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Specificity of transport 



General 



The minimal requirement of a chemical site mediating transport, 

 under the simplest conception, is that it be able to bind the trans- 

 ported solute in such a way that it can be released into either of two 

 phases. We can study transport, as we study the specificity of bind- 

 ing by an enzyme or other protein, by examining analogous solute 

 molecules to see if they appear to be transported by the same site, 

 and by proceeding to structures less and less similar to the solute 

 until we encounter structural modifications that eliminate affinity 

 for the transport site. In this way we should be able to map the 

 active site. 



For enzymes, we have come to expect rather high (although not 

 necessarily absolute) structural and steric specificity in the binding 

 of substrate molecules and in the consequent catalytic action on 

 them. For comparison with transport sites we should perhaps limit 

 the analogy with enzymes to the specificity of binding and not to 

 their action as catalysts; the requirements are then obviously not as 

 high. On the other hand, we can sense that transport may also intro- 

 duce additional specificity requirements. In the simplest case, if the 

 solute does not readily dissociate from the site, no significant trans- 

 port will occur; this situation is analogous to the inhibition pro- 

 duced when the product of an enzymatic catalysis fails to dissociate 

 from the enzyme. We have seen also that complex events may inter- 

 vene between the binding and release of the solute, so that if we 



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