Protein structure transmembrane helices

Figure 12.14 The three-dimensional structure of a photosynthetic reaction center of a purple bacterium was the first high-resolution structure to be obtained from a membrane-bound protein. The molecule contains four subunits L, M, H, and a cytochrome. Subunits L and M bind the photosynthetic pigments, and the cytochrome binds four heme groups. The L (yellow) and the M (red) subunits each have five transmembrane a helices A-E. The H subunit (green) has one such transmembrane helix, AH, and the cytochrome (blue) has none. Approximate membrane boundaries are shown. The photosynthetic pigments and the heme groups appear in black. (Adapted from L. Stryer, Biochemistry, 3rd ed. New York ...

Fig. 8. (a) Structure of the full-length Rieske protein from bovine heart mitochondrial bci complex. The catalytic domain is connected to the transmembrane helix by a flexible linker, (b) Superposition of the three positional states of the catalytic domain of the Rieske protein observed in different crystal forms. The ci state is shown in white, the intermediate state in gray, and the b state in black. Cytochrome b consists of eight transmembrane helices and contains two heme centers, heme and Sh-Cytochrome c i has a water-soluble catalytic domain containing heme c i and is anchored by a C-terminal transmembrane helix. The heme groups are shown as wireframes, the iron atoms as well as the Rieske cluster in the three states as space-filling representations. 

Neoxanthin and the two lutein molecules have close associations with three transmembrane helixes, A, B, and C, forming three chlorophyll-xanthophyll-protein domains (Figure 7.5). Considering the structure of LHCII complex in terms of domains is useful for understanding how the antenna system works, and the functions of the different xanthophylls. Biochemical evidence suggests that these xanthophylls have a much stronger affinity of binding to LHCII in comparison to violaxanthin... 

Bitopic proteins with a single transmembrane helix are more common. If oriented with the N-terminus extra-cytoplasmic, they are classified as type I or, if cytoplasmic, type II (Fig. 2-4). Bitopic membrane proteins are often involved in signal transduction, as exemplified by receptor-activated tyrosine kinases (Ch. 24) agonist occupation of an extracytoplasmic receptor domain can transmit structural changes via a single transmembrane helix to activate the latent kinase activity in a cytoplasmic domain. 

Turner, R., and Weiner,J. (1993). Evaluation of transmembrane helix prediction methods using the recently defined NMR structures of the coat proteins from bacteriophages ml3 and pfl. Biochim. Biophys. Acta 1202, 161 — 168. 

Type i and ii membrane proteins only contain one transmembrane helix of this type, while type ill proteins contain several. Rarely, type i and ii polypeptides can aggregate to form a type iV transmembrane protein. Several groups of integral membrane proteins—e.g., the porins (see p. 212)—penetrate the membrane with antiparallel (3-sheet structures. Due to its shape, this tertiary structure is known as a P-barrel. ... 

Fig. 5.2. Structural principles of transmembrane receptors, a) Representation of the most important functional domains of transmembrane receptors, b) Examples of subunit structures. Transmembrane receptors can exist in a monomeric form (1), dimeric form (2) and as higher oligomers (3,4). Further subunits may associate at the extracellular and cytosohc domains, via disulfide bridges (3) or via non-covalent interactions (4). c) Examples of structures of the transmembrane domains of receptors. The transmembrane domain may be composed of an a-hehx (1) or several a-helices linked by loops at the cytosolic and extracellular side (2). The 7-helix transmembrane receptors are a frequently occurring receptor type (see 5.3). Several subunits of a transmembrane protein may associate into an ohgomeric structure (3), as is the case for voltage-controUed ion channels (e.g., K channel) or for receptors with intrinsic ion channel function (see Chapter 17).

The nature of the unfolded state in denaturant and how it relates to the denatured state under native conditions in the bilayer is a major issue in all denaturation experiments. Thermodynamic arguments from the two-stage model suggest that the relevant denatured state has lost its tertiary structure and maintained the transmembrane helix secondary structure. As noted above, CD spectra on thermally denatured bacteriorhodopsin suggest that the denatured protein maintains most of its helical secondary structure. The extent to which tertiary structure is disrupted is unclear, however. It is possible that some stable interhelical interactions are maintained even at high temperature. The helical secondary structure content is also maintained in SDS micelles, and near-UV circular dichroism (CD) spectra suggest substantial loss or... 

The crystal structure of Tb MscL established that this protein assembles as a homopentamer that is organized into two domains, the transmembrane domain and the cytoplasmic domain. The transmembrane domain consists of 10 helices (2 per subunit) connected by an extracellular loop, while the cytoplasmic domain contains 5 helices that form a left-handed pentameric bundle. The sequence of the Tb.MscL subunit has 151 amino acids and can be further subdivided into five segments the N terminus, the first transmembrane helix (TM1), an extracellular loop, the second transmembrane helix (TM2), and a cytoplasmic domain (Fig. 4, see color insert). Each of the segments is discussed in more detail below. The pore is aligned along the fivefold symmetry axis and is formed by the first transmembrane helix (TM1) and an extracellular loop from each subunit. The channel has overall dimensions of approximately 85 x 50 x 50 A and both the N and C termini reside on the... 

As seen in Figure 7, the transmembrane helix of the Rieske protein assoeiates with those of other subunits from the same monomer. However, the extrinsic domain of the protein extends out so that the Fe2S2 cluster eomes elose to the haem group of cytochrome c i from the other monomer, providing the pathway for electron transfer between these two subunits. This feature provides the structural basis for understanding that the dimerie state of the bci complex is necessary for the eleetron transfer funetion sinee eleetrons are cross-transferred between the two bc monomers. 

Subunit 6 of the bovine bc complex consists of four helices and connecting loops. Located in the matrix, it contacts the exposed part of helix F, G, and H of cytochrome b of one monomer and core 1 and core 2 from the other monomer. It has been suggested this subunit is involved in quinone binding (Yu and Yu, 1982) and proton translocation (Cocco et al., 1991) but no supporting evidences has been observed in the crystal structures. Subunit 7 of the complex is anchored in the matrix side as its N-terminal end associates with the core 1 protein as part of a P-sheet its C-terminal 50 residues form a long, bent transmembrane helix (see Figure 3). 

Great excitement has been generated in the signal transduction field by the first determination of the structure of a seven-transmembrane-helix receptor—the visual system protein rhodopsin—discussed in Chapters 15 and ... 

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