Experimental Studies on Specific Proteins

This enzyme from E. coli is a tetramer of four identical subunits, each of molecular weight 116,500. Amber and ochre (premature termination) mutants of the enzyme provide a number of enzymically inactive, incomplete peptide chains, identical in sequence with the N-term nal part of the wild-type chains. A subset of these N-terminal peptides, called acceptor peptides, can combine with so-called a-peptides identical in sequence with the C-terminal part of the wild-type chain, to restore enzymic activity (Ullmann et al., 1965, 1967 Ullmann and Perrin, 1970 see also the review by Zabin and Villarejo, 1975). Goldberg (1969) suggested that the acceptor peptides and the u-peptides are presumably folded around independent nucleation centers as evidenced by the following facts  

We suggest that the specificity of the chromatographic adsorption might have been more strongly demonstrated by specific elution by a substrate for the native enzyme. However, the evidence obtained is consistent with the conclusion that the retarded fragments contain structures similar to the substrate-binding site of the native enzyme. 

In summary, on the basis of these findings, we are led to conclude that the o polypeptide, even though it corresponds to only 30% of the complete wild-type polypeptide, is able by itself (i.e., in the absence of any interactions with other segments or parts of the wild-type molecule) to fold into the correct wild-type structure, or one very close to it. It seems reasonable, on this basis, to assume that the normal in vivo mechanism of folding involves the stepwise activation of several virtually independent centers of nucleation. 

The underlying idea of the above experiments has been extended in the recent work of Celada et al. (1978), who determine the immuno- 

Antibody to peptide Size (amino acid residues) Binding capacity (nmol of antigen per ml of serum) Avidity (liters/mol) Relative binding capacity (%) 

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