Cd2+-induced aggregation of Escherichia coli pyrophosphatase

We report here that Escherichia coli pyrophosphatase aggregates in the presence of millimolar Cd(2+). This highly cooperative process was specific to both the metal ion and the protein and could be reversed fully by decreasing the Cd(2+) concentration. Aggregation was enhanced by Mg(2+), the natural cofactor of pyrophosphatase, and Mn(2+). Mutations at the intersubunit metal-binding site had no effect, whereas mutation at Glu139, which is part of the peripheral metal-binding site found in pyrophosphatase crystals near the contact region between two enzyme molecules, suppressed aggregation. These findings indicate that aggregation is affected by Cd(2+) binding to the peripheral metal-binding site, probably by strengthening intermolecular Trp149-Trp149' stacking interactions.

Modulation of dimer stability in yeast pyrophosphatase by mutations at the subunit interface and ligand binding to the active site

Yeast (Saccharomyces cerevisiae) pyrophosphatase (Y-PPase) is a tight homodimer with two active sites separated in space from the subunit interface. The present study addresses the effects of mutation of four amino acid residues at the subunit interface on dimer stability and catalytic activity. The W52S variant of Y-PPase is monomeric up to an enzyme concentration of 300 microm, whereas R51S, H87T, and W279S variants produce monomer only in dilute solutions at pH > or = 8.5, as revealed by sedimentation, gel electrophoresis, and activity measurements. Monomeric Y-PPase is considerably more sensitive to the SH reagents N-ethylmaleimide and p-hydroxymercurobenzosulfonate than the dimeric protein. Additionally, replacement of a single cysteine residue (Cys(83)), which is not part of the subunit interface or active site, with Ser resulted in insensitivity of the monomer to SH reagents and stabilization against spontaneous inactivation during storage. Active site ligands (Mg(2+) cofactor, P(i) product, and the PP(i) analog imidodiphosphate) stabilized the W279S dimer versus monomer predominantly by decreasing the rate of dimer to monomer conversion. The monomeric protein exhibited a markedly increased (5-9-fold) Michaelis constant, whereas k(cat) remained virtually unchanged, compared with dimer. These results indicate that dimerization of Y-PPase improves its substrate binding performance and, conversely, that active site adjustment through cofactor, product, or substrate binding strengthens intersubunit interactions. Both effects appear to be mediated by a conformational change involving the C-terminal segment that generally shields the Cys(83) residue in the dimer.

Reciprocal effects of substitutions at the subunit interfaces in hexameric pyrophosphatase of Escherichia coli. Dimeric and monomeric forms of the enzyme

A homohexameric molecule of Escherichia coli pyrophosphatase is arranged as a dimer of trimers, with an active site present in each of its six monomers. Earlier we reported that substitution of His(136) and His(140) in the intertrimeric subunit interface splits the molecule into active trimers (Velichko, I. S., Mikalahti, K., Kasho, V. N., Dudarenkov, V. Y., Hyytiä, T., Goldman, A., Cooperman, B. S., Lahti, R., and Baykov, A. A. (1998) Biochemistry 37, 734-740). Here we demonstrate that additional substitutions of Tyr(77) and Gln(80) in the intratrimeric interface give rise to moderately active dimers or virtually inactive monomers, depending on pH, temperature, and Mg(2+) concentration. Successive dissociation of the hexamer into trimers, dimers, and monomers progressively decreases the catalytic efficiency (by 10(6)-fold in total), and conversion of a trimer into dimer decreases the affinity of one of the essential Mg(2+)-binding sites/monomer. Disruptive substitutions predominantly in the intratrimeric interface stabilize the intertrimeric interface and vice versa, suggesting that the optimal intratrimeric interaction is not compatible with the optimal intertrimeric interaction. Because of the resulting "conformational strain," hexameric wild-type structure appears to be preformed to bind substrate. A hexameric triple variant substituted at Tyr(77), Gln(80), and His(136) exhibits positive cooperativity in catalysis, consistent with this model.

Functional characterization of Escherichia coli inorganic pyrophosphatase in zwitterionic buffers

Catalysis by Escherichia coli inorganic pyrophosphatase (E-PPase) was found to be strongly modulated by Tris and similar aminoalcoholic buffers used in previous studies of this enzyme. By measuring ligand-binding and catalytic properties of E-PPase in zwitterionic buffers, we found that the previous data markedly underestimate Mg(2+)-binding affinity for two of the three sites present in E-PPase (3.5- to 16-fold) and the rate constant for substrate (dimagnesium pyrophosphate) binding to monomagnesium enzyme (20- to 40-fold). By contrast, Mg(2+)-binding and substrate conversion in the enzyme-substrate complex are unaffected by buffer. These data indicate that E-PPase requires in total only three Mg2+ ions per active site for best performance, rather than four, as previously believed. As measured by equilibrium dialysis, Mg2+ binds to 2.5 sites per monomer, supporting the notion that one of the tightly binding sites is located at the trimer-trimer interface. Mg2+ binding to the subunit interface site results in increased hexamer stability with only minor consequences for catalytic activity measured in the zwitterionic buffers, whereas Mg2+ binding to this site accelerates substrate binding up to 16-fold in the presence of Tris. Structural considerations favor the notion that the aminoalcohols bind to the E-PPase active site.

Directed mutagenesis studies of the metal binding site at the subunit interface of Escherichia coli inorganic pyrophosphatase

Recent crystallographic studies on Escherichia coli inorganic pyrophosphatase (E-PPase) have identified three Mg2+ ions/enzyme hexamer in water-filled cavities formed by Asn24, Ala25, and Asp26 at the trimer-trimer interface (Kankare, J., Salminen, T., Lahti, R., Cooperman, B., Baykov, A. A., and Goldman, A. (1996) Biochemistry 35, 4670-4677). Here we show that D26S and D26N substitutions decrease the stoichiometry of tight Mg2+ binding to E-PPase by approximately 0.5 mol/mol monomer and increase hexamer stability in acidic medium. Mg2+ markedly decelerates the dissociation of enzyme hexamer into trimers at pH 5.0 and accelerates hexamer formation from trimers at pH 7.2 with wild type E-PPase and the N24D variant, in contrast to the D26S and D26N variants, when little or no effect is seen. The catalytic parameters describing the dependences of enzyme activity on substrate and Mg2+ concentrations are of the same magnitude for wild type E-PPase and the three variants. The affinity of the intertrimer site for Mg2+ at pH 7.2 is intermediate between those of two Mg2+ binding sites found in the E-PPase active site. It is concluded that the metal ion binding site found at the trimer-trimer interface of E-PPase is a high affinity site whose occupancy by Mg2+ greatly stabilizes the enzyme hexamer but has little effect on catalysis.