FREE AMINO ACIDS IN PROTEIN SYNTHESIS WSS) 
protein synthesis and indeed peptides are intermediates in the sense that the entire 
protein molecule does not spring into existence in an instant. However, the unit 
being added to the growing chain is a monomeric amino acid, not a peptide. Further- 
more, as BURNETT!" has shown for hemoglobin synthesis, the growing peptide chain 
does not exist as a free peptide but it is distinguished by some chemical or physical 
conditions which make it non-interchangeable with a corresponding free peptide. 
In a general way I have tried to show the flaws in most of the arguments and 
experiments offered in support of the proposition that peptides are intermediates in 
protein synthesis. I have quite deliberately avoided invoking the current theories of 
protein synthesis, for my argument would have seemed cyclic. That is to say, most 
n VALINE SITES 
a 
mers ee i Ae eee cipcgeabgorn 
eee eles SN, Se! ae Ate == Syese Atm ohne ie Atn KH Hew 
a SO ef 
que we See wee vee cee —* eee ese cos ese — Kx tee eee eae —KHKK ose Ke nee eee wee eee 
Sige By inne APA se 
x  , W x ,W *_ W aCe. 
[Ay oo LNW uo Byam AY «= ir 
Fig. 1. Scheme for the labeling of protein molecules formed by the stepwise assembly of activated 
amino acids. Each horizontal line represents one ferritin template with valine sites occupied by 
(#2C)valine (—), by [@Cjvaline (*), or unoccupied (...). An array is shown such that one ferritin 
chain is to be found in each stage of assembly. In each time interval, At, one valine residue is 
added to each growing chain and one ferritin molecule (indicated by the box) is completed. The 
increment in radioactivity in ferritin in any interval, /\t, is seen to be proportional to the number 
of such intervals and hence proportional to the time since the labeled amino acid was firSt 
introduced (from ref. 32). 
of the contemporary theories begin with the assumption that the immediate precur- 
sors of protein are monomeric activated amino acids and hence neither the theories 
nor the experiments are likely to provide evidence for peptide intermediates. However, 
it might be worth considering what has been established experimentally and asking 
whether it would be difficult to accommodate the experimental facts to a theory in 
which free peptides were intermediates in protein synthesis. 
The first and most solid fact is that proteins are chemical entities each with a 
unique arrangement of amino acid residues. Not only have such unique structures 
been established for several proteins but the fact that specific genetic changes result 
in the substitution of a single residue at a single locus*4 convinces one that the bio- 
synthetic mechanism is capable of extremely high precision, making errors only 
when these are, so to speak, directed by the genetic blueprint. 
Beyond this there is strong evidence that there exists in every organism for every 
natural amino acid one and only one activating enzyme whose function is to convert 
the free amino acid first to an aminoacyladenylate and then to an aminoacyl S-RNA 
in which the natural amino acid is bound to a terminal ribose of a transfer ribonucleic 
acid molecule that is specific for the particular amino acid. Then the aminoacyl-S- 
RNA diffuses to the ribosome where, as far as can be told, the aminoacyl-S-RNA is 
oriented and the amino acid residue is incorporated into a polypeptide. The mecha- 
nism for orientation is uncertain but presumably it involves some sort of base-pairing 
References p. 737 
