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Bacteriophage lambda protein phosphatase (lambdaPP) with Mn(2+) as the activating metal cofactor was studied using phosphatase inhibition kinetics and electron paramagnetic resonance (EPR) spectroscopy. Orthophosphate and the oxoanion analogues orthovanadate, tungstate, molybdate, arsenate, and sulfate were shown to inhibit the phosphomonoesterase activity of lambdaPP, albeit with inhibition constants (K(i)) that range over 5 orders of magnitude. In addition, small organic anions were tested as inhibitors. Phosphonoacetohydroxamic acid (PhAH) was found to be a strong competitive inhibitor (K(i) = 5.1 +/- 1.6 microM) whereas phosphonoacetic acid (K(i) = 380 +/- 45 microM) and acetohydroxamic acid (K(i) > 75 mM) modestly inhibited lambdaPP. Low-temperature EPR spectra of Mn(2+)-reconstituted lambdaPP in the presence of oxoanions and PhAH demonstrate that inhibitor binding decreases the spin-coupling constant, J, compared to the native enzyme. This suggests a change in the bridging interaction between Mn(2+) ions of the dimer due to protonation or replacement of a bridging ligand. Inhibitor binding also induces several spectral shifts. Hyperfine splitting characteristic of a spin-coupled (Mn(2+))(2) dimer is most prominent upon the addition of orthovanadate (K(i) = 0.70 +/- 0.20 microM) and PhAH, indicating that these inhibitors tightly interact with the (Mn(2+))(2) form of lambdaPP. These EPR and inhibition kinetic results are discussed in the context of establishing a common mechanism for the hydrolysis of phosphate esters by lambdaPP and other serine/threonine protein phosphatases.
Although intensive investigations have been conducted on the allosteric enzyme, aspartate transcarbamoylase, which catalyzes the first committed reaction in the biosynthesis of pyrimidines in Escherichia coli, little is known about the role of individual amino acid residues in catalysis or regulation. Two inactive enzymes produced by random mutagenesis have been characterized previously but the loss of activity is probably attributable to changes in the folding of the chains stemming from the introduction of charged and bulky residues (Asp for Gly-128 and Phe for Ser-52). Site-directed mutagenesis of pyrB, which encodes the catalytic chains of the enzyme, was used to probe the functional roles of several amino acids by making more conservative substitutions. Replacement of Lys-84 by either Gln or Arg leads to virtually inactive enzymes, confirming chemical studies indicating that Lys-84 is essential for catalysis. In contrast, substitution of Gln for Lys-83 has only a slight effect on enzyme activity, whereas chemical modification causes considerable inactivation. Gln-133, which has been shown by x-ray crystallography to reside near the contact region between the catalytic and regulatory chains, was replaced by Ala. This substitution has little effect on catalytic activity but leads to a marked increase in cooperativity. The Gln-83 mutant, in contrast, exhibits much less cooperativity. Since a histidine residue may be involved in catalysis and His-134 has been shown by x-ray diffraction studies to be in close proximity to the site of binding of a bisubstrate analog, His-134 was replaced by Ala, yielding a mutant with only 5% wild-type activity, considerable cooperativity, and lower affinity for aspartate and carbamoylphosphate. All of the mutants, unlike those in which charged or bulky residues replaced small side chains, bind the bisubstrate analog, which promotes the characteristic "swelling" of the enzymes indicative of the allosteric transition.
Site-directed mutagenesis was used to determine how the allosteric properties of aspartate transcarbamoylase (ATCase) are affected by amino acid replacements in the nucleotide binding region of the regulatory polypeptide chains. Amino acid substitutions were made for both Lys-60 and Lys-94 in the regulatory chain since those residues have been implicated by x-ray diffraction studies, chemical modification experiments, and site-directed mutagenesis as playing a role in binding CTP and ATP. Lys-60 was replaced by His, Arg, Gln, and Ala, and Lys-94 was changed to His. These mutant forms of ATCase exhibit bewildering changes in the allosteric properties compared to the wild-type enzyme as well as altered affinities for the nucleotide effectors. The enzyme containing His-60 lacks both homotropic and heterotropic effects and exhibits no detectable binding of nucleotides. In contrast, the holoenzymes containing either Gln-60 or Arg-60 retain both homotropic and heterotropic effects. Replacement of Lys-60 by Ala yields a derivative exhibiting altered heterotropic effects involving insensitivity to CTP and activation by ATP. The mutant enzyme containing His-94 in place of Lys exhibits cooperativity with reduced affinity for nucleotides. The multiple substitutions at Lys-60 in the nucleotide binding region of the regulatory chains of ATCase demonstrate that different amino acids in the same location can alter indirectly the delicate balance of interactions responsible for the allosteric properties of ATCase. The studies show that it is hazardous and frequently unwarranted from single amino acid replacements of a specific residue to attribute to that residue the properties observed for the wild-type enzyme.