Catalytic efficiency (kcat / Km) of an enzyme for any substrate can allow comparison of substrate preference. Table shows the kcat/Km values for both GRLMR and SeR for the substrates GS-Se-SG, GSSG, selenite, and selenate. The parent enzyme (GRLMR) has catalytic efficiency for the 4 substrates that follows the trend GS-Se-SG > GSSG >> SeO3 2-. In comparison, SeR ), and Kcat/Km (𝐌 -𝟏 𝐬 -𝟏 ) for GRLMR and SeR. Algorithmic fits to Michaelis-Menten models failed for some datasets due to marked product inhibition. Values marked with † were estimated from plots in figure . Examples of failed algorithmic fits may be found in figures S1 and S2. is much more promiscuous in substrate catalytic efficiency. For SeR, considering quantifiable experimental error, the kcat/Km are indistinguishable (GSSG = GS-Se-SG = SeO3 2->> SeO4 2-).