Peptide-Bond Synthesizing Systems

The conjugation of bile acids with added hydroxylamine by rat liver microsomes in the presence of ATP, CoA, and fluoride to yield bile acid hydroxamates has been used to demonstrate the formation of cholyl-CoA as an intermediate in the enzymatic system catalyzing the synthesis of the natural conjugates (12,13). In pig liver, there appear to be two distinctly different enzyme systems capable of the synthesis of cholylhydroxamic acid (15). A partially solubilized preparation from pig liver catalyzed the rapid formation of cholylhydroxamic acid in the presence of cholic acid and hydroxylamine but in the absence of ATP and CoA. This is in contrast to the synthesis of hydroxamates of bile acids by the microsomes, which required both ATP and CoA. Since fluoride inhibits the system that functions in the absence of ATP and CoA, the reaction appears to be similar to that of a lipase (15) 

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Electron-Transfer in Simple Binuclear Complexes. In trying to understand the electron transfer mediation effects of peptide bonds and amino acid side chains on rates of electron transfer in simple systems that are amenable to detailed investigation, we have designed and synthesized a series of complexes which contain within a single molecule two different oxidizing agents —both of which are inert to substitution. The series of complexes we have synthesized is represented schematically by the general structure 1. 

We thus have a protein synthesizing system in which suitably activated amino acids are brought into contact with an RNA chain where each individual amino acid is recognized by a group of three bases and held in position by them. When enough of the amino acids required to match all the groups of bases on the RNA chain are present, the peptide bonds between adjacent amino acids can be formed, the activating t-RNA-molecule released into solution, and the new protein molecule unpeeled from its RNA template. 

Aso et al. [95] studied a model system in order to obtain basic information on the mechanism of amino acid incorporation during an enzymatic modification reaction in the presence of papain. They found that the amino acid ester reacted as a nucleophile in the aminolysis of the acyl-enzyme intermediate to result in the formation of new peptides. Several proteases used in enzymatic peptide bond synthesis are known to form transitory acyl-enzyme intermediates during the hydrolysis of proteins. However, the acyl groups can be transferred to other nucleophiles (amino terminals of peptides or amino acids), synthesizing new peptide bonds [71]. With full knowledge of the above-mentioned facts, covalent amino acid enrichment of proteins can result in... 

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