Active Site of Sulfite Reductase

Assimilatory sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide without release of intermediate reduction products. Owing to the now-classic investigations of Siegel and co-workers, E. coli sulfite reductase is the best understood of these enzymes. The enzyme is a complex hemoprotein M, == 685,000) with an as(J4 subunit composition (191). The a subunit M, 59,000) contains a NADPH binding site and a FAD or FMN prosthetic group, there being four of each group per holoenzyme. 

The 3 subunit (M, 54,600) contains a special heme, siroheme (192, 193), of the isobacteriochlorin type, and one [4Fe-4S] cluster. The minimal electron-transfer sequence is NADPH--------- FAD-------FMN--------- siroheme --------------------------------------------------------------- sulfite. The minimal catalytic system is a p subunit, substrate, 

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