BIOLOGICAL TRANSPORT 



tor preferring alanine, glycine, and a-aminoisobutyric acid is here 

 designated C. It is visualized as a reactive form of the other site. 

 Its combination with amino acid A from outside triggers a transloca- 

 tion so that the resultant complex C'A appears, in orientation to 

 the internal phase, in the reorganized form CA, which can dissociate 

 to produce a higher level of A than existed in the external phase. 

 The scheme supposes, however, that site C retains considerable 

 affinity especially for certain amino acids such as L-leucine and 

 L-valine, so that for these it can operate both for exit and entry. 

 Amino acids such as glycine and a-aminoisobutyric acid have such 



Outside 



-Barrier 





C + A 



mm* i n 



^ AC 



Inside 



I 



Figure 23 Scheme for /3-galactoside entry and exit, redrawn from 

 Kepes (1960) to permit comparison with the scheme of Figure 22. The 

 mode of activation of the carrier C is more closely specified in this case 

 as the addition of R with a high-energy bond to C. R is then displaced 

 by the entering solute molecule. In both schemes the lower sequence, 

 A + C ?=* AC ?=* AC <=* C + A, serves for passive migration. 



62 



