The effects of chemical modifications on the reconstitution, activity, and stability of clostridial ferredoxin.

Native ferredoxin isolated from Clostridium acidi-urici cannot be acetylated and is not digested by carboxypeptidase A. However, apoferredoxin, free of iron and sulfide, can be acetplated and the COOH-terminal alanine and glutamine residues are quantitatively removed by carboxypeptidase A. Ferredoxin derivatives can be reconstituted from both modified proteins. The following derivatives were also prepared: N-Acetimido-, N-succinyl-, tetraiodo-, various N-aminoacyl derivatives (glycyl-, phenylalanyl-, lysyl-, glutamyl-, and methionyl-) and N-t-butyloxycarbonyl-. All modified apoferredoxins, except the N-succinyland the tetraiododerivatives, could be converted to the corresponding ferredoxin derivatives. These had both lower biological activity and stability than the native protein. Those with a positive charge at the NH2-terminal end were more stable than those with no charge (acetyl-, t-butyloxycarbonyl-). Steric effects of the added amino acid derivatives were also detected. It is concluded that both the NHz-terminal and the COOHterminal amino acids are important for conferring stability to the ferredoxin structure.