CONCEPTS AND TERMS 



These special features of position and configuration could per- 

 haps be attributed equally well to proteins that are not ordinary- 

 enzymes and which accomplish the vectorial part of the transfer 

 with only a transient chemical alteration. Specific binding sites are 

 recognized to occur on protein molecules possessing little or no 

 catalytic activity. We shall consider later whether the specificity 

 patterns of transport suggest that ordinary enzymes contribute spe- 

 cific transport sites. 



Group translocations may perhaps also be effected from pep- 

 tides, which in some instances appear to donate a contained free 

 amino acid to cells (Gale and Van Halteren, 1952; Leach and Snell, 

 1959, 1960; Newey and Smyth, 1959a, 1959b). The peptide cleavage 

 does not occur outside the cell. If the peptidase is located in the 

 plasma membrane and if it releases one of the hydrolvsis products 

 into the cell, we may say that the enzyme has mediated a group 

 translocation. 



Definition of transport 



The transfer of chemical groups from a donor to an acceptor 

 and, at the same time, from one phase to another is a subject that 

 certainly lies in the area of our biological interest in transport. Fur- 

 thermore, the proposal by Mitchell and Moyle concerning this type 

 of enzyme behavior stands as one of the important suggestions as to 

 how the actual vectorial component of transport may perhaps be 

 achieved. From the physicochemical point of view, however, an im- 

 portant distinction needs to be made between group translocations 

 per se and the events that have classically been understood as trans- 

 ports. 



Historically, central interest in transport has gone to the 

 challenging problem— how a solute can be propelled from one 

 phase into another against a concentration gradient, as in the secre- 

 tion process. This interest focused careful attention on the reality 

 of the gradient, that is, on the identity of the states of the migrating 

 solute in the two phases. The realization then followed that in- 

 herently similar processes might operate across a barrier between 

 phases without producing gradients. The broadened interest never- 

 theless remained in the transfer of a distinct molecular species. Let 

 me therefore urge, without any wish to set arbitrary limits to the 

 scope of our biological interest, that we continue to define transport 

 as the process by which a solute is transferred from one phase to 



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