Discussion
The number of equivalents of H2S gas released from the active site of rubredoxin, 2-iron ferredoxin, and 4-iron ferredoxin when treated with mineral acid, respectively, are
Answer: B
The answer is 0, 2, 4.
Mineral acid releases H2S only from acid-labile inorganic sulfide (S2-) in an iron-sulfur cluster, not from the cysteine thiolates that bind the cluster to the protein.
- Rubredoxin has a single iron held by four cysteine sulfurs, Fe(S-Cys)4, with no inorganic sulfide: 0.
- A 2-iron ferredoxin has an [Fe2S2] core with two bridging sulfides: 2.
- A 4-iron ferredoxin has a cubane [Fe4S4] core with four sulfides: 4.
'4, 6, 8' counts the total sulfur atoms including the cysteines, and '1, 2, 4' wrongly assigns a sulfide to rubredoxin. '0, 2, 3' undercounts the four sulfides of the [Fe4S4] cube.
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