In biology, a sulfonucleotide reductase is actually a class of enzymes involved in reductive sulfur assimilation. This reaction consists of a conversion from activated sulfate to sulfite. (Inorganic sulfate occurs abundantly on Earth; terrestrial organisms must use sulfate assimilation to convert it to sulfide.) The sulfite is used in essential biomolecules such as cysteine. Sulfonucleotide reduction by adenosine-5'-phosphosulfate uses a mechanism of nucleophilic attack. Carroll et al., after studying that mechanism, suggested that sulfonucleotide reductases evolved from a common ancestor.
Further, Carroll, et al. found some key characteristics of adenosine-5'-phosphosulfate in M. tuberculosis. For example, it's a monomer. Also, it does not possess an "intramolecular disulfide bond instead of an intermolecular disulfide bond." In addition, they proposed a role of the iron-sulfur cluster in adenosine-5'-phosphosulfate reduction.