PROTEIN METABOLISM 487 



( oiiipouiuls t;iii br inailc in ihc laboraloi) antl arc analogous to other 

 high-energy phosphates. With these as the reactants, peptide bond 

 formation would be highly favored and the free-energy change would 

 be negative. This oi a similar rcac (ion was lavored for a time because 



R R 



1 1 



+H3NCHCOO- + ATP ±^ +H3NCHCOPOv,H- + ADP 



R' 



I 

 +H3NCHCOO- 



R R' 



I 1 



+H:jNCHCONHCHCOO + H.,P04- 



a dependence i)i peptide bond formation on ATP has been observed 

 in studies on both simple peptides and proteins. However, diligent 

 searches for amino acid phosphates have been to no avail. Hence 

 these compounds may not be biologically important. 



Work with isolated peptide syntheses has led to a different theory. 

 The peptide glutathione occurs in most plant and animal cells and is 

 believed to form in part as follows: 



Enzyme + ATP t=^ enzyme— PO3H- -f ADP 



Enzyme — PO^H" + glutamate ^ enzyme — glutamate + H2P04~ 



Enzyme — glutamate + amino acid -^ enzyme + peptide 



(or peptide) 



Although the reaction is rather well established for glutathione, this 

 substance is not a peptide typical of proteins because one of the 

 peptide bonds is y rather than a. 



-OOCCH(CHo).CO i NHCHCO j NHCH.COO- 



I 111' 



NH:j+ CH2SH i 



glutamate cysteine Klycinc 



glutathione 



Unfortunately there is yet no good evidence of this type of mechanism 

 in protein synthesis. However, the following extension of these 

 reactions has made them of greater interest. 



Enzymes have been isolated that catalyze exchanges of amino acids 

 between peptides and might allow utilization of glutathione and 

 other peptides made as above. This process is called transpeptidation 

 and may be represented as on page 488. Apparently a variety of similar 



