane-spanning 3 strands. OmpLA (derived from PDB ID 1QD5), a phospholipase, is a 12-stranded 3 barrel that exists as a dimer in the membrane. Maltoporin (derived from PDB ID 1MAL), a maltose transporter, is a trimer, each monomer constructed of 16 3 strands. TolC (PDB ID 1EK9), another transporter, has three separate subunits, each contributing four 3 strands in this 12-stranded barrel. The Staphylococcus aureus «-hemolysin toxin (PDB ID 7AHL; top view below) is composed of seven identical sub-units, each contributing one hairpin-shaped pair of 3 strands to the 14-stranded barrel.

The hydrophobic domains of some integral membrane proteins penetrate only one leaflet of the bilayer. Cyclooxygenase, the target of aspirin action, is an example; its hydrophobic helices do not span the whole membrane but interact strongly with the acyl groups on one side of the bilayer (see Box 21-2, Fig. 1a).

Not all integral membrane proteins are composed of transmembrane a helices. Another structural motif common in membrane proteins is the ft barrel (see Fig. 4-20d), in which 20 or more transmembrane segments form 3 sheets that line a cylinder (Fig. 11-13). The same factors that favor a-helix formation in the hydrophobic interior of a lipid bilayer also stabilize 3 barrels. When no water molecules are available to hydrogen-bond with the carbonyl oxygen and nitrogen of the peptide bond, maximal intrachain hydrogen bonding gives the most stable conformation. Planar 3 sheets do not maximize these interactions and are generally not found in the membrane interior; 3 barrels do allow all possible hydrogen bonds and are apparently common among membrane proteins. Porins, proteins that allow certain polar solutes to cross the outer membrane of gramnegative bacteria such as E. coli, have many-stranded 3 barrels lining the polar transmembrane passage.

A polypeptide is more extended in the 3 conformation than in an a helix; just seven to nine residues of 3 conformation are needed to span a membrane. Recall that in the 3 conformation, alternating side chains project above and below the sheet (see Fig. 4-7). In 3 strands of membrane proteins, every second residue in the membrane-spanning segment is hydrophobic and interacts with the lipid bilayer; aromatic side chains are commonly found at the lipid-protein interface. The other residues may or may not be hydrophilic. The hydropathy plot is not useful in predicting transmembrane segments for proteins with 3 barrel motifs, but as the database of known 3 barrel motifs increases, sequence-based predictions of transmembrane 3 conformations have become feasible. For example, a number of outer membrane proteins of gram-negative bacteria (Fig. 11-13) have been correctly predicted, by sequence analysis, to contain 3 barrels.

Covalently Attached Lipids Anchor Some Membrane Proteins

Some membrane proteins contain one or more covalently linked lipids of several types: long-chain fatty acids, isoprenoids, sterols, or glycosylated derivatives of phosphatidylinositol, GPI (Fig. 11-14). The attached lipid provides a hydrophobic anchor that inserts into the lipid bilayer and holds the protein at the membrane surface. The strength of the hydrophobic interaction between a bilayer and a single hydrocarbon chain linked to a protein is barely enough to anchor the protein securely, but many proteins have more than one attached q

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