Characterization of the complex of glutathione S-transferase pi and 1-cysteine peroxiredoxin

Glutathione S-transferase pi has been shown to reactivate 1-cysteine peroxiredoxin (1-Cys Prx) by formation of a complex [L.A. Ralat, Y. Manevich, A.B. Fisher, R.F. Colman, Biochemistry 45 (2006) 360-372]. A model of the complex was proposed based on the crystal structures of the two enzymes. We have now characterized the complex of GST pi/1-Cys Prx by determining the M(w) of the complex, by measuring the catalytic activity of the GST pi monomer, and by identifying the interaction sites between GST pi and 1-Cys Prx. The M(w) of the purified GST pi/1-Cys Prx complex is 50,200 at pH 8.0 in the presence of 2.5mM glutathione, as measured by light scattering, providing direct evidence that the active complex is a heterodimer composed of equimolar amounts of the two proteins. In the presence of 4M KBr, GST pi is dissociated to monomer and retains catalytic activity, but the K(m) value for GSH is increased substantially. To identify the peptides of GST pi that interact with 1-Cys Prx, GST pi was digested with V8 protease and the peptides were purified. The binding by 1-Cys Prx of each of four pure GST pi peptides (residues 41-85, 115-124, 131-163, and 164-197) was investigated by protein fluorescence titration. An apparent stoichiometry of 1mol/subunit 1-Cys Prx was measured for each peptide and the formation of the heterodimer is decreased when these peptides are included in the incubation mixture. These results support our proposed model of the heterodimer.

Communication between the two active sites of glutathione S-transferase A1-1, probed using wild-type-mutant heterodimers

To study the communication between the two active sites of dimeric glutathione S-transferase A1-1, we used heterodimers containing one wild-type (WT) active site and one active site with a single mutation at either Tyr9, Arg15, or Arg131. Tyr9 and Arg15 are part of the active site of the same subunit, while Arg131 contributes to the active site of the opposite subunit. The V(max) values of Tyr9 and Arg15 mutant enzymes were less than 2% that of WT, indicating their importance in catalysis. In contrast, V(max) values of Arg131 mutant enzymes were about 50-90% of that of WT enzyme while K(m)(GSH) values were approximately 3-8 times that of WT, suggesting that Arg131 plays a role in glutathione binding. The mutant enzyme (with a His(6) tag) and the WT enzyme (without a His(6) tag) were used to construct heterodimers (WT-Y9F, WT-Y9T, WT-R15Q, WT-R131M, WT-R131Q, and WT-R131E) by incubation of a mixture of wild-type and mutant enzyme at pH 7.5 in buffer containing 1,6-hexanediol, followed by dialysis against buffer lacking the organic solvent. The resultant heterodimers were separated from the wild-type and mutant homodimers using chromatography on nickel-nitrilotriacetic acid agarose. The V(max) values of all heterodimers were lower than expected for independent active sites. Our experiments demonstrate that mutation of an amino acid residue in one active site affects the activity in the other active site. Modeling studies show that key amino acid residues and water molecules connect the two active sites. This connectivity is responsible for the cross-talk between the active sites.

Human pituitary glutaminyl cyclase: expression in insect cells and dye affinity purification

Human pituitary glutaminyl cyclase (hQC) was expressed in Drosophila S2 cells under the control of an inducible metallothionene promoter and fused to the Drosophila immunoglobulin-binding protein signal sequence to enable secretion into the culture media. Expression levels reached 50 microg/mL culture media after 7 days of induction. The enzyme was purified to homogeneity directly from culture media by affinity chromatography on Reactive Blue 4-agarose using a step pH elution. The identity of the expressed protein was confirmed by peptide mass mapping and Western blotting. Glutaminyl cyclase was expressed as a fully active 37 kDa enzyme with kcat/Km values of 14.3, 9.3, and 2.4 mM(-1)s(-1) for the substrates Gln-Gln, Gln-NH(2), and Gln-t-butyl ester, respectively. The two cysteines were disulfide bonded, and the lone predicted glycosylation site, asparagine 49, was shown by both enzymatic deglycosylation of the expressed enzyme and site-directed mutagenesis to be glycosylated.

Evidence for essential histidines in human pituitary glutaminyl cyclase

Glutaminyl cyclase (QC, EC 2.3.2.5) catalyzes the formation of the pyroglutamyl residue present at the amino terminus of numerous secretory peptides and proteins. Treatment with diethyl pyrocarbonate inactivated recombinant human QC with the apparent modification of three essential histidine residues. Comparisons of the protein sequences of QC from a variety of eukaryotic species show four completely conserved histidine residues. Mutation of each of these residues to glutamine resulted in two mutant enzymes that were inactive (H140Q and H330Q), suggesting a role in catalysis, and two that exhibited increased Km values (H307Q and H319Q), suggesting a role in substrate binding. Consistent with these results is the prediction that QC possesses a zinc aminopeptidase domain in which the four histidines identified here are present in the active site. Mammalian glutaminyl cyclases may, therefore, have structural and catalytic similarities to a family of bacterial zinc aminopeptidases.