AMINO ACID POOL FORMATION IN Escherichia colt 603 
The stoichiometric site model 
The central feature of any “site” model is that the amino acid is held by association 
with the macromolecules of the cell. For the moment the nature of this association is 
unspecified and it is presumed that the osmotic barrier which may exist near the 
surface of the cell is of minor importance in the maintenance of the pool. The simplest 
possible model of this type is shown schematically in Fig. 7. 
It is immediately obvious that this model is unsatisfactory since it does not ex- 
plicitly contain a step equivalent to the catalytic site. Further, the rate of pool 
formation should be proportional to the amino acid concentration and the number 
of unoccupied sites. 
Via sites 
External amino acid = A — 
Ry x 
lays > pool 
wh 
Ye 
x 
AR 
ky 
A -- R |<] AR 
ky 
Vie 
Ue 
Wie 
Energy coupled 
Fig. 7. Stoichiometric site model. 
In order that the pool size increases with the external concentration (at low con- 
centrations), there must be a process by which the site-amino acid complexes dis- 
sociate. The fraction occupied would then be determined by a balance between the 
rate of formation and dissociation. The rate of dissociation, however, would have 
to be energy-dependent, in order that the pool be maintained as implied by items 
2, 3, 4 and 5. Since all of these difficulties can be resolved together, we will im- 
mediately pass to the discussion of the “carrier” model. 
The carrier model 
The carrier model is consistent with practically all of the known facts concerning 
amino acid pools. We have been led to postulate its central features by the failure 
of the previously discussed models. 
The fact that pools are maintained under adverse conditions where they might 
be expected to leak out, combined with other evidence, has led us to include sites 
as the major mechanism for maintaining the pool. The strong evidence that a catalytic 
site participates in pool formation has led us to include such an intermediate step 
in the formation of the site-amino acid complex (pool). We have, therefore, postu- 
lated that the catalytic site is part of a molecule of moderate molecular weight 
References p. 609 
