376 



HICIIAKI) SCIIWKKT AND JOHN UISHOI' 



foriiK'd Ix'twrcn the aiuiiio ^louj) of one amino acid and the carboxyl 

 group of the other. The projjer condition for this is shown in Fig. lA. 

 Two carboxyl-activated amino acids arc attached to the ribosomc ready 

 for peptide bond formation. These may be attached to transfer RNA, or 

 in some other linkage. 



NH-CH NH-CH 



c=o c=o 



RIBOSOME 



I 

 NH,CH 



I 



c=o 



NH— CH 



I 



c = o 



R/BOSOME 



RIBOSOME 



INTERRUPTED GROWTH 



RIBOSOME 



GROWTH FROM C-TERMtNUS 



NKaa, 



NHaa. 



Fig. 1. Proposed mechanisin.s of pei)tide chain growth in protein synthcsi.s. 



2. When the peptide bond is formed, one of the amino acids is then 

 no longer attached to the ribosome (Fig. lA, on the right). From the 

 previous assumption, if a third amino acid is to be added to tlie chain, 

 it will attach to the ribosome by its carboxyl group, but then will not 

 be able to link if it attaches to the left of amino acid R', since R' is not 

 attached to the ribosome. It can link only by attaching to the right of 

 amino acid R- and attaching to this amino acid. 



Thus, if peptides would start to form at various points, at some stage 

 (Fig. IB), aag (attached to the ribosome) would need to form a peptide 

 bond with aa^ of another peptide; according to our assumption this is 



