pes (Fig 14). The structure of subunits that can make up a subtype differs some what to allow differentiation in binding properties of the specific receptors example (Fig. 15 & Fig. 16). The end result is a combination of subunits creating a specifically shaped receptor allowing only certain ligands to bind (Fig. 17). Figure 14 A Table comprising some of the combinatorial subunits needed to assemble a receptor subtype Figure 15 Four TM subunit Figure 16 Three TM subunit plus pore Figure 17 Assembled subunits The structures of receptors can be extremely complex to view as demonstrated by the glycine receptor (Fig. 18) Figure 18 The molecular structure of a glycine receptorBelow, the evolutionary tree for the rhodopsin-like receptors indicates that they have evolved both as a consequence of selection for coupling to different G proteins and selection for reaction with different ligands. However these two developments would appear to have occurred independantly and through different mechanisms. Subtypes of receptors which bind the same ligand generally have evolved within a given branch of the tree through ordinary divergent evolution. (see figure D2, D3, D4 dopamine receptors and also the muscarinic receptors). However subtypes of receptors are also frequently found in separate branches of the tree. Aminergic (ie binding monoamines) receptors are examples of how receptors that couple to a particular G protein, but bind different ligands, can be more homologous to each other than receptor subtypes which bind to the same transmitter. Thus it appears that the ability to bind, for example dopamine has evolved in different evolutionary branches of G protein coupled receptors, which through divergent evolution had already segregated from each other (D1 and D5 versus D2, D3 and D4 in the figure). Similarly, histamine H1 and H2 receptors are only approximatel...
