TY - JOUR
T1 - Analysis of Reaction Intermediates in Tryptophan 2,3-Dioxygenase
T2 - A Comparison with Indoleamine 2,3-Dioxygenase
AU - Basran, Jaswir
AU - Booth, Elizabeth S.
AU - Lee, Michael
AU - Handa, Sandeep
AU - Raven, Emma L.
PY - 2016/12/13
Y1 - 2016/12/13
N2 - Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme-containing enzymes that catalyze the O2-dependent oxidation of l-tryptophan (l-Trp) in biological systems. Although many decades have passed since their discovery, the mechanism of tryptophan oxidation has not been established. It has been widely assumed that IDO and TDO react using the same mechanism, although there is no evidence that they do. For IDO, a Compound II (ferryl) species accumulates in the steady state and is implicated in the mechanism; in TDO, no such species has ever been observed. In this paper, we examine the kinetics of tryptophan oxidation in TDO. We find no evidence for the accumulation of Compound II during TDO catalysis. Instead, a ternary [Fe(II)-O2, l-Trp] complex is detected under steady state conditions. The absence of a Compound II species in the steady state in TDO is not due to an intrinsic inability of the TDO enzyme to form ferryl heme, because Compound II can be formed directly through a different route in which ferrous heme is reacted with peroxide. We interpret the data to mean that the rate-limiting step in the IDO and TDO mechanisms is not the same.
AB - Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme-containing enzymes that catalyze the O2-dependent oxidation of l-tryptophan (l-Trp) in biological systems. Although many decades have passed since their discovery, the mechanism of tryptophan oxidation has not been established. It has been widely assumed that IDO and TDO react using the same mechanism, although there is no evidence that they do. For IDO, a Compound II (ferryl) species accumulates in the steady state and is implicated in the mechanism; in TDO, no such species has ever been observed. In this paper, we examine the kinetics of tryptophan oxidation in TDO. We find no evidence for the accumulation of Compound II during TDO catalysis. Instead, a ternary [Fe(II)-O2, l-Trp] complex is detected under steady state conditions. The absence of a Compound II species in the steady state in TDO is not due to an intrinsic inability of the TDO enzyme to form ferryl heme, because Compound II can be formed directly through a different route in which ferrous heme is reacted with peroxide. We interpret the data to mean that the rate-limiting step in the IDO and TDO mechanisms is not the same.
UR - http://www.scopus.com/inward/record.url?scp=85006136166&partnerID=8YFLogxK
U2 - 10.1021/acs.biochem.6b01005
DO - 10.1021/acs.biochem.6b01005
M3 - Article (Academic Journal)
C2 - 27951658
AN - SCOPUS:85006136166
SN - 0006-2960
VL - 55
SP - 6743
EP - 6750
JO - Biochemistry
JF - Biochemistry
IS - 49
ER -