CONCEPTS AND TF.RMS 



In carrier transport the actual movement of the carrier-solute 

 complex is usually pictured as a diffusion, but other possibilities 

 have been considered. Goldacre (1952) first suggested that the 

 unfolding and folding of protein molecules could cyclically gen- 

 erate and destroy binding sites to produce transport. This effect was 

 imagined to occur in the cytoplasm, as well as in the membrane, 

 and to be the basis of protoplasmic streaming, to which transport 

 was considered to be a side effect. The following separate sections 

 of Goldacre's discussions (1952) are quoted to portray the nature 

 of his proposal: 



When protein molecules are unfolded they have more surface 

 area available for adsorption of other molecules than when they 

 are folded up. The side-chains and other groups which are used 

 to hold one part of the polypeptide chain to another part in a 

 folded globular protein become free and turn toward the solu- 

 tion and can then adsorb other substances. . . . 



In the ameba the unfolded molecules in the cortical gel and 

 plasma membrane should adsorb the material from the environ- 

 ment, and when they fold up in the tail, they desorb it. . . . 



Protein folding and unfolding appear therefore to be required 

 for something much more fundamental [than osmotic work and 

 cytoplasmic streaming] in the cell. This probably involves en- 

 zyme reactions which are assisted by this rhythmic change in 

 the form of the molecule. The products of the reaction would 

 be squeezed off the surface of the molecule when it folded up, 

 but while adsorbed it would be kept out of the back reac- 

 tion. . . . 



In this way ATP, which works the folding-unfolding cycle, 

 could be harnessed to drive any other reaction which involves 

 the folding-unfolding cycle (coupling) ... A muscle cell for 

 example is not something apart but an ordinary cell in which the 

 already contractile protein chains are aligned so that the con- 

 traction of the cell all at once becomes a conspicuous feature. 

 The same contractile chains can do other things as well, e.g., 

 osmotic work and secretion. 



The potentialities for transport inherent in changes in the ter- 

 tiary structure of protein molecules are now widely appreciated, 

 although today we may be inclined to think that denaturation might 

 destroy or translocate active sites rather than unmask them. In gen- 

 eral, models in which the solute molecule is directly propelled 

 through the osmotic barrier by an event stimulated by its entry 

 into the binding site, and in which the site becomes externally avail- 

 able again after reactivation, appear to have two advantages: first, 



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