This increase in promiscuity arises from a ~50% decrease in Vmax for GSH based substrates, along with a ~300% increase in Km for GS-Se-SG. Combined the kcat/Km for the GSH based substrates becomes less favorable by a factor of ~20 for GS-Se-SG and a factor of ~5 for GSSG. In contrast the Vmax for SeO3 2-increases by a factor of ~5 and the Km decreases by a factor of 4. The overall catalytic efficiency of SeR increases from 8.82 x 10 4 to 1.25x10 6 M -1 s -1 for SeO3 2-. Overall, the catalytic efficiency of SeR is similar for the GSSG, GS-Se-SG, and SeO3 2-substrates. Superficially, the overall effect of the enzyme evolution appears to broaden Se substrate specialization (given that all 3 substrates show similar catalytic efficiency).