Primary structure of alfalfa ferredoxin.

The amino acid sequences of the six peptides obtained from tryptic digests of alfalfa ferredoxin were determined by chemical and enzymic degradations. The order of tryptic peptides was established from the properties of the two segments of the succinylated protein obtained by tryptic hydrolysis at the single arginyl residue. Alfalfa ferredoxin consists of a polypeptide chain of 9’7 residues, characterized by clusters of acidic and hydrophobic amino acids. It is suggested that this type of distribution of hydrophobic and strongly hydrophilic residues implies a low a-helical content of the native structure. Of the 5 cysteinyl residues only 2, separated by 2 serine residues, show a grouped arrangement similar to that observed in clostridial ferredoxins. Alfalfa ferredoxin differs from spinach ferredoxin at 19 residue positions. The over-all effect of these substitutions is largely conservative in terms of the total numbers of charged, hydroxyand hydrophobic residues. Most of the variant positions are clustered in the vicinity of Ala-Ala sequences, leaving several long stretches unaltered. Comparisons of peptide segments based on minimal mutation distances indicate the occurrence of internal repetitions of similar sequences 27 and 14 residues long in alfalfa ferredoxin. A rigorous statistical comparison of alfalfa and clostridial ferredoxins demonstrates a degree of amino acid sequence similarity clearly greater than would be expected on the basis of random occurrences. Although this raises the possibility of a common evolutionary origin for all ferredoxins, without knowledge of the minimal number of invariant residues required for function, such analyses cannot conclusively prove ancestral homology.