In 3‐iron ferredoxins, the number of sulfide bridges and cysteinyl ligands, respectively, are:

Concept:

• Ferredoxins are soluble and iron based proteins containin iron-sulphur clusters.
• These function in electron transfer in photosystem during photosynthesis.
• Three types of ferredoxin proteins have been isolated. These are [2Fe-2S], [3Fe-4S] and [4Fe-4S].
• S in the presentation are labile sulphurs of cysteine which are removed easily on treatment with acid.

Explanation:

Structure of 3-iron ferredoxin is as given below :

Clearly, 3 cysteinyl ligands and 4 sulphur bridges are present in each unit

Conclusion:

Therefore, 3‐iron ferredoxins consist of 4 sulfide bridges and 3 cysteinyl ligands.