Spinach Ferredoxin

The complete amino acid sequence of spinach ferredoxin was determined by analyses of tryptic, chymotryptic, and thermolytic digests of various derivatives of the protein. There are 97 amino acid residues in the molecule. These include 19 different amino acids. Methionine is not a member of the major sequence, but it is suggested that another species of ferredoxin, containing methionine, might exist. Spinach ferredoxin has 5 cysteine residues, 3 of which are near the center of the molecule. The distribution of the cystein residues in spinach ferredoxin differs from that in nonphotosynthetic bacterial ferredoxins, and this suggested some distinctive features in the mechanisms of electron transfer and in the molecular surroundings of iron atoms. Spinach ferredoxin contains a region similar to the known bacterial ferredoxins, extending over a distance of 19 consecutive amino acid residues. This suggests a common archetype for plant and bacterial ferredoxins. An internal repetition of 9 residues occurs in the molecule of spinach ferredoxin.