This enzyme catalyzes the 6-electron reduction of sulfite to sulfide. This is one of several activities required for the biosynthesis of L-cysteine from sulfate (By similarity). 3 H2O + hydrogen sulfide + 3 NADP(+) = 4 H(+) + 3 NADPH + sulfite Binds 1 FAD per subunit. Binds 1 FMN per subunit. Sulfur metabolism; hydrogen sulfide biosynthesis; hydrogen sulfide from sulfite (NADPH route): step 1/1.