Bridged dimer, covalent structure

Covalent structure of a bridged dimer. The extra ring prohibits rotation about the interflavan bond. Hydrogen atoms bonded to carbon atoms are omitted. 

Structural and functional evidence clearly demonstrates that family C receptors function as dimers, either as homodimers or as heterodimers. The metabotropic glutamate receptors and the calcium sensors, as discussed in Section 2.6.1, are found as covalently connected dimers in which there is a disulfide bridge between a Cys residue located in a loop in the N-terminal extracellular domain of each monomer. This disulfide bridge apparently serves only to hold the monomers in close proximity, as the loop is so unstructured that it does not resolve in the x-ray structure. 

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).

Another Ptm dimer unsupported by covalent bridges has been reported and structurally characterized by X-ray crystallography [50]. Each platinum unit carries two l,2-(cyclooctane)dioximate ligands and is capped by chloride ligands. Structural features are very similar to those found for the previous three species a part from slightly shorter Pt-Cl axial bonds (mean value of 2.40 A as compared with a mean value of 2.46 A of the previous compounds) which reveals that also axial ligands can be very sensitive to steric interactions with the equatorial ligands. 

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