Info

Figure 11.16. (Left) Schematic representation of reaction scheme B in Fig. 11.10, with examples of the sort of channel activations that are produced by the wild-type receptor with either one or both of the bindingsites occupied by ACh. (Middleand right) Reaction scheme B (Fig. 11.10), with the results of a particular HJCFIT fitting marked on the arrows (the binding sites were assumed to be independent). See color insert.

Figure 11.17. The predicted macroscopic current. The rate constants that have been fitted to results from equilibrium recordings (see Figs. 11.14-11.16) were used to calculate the macroscopic response to a 0.2-ms pulse of ACh (lmM), as in Colquhoun and Hawkes (44).This calculation predicts that the mutation will cause a sevenfold slowing of the decay of the synaptic current, much as observed (102). See color insert.

Figure 11.17. The predicted macroscopic current. The rate constants that have been fitted to results from equilibrium recordings (see Figs. 11.14-11.16) were used to calculate the macroscopic response to a 0.2-ms pulse of ACh (lmM), as in Colquhoun and Hawkes (44).This calculation predicts that the mutation will cause a sevenfold slowing of the decay of the synaptic current, much as observed (102). See color insert.

accurate picture of the site appears to have emerged, even when the arguments have been somewhat irrational (like looking only at the EC,, value of agonists). However, as expected, this approach has also led to some wrong conclusions. The less optimistic conclusion that has to be drawn is that, even when results are analyzed by the best available methods, it is not possible to infer that a residue is part of the binding site, as exemplified by the cases of the aG153S, ┬źN217K, and eL221F mutations. Perhaps now that we arejust beginning to understand the physical movements of chains and residues that accompany the process by which binding is translated into a conformation change, it may become possible to explain, and even predict, some of the effects that are seen. For the moment, though, it is still necessary to engage in quite a lot of post hoc rationalization. As always, comparisons with enzymes are interesting. Functional assessment of enzymes has to be done at a much cruder level than can be done with channels. However, enzymes have the advantage that complete structures of many mutants have been determined. Despite this, the ability to rationalize the effect of mutations is still limited (105).

Was this article helpful?

0 0

Post a comment