DEAX B. COWIE AND RICHARD B. ROBERTS T,^ 



wliich accumulate amino acids an energy source (glucose) is required for net 

 accumulation, but an exchange of the internally bound amino acids with 

 external amino acids occurs even in the absence of glucose. The glucose re- 

 quirement can be taken to indicate a possible energy storage in the bound 

 forms which might be useful in forming peptide bonds. 



In considering what molecules are involved as 'R' groups there are two 

 limitations: the molecule should be large enough to keep the amino acids within 

 the cellular membrane but small enough to permit the amino acids to reach 

 the sites where they are utilized in protein synthesis. Large peptides or even 

 proteins are possibilities in addition to smaller molecules such as coenzymes. 

 Further work may disclose the nature of the binding molecules. 



At present, however, by assuming that binding does occur, it becomes pos- 

 sible to postulate a very simple model of the cell. According to this model the 

 small molecules of the medium can diffuse into the cell and reach the reactive 

 centers. At the same time, any small molecules /ree in the cell can diffuse out 

 into the medium. During the course of the metabolic reactions of the cell, 

 however, the intermediates are not free but bound to some larger molecule. 

 Accordingly, they can move from one reactive site to another, but they cannot 

 diffuse out of the cell. The binding molecule also serves to limit the range of 

 enzymatic sites available to the intermediates. Special organization of the 

 enzymes, as suggested by studies of mitochondria, may well occur in these 

 cells. In our work, however, there is no direct evidence for it, but only the 

 indirect evidence that the cell is unbelievably efficient in carrying out a multi- 

 tude of simultaneous reactions. 



REFERENCES 



1. Abelson, p. H. Amino acid biosynthesis in Esclierichia coli: isotopic competition with 

 C'^ glucose. J. Biol. Cliem. 206: 335, 1954. 



2. Abelson, P. H., E. T. Bolton and E. Aldous. The utilization of carbon dioxide in the 

 synthesis of proteins by Esclierichia coli. J. Biol. Chem. 198: 165, 1952. 



3. .\belson, p. H., E. T. Bolton, R. Briteen, D. B. Cowie and R. B. Roberts. Synthesis 

 of the aspartic and glutamic families of amino acids in Escherichia coli. Proc. Nat. Acad. 

 Sc. 39: I020, 1953. 



4. Bolton, E. T. Biosynthesis of nucleic acid in Esclierichia coli. Proc. Nat. Acad, of Sc. 40: 

 764, 1954. 



5. Bolton, E. T. Potassium compounds associated with carbohydrate metabolism of 

 Escherichia coli. Federation Proc. 9: 153, 1950. 



6. Britten, R. Extracellular metabolic products of Escherichia coli during rapid growth. 

 Science 119: 578, 1954. 



7. CoNw AY, E. J. AND M. DowNEY. .\n outcr metabolic region of the yeast cell. Biocliem J . 

 47: 347, 1950. 



8. Cowie, D. B. and E. T. Bolton. Sulfur metabolism in Escherichia coli. IV. Influence of 

 nonmethionine sulfur compounds upon a methionine requiring mutant. /. Bad. 64: 87, 

 1952. 



9. Cowie, D. B., E. T. Bolton and M. K. Sands. Sulfur metabolism in Escherichia coli. 

 II. Competitive utilization of labled and nonlabeled sulfur compounds. J. Bad. 62: 63, 

 1951- 



