The role of the active site cysteine in catalysis by type 1 iodothyronine deiodinase

Abstract

Type 1 iodothyronine deiodinase (deiodinase 1) is a selenoenzyme that converts the prohormone T4 to the active thyroid hormone T3 by outer ring deiodination or to the inactive metabolite rT3 by inner ring deiodination. Although selenocysteine has been demonstrated to be essential for the biochemical profile of deiodinase 1, the role era highly conserved, active site cysteine (C124 in rat deiodinase 1) has not been defined. The present studies examined the effects of a Cys124Ala mutation on rat deiodinase 1 enzymatic function and substrate affinity. At a constant 10-mM concentration of dithiethreitol (DTT), the C124A mutant demonstrated a 2-fold lower apparent maximal velocity (V(max)) and K(m) for rT3 (K(m)rT3)) than the wild type for outer ring deiodination, whereas the V(max)/K(m) ratio was unchanged. Similarly, the apparent V(max) and K(m)T3 sulfate for inner ring deiodination were 2-fold lower in the C124A mutant relative to those in the wild type, with no change in the V(max)/K(m) ratio. The C124A mutant exhibited ping-pong kinetics in the presence of DTT, and substitution of the active site cysteine increased the K(m)DTT by 14-fold relative to that of the wild-type enzyme, with no significant effects on K(m)rT3 or V(max). The C124A mutant was inhibited by propylthiouracil in an uncompetitive fashion and exhibited a 2-fold increase in K1propylthiouracil compared with that of the wild type. K(m)rT3 was also reduced for the C124A mutant when 5 mM reduced glutathione, a potential physiological monothiol cosubstrate, was used in outer ring deiodination assays. These results demonstrate that thiol cosubstrate interactions with C124 in type 1 deiodinase play an important role in enhancing catalytic efficiency for both outer and inner ring deiodination.