Membrane Proteins Contain Transmembrane a Helices

Integral Membrane Proteins Contain Transmembrane a Helices 

Very few integral membrane proteins have been crystallized. The reaction-center proteins purified from membranes of photosynthetic bacteria are a notable exception. These proteins were discussed in chapter 15. Before their crystal structures were elucidated, analysis of hydropathy plots suggested that each of the two main protein subunits is folded into five transmembrane a helices, and one such helix was predicted to occur in another subunit. The crystal structures provided a beautiful confirmation of these predictions (see fig. 15.11a). Successful crystallization of the reaction-center proteins was achieved by including small, 

A three-dimensional structure also has been elucidated for bacteriorhodopsin, an integral membrane protein of the halophilic (salt-loving) bacterium Halobacterium halobium. This protein has been studied intensively because of its remarkable activity as a light-driven proton pump (see chapter 14). It forms well-ordered arrays in two-dimensional sheets that can be studied by electron diffraction. Measurements of the diffraction patterns show clearly that bacteriorhodopsin has seven transmembrane helices (fig. 17.12). 

Another microscopic technique is to freeze the specimen and then fracture it with a knife. A knife cutting through the frozen specimen splits the membrane down the middle, exposing the inside of the bilayer (fig. 17.13a). If the Davson-Danielli model for membrane structure were correct, the two exposed surfaces would be featureless. However, electron micrographs of metallic casts of such samples reveal surfaces studded with particles of various sizes (fig. 17.13(f)- Additional studies indicate that these particles are proteins that are deeply embedded in the membrane. The particles seen on the inner and outer leaflets of the bilayer usually differ in size and distribution because of an asymmetrical disposition of the proteins across the bilayer. 

Proteins and Lipids Can Move around within Membranes 

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Integral Membrane Proteins Contain Transmembrane a Helices Proteins and Lipids Can Move around within... 

Membranes contain proteins that merely bind to their surface (peripheral proteins) and those that are embedded in the lipid matrix (integral proteins). Integral membrane proteins contain transmembrane a-helices. 

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