CHEMICAL PATHWAYS 95 



incorporated into acid-insoluble material in the absence of other amino 

 acids, is actually incorporated into cell wall material rather than into 

 protein (Gale and Folkes, 1958; Mandelstam and Rogers, 1959). Thus, the 

 apparent evidence provided by the quoted experiments in favour of direct 

 exchange of free amino acids with amino acid residue in a protein cannot 

 any more be regarded as valid. 



It has been most clearly shown that in exponentially growing bacteria 

 amino acid incorporation into protein is essentially irreversible (Rotman 

 and Spiegelman, 1954; Hogness et al., 1955). In animal or plant tissue, 

 in yeast and in resting bacteria, a protein turnover does occur (Halvorson, 

 1958; Mandelstam, 1957, 1958), but there is no reason to assume that it is 

 due to an amino acid exchange; protein turnover probably reflects the 

 continued formation of new molecules from amino acids released by des- 

 truction of other protein molecules. Exchange process can still be called 

 upon for explaining unequal labelling of proteins in various positions, but 

 it is only one of several possible interpretations of this phenomenon which 

 is still completely obscure (see p. 115). 



In conclusion, it may be assumed that amino acid incorporation within 

 polypeptide chains is due to net synthesis of protein material; the only 

 restriction being that it may not always correspond to the formation of 

 complete or perfect protein molecules. 



ATP requirement for amino acid incorporation confirmed that protein 

 synthesis is driven by energy yielding reactions, and indicated ATP as the 

 essential energy distributor in this process as well as in many other synthe- 

 ses. The mechanism of the energy coupling has been the object of many 

 studies. An early working hypothesis assumed that ATP was used in the 

 making of small peptides, and that polypeptides resulted from rearrange- 

 ments of amino acids between these peptides by a series of transpeptidation 

 reactions (Fruton, 1950, 1952, 1957; Hanes et al, 1950; Waelsch, 1952; 

 Linderstrom-Lang, 1949, 1952; Virtanen, 1950). The justification of this 

 hypothesis was that it accounted for the energy coupling and ATP require- 

 ment on one hand, and that it implicated on the other hand many well 

 observed transpeptidation processes catalysed by proteolytic enzymes. 

 Transpeptidations involving esters or amides of amino acids or peptides can 

 indeed result in the formation of rather long peptides in vitro (Fruton et al., 

 1951, 1953 ; Jones ^^«/., 1952; Durell and Fruton, 1954; Waley and Watson, 

 1954; Blau and Waley, 1954; Schweet et al, 1948; Brenner et al, 1950; 

 Tauber, 1952; Kaganova and Orekhovich, 1954; Watson, 1956; Neumann 

 et al , 1 959). Glutamine and glutathione received special attention as possible 

 initiators of protein formation, because they can transfer their glutamyl 

 residue to a variety of amino acids and peptides (Hanes et al, 1950, 1952, 

 1953; Waelsch, 1952; Hird and Springell, 1954) and it was conceivable 

 that a series of transfers of carboxyl or amino moieties of small peptides could 



