TY - JOUR
T1 - Trypanosoma brucei γ-glutamylcysteine synthetase. Characterization of the kinetic mechanism and the role of Cys-319 in cystamine inactivation
AU - Brekken, Deirdre L.
AU - Phillips, Margaret A.
PY - 1998/10/9
Y1 - 1998/10/9
N2 - The parasitic protozoan Trypanosoma brucei utilizes a conjugate of glutathione and spermidine, termed trypanothione, in place of glutathione to maintain cellular redox balance. The first committed step in the biosynthesis of glutathione and thereby trypanothione, is catalyzed by γ-glutamylcysteine synthetase (γ-GCS). We have determined the kinetic mechanism for T. brucei γ-GCS. The kinetics are best described by a rapid equilibrium random ter- reactant mechanism, in which the model derived K(d) values for the binding of L-Glu, L-α-aminobutyrate, and ATP to free enzyme are 2.6, 5.1, and 1.4 mM, respectively. However, significant dependences exist between the binding of some of the substrate pairs. The binding of either ATP or L-Glu to the enzyme increases the binding affinity of the other by 18-fold, whereas the binding of L-Glu or L-α-aminobutyrate decreases the binding affinity of the other by 6-fold. Similarly to the mammalian enzyme, cystamine is a time-dependent, irreversible inhibitor of T. brucei γ-GCS. It has been suggested by several studies that cystamine labels an active site Cys residue essential for catalysis. Among the enzymes reported to be inactivated by cystamine, only one Cys residue is invariant (Cys-319 in T. brucei γ-GCS). Mutation of Cys- 319 to Ala in T. brucei γ-GCS renders the enzyme insensitive to cystamine inactivation without significantly affecting the enzyme's catalytic efficiency, kinetic mechanism, or substrate affinities. These studies suggest that cystamine inactivates the enzyme by blocking substrate access to the active site and not by labeling an essential active site residue.
AB - The parasitic protozoan Trypanosoma brucei utilizes a conjugate of glutathione and spermidine, termed trypanothione, in place of glutathione to maintain cellular redox balance. The first committed step in the biosynthesis of glutathione and thereby trypanothione, is catalyzed by γ-glutamylcysteine synthetase (γ-GCS). We have determined the kinetic mechanism for T. brucei γ-GCS. The kinetics are best described by a rapid equilibrium random ter- reactant mechanism, in which the model derived K(d) values for the binding of L-Glu, L-α-aminobutyrate, and ATP to free enzyme are 2.6, 5.1, and 1.4 mM, respectively. However, significant dependences exist between the binding of some of the substrate pairs. The binding of either ATP or L-Glu to the enzyme increases the binding affinity of the other by 18-fold, whereas the binding of L-Glu or L-α-aminobutyrate decreases the binding affinity of the other by 6-fold. Similarly to the mammalian enzyme, cystamine is a time-dependent, irreversible inhibitor of T. brucei γ-GCS. It has been suggested by several studies that cystamine labels an active site Cys residue essential for catalysis. Among the enzymes reported to be inactivated by cystamine, only one Cys residue is invariant (Cys-319 in T. brucei γ-GCS). Mutation of Cys- 319 to Ala in T. brucei γ-GCS renders the enzyme insensitive to cystamine inactivation without significantly affecting the enzyme's catalytic efficiency, kinetic mechanism, or substrate affinities. These studies suggest that cystamine inactivates the enzyme by blocking substrate access to the active site and not by labeling an essential active site residue.
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U2 - 10.1074/jbc.273.41.26317
DO - 10.1074/jbc.273.41.26317
M3 - Article
C2 - 9756861
AN - SCOPUS:0032500534
SN - 0021-9258
VL - 273
SP - 26317
EP - 26322
JO - Journal of Biological Chemistry
JF - Journal of Biological Chemistry
IS - 41
ER -