Limited proteolysis of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

Two phosphoenolpyruvate carboxykinases coexist in the Crassulacean Acid Metabolism plant Ananas comosus. Isolation and characterization of the smaller 65 kDa form

Two phosphoenolpyruvate carboxykinase (PEPCK, EC 4.1.1.49) isoforms of 74 and 65 kDa were found to coexist in vivo in pineapple leaves, a constitutive Crassulacean Acid Metabolism plant. The 65 kDa form was not the result of proteolytic cleavage of the larger form since extraction methods reported to prevent PEPCK proteolysis in other plant tissues failed to yield a single immunoreactive PEPCK polypeptide in leaf extracts. In this work, the smaller form of 65 kDa was purified to homogeneity and physically and kinetically characterized and showed parameters compatible with a fully active enzyme. The specific activity was nearly twice higher for decarboxylation of oxaloacetate when compared to carboxylation of phosphoenolpyruvate. Kinetic parameters fell within the range of those estimated for other plant PEPCKs. Its activity was affected by several metabolites, as shown by inhibition by 3-phosphoglycerate, citrate, malate, fructose-1,6-bisphosphate, l-asparagine and activation of the decarboxylating activity by succinate. A break in the Arrhenius plot at about 30°C indicates that PEPCK structure is responsive to changes in temperature. The results indicate that pineapple leaves contain two PEPCK forms. The biochemical characterization of the smaller isoform performed in this work suggests that it could participate in both carbon and nitrogen metabolism in vivo by acting as a decarboxylase.

Ligand interactions and protein conformational changes of phosphopyridoxyl-labeled Escherichia coli phosphoenolpyruvate carboxykinase determined by fluorescence spectroscopy

Escherichia coli phosphoenolpyruvate (PEP) carboxykinase catalyzes the decarboxylation of oxaloacetate and transfer of the gamma-phosphoryl group of ATP to yield PEP, ADP, and CO2. The interaction of the enzyme with the substrates originates important domain movements in the protein. In this work, the interaction of several substrates and ligands with E. coli PEP carboxykinase has been studied in the phosphopyridoxyl (P-pyridoxyl)-enzyme adduct. The derivatized enzyme retained the substrate-binding characteristics of the native protein, allowing the determination of several protein-ligand dissociation constants, as well as the role of Mg2+ and Mn2+ in substrate binding. The binding affinity of PEP to the enzyme-Mn2+ complex was -8.9 kcal.mol-1, which is 3.2 kcal.mol-1 more favorable than in the complex with Mg2+. For the substrate nucleotide-metal complexes, similar binding affinities (-6.0 to -6.2 kcal.mol-1) were found for either metal ion. The fluorescence decay of the P-pyridoxyl group fitted to two lifetimes of 5.15 ns (34%) and 1.2 ns. These lifetimes were markedly altered in the derivatized enzyme-PEP-Mn complexes, and smaller changes were obtained in the presence of other substrates. Molecular models of the P-pyridoxyl-E. coli PEP carboxykinase showed different degrees of solvent-exposed surfaces for the P-pyridoxyl group in the open (substrate-free) and closed (substrate-bound) forms, which are consistent with acrylamide quenching experiments, and suggest that the fluorescence changes reflect the domain movements of the protein in solution.

Urea-induced unfolding studies of free- and ligand-bound tetrameric ATP-dependent Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase. Influence of quaternary structure on protein conformational stability

ATP-dependent phosphoenolpyruvate (PEP) carboxykinases are found in plants and microorganisms, and catalyse the reversible formation of PEP, ADP, and CO(2) from oxaloacetate plus ATP. These enzymes vary in quaternary structure although there is significant sequence identity among the proteins isolated from different sources. To help understand the influence of quaternary structure in protein stability, the urea-induced unfolding of free- and substrate-bound tetrameric Saccharomyces cerevisiae PEP carboxykinase is described and compared with the unfolding characteristics of the monomeric Escherichia coli enzyme [Eur. J. Biochem. 255 (1998) 439]. The urea-induced denaturation of S. cerevisiae PEP carboxykinase was studied by monitoring the enzyme activity, intrinsic protein fluorescence, circular dichroism (CD) spectra, and 1-anilino-8-naphthalenesulfonate (ANS) binding. The unfolding profiles were multi-steps, and formation of hydrophobic structures were detected. The data indicate that unfolding and dissociation of the enzyme tetramer are simultaneous events. Ligand binding, most notably PEP in the presence of MnCl(2), conferred a marked protection against urea-induced denaturation. A similar protection effect was found when N-iodoacetyl-N'-(5-sulfo-1-napthyl)ethylene diamine (1,5-I-AEDANS) was covalently bound at Cys(365), within the active site region. Refolding experiments indicated that total recovery of tertiary structure was only obtained from samples previously unfolded to less than 30%. In the presence of substrates, complete refolding was achieved from samples originally denatured up to 50%. The unfolding behaviour of S. cerevisiae PEP carboxykinase was found to be similar to that of E. coli PEP carboxykinase, however all steps take place at lower urea concentrations. These findings show that, at least for monomeric and tetrameric ATP-dependent PEP carboxykinases, quaternary structure does not contribute to protein conformational stability.

The strongly conserved lysine 256 of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase is essential for phosphoryl transfer

Lysine 256, a conserved amino acid of Saccharomycescerevisiae phosphoenolpyruvate (PEP) carboxykinase located in the consensus kinase 1a sequence of the enzyme, was changed to alanine, arginine, or glutamine by site-directed mutagenesis. These substitutions did not result in gross changes in the protein structure, as indicated by circular dichroism, tryptophan fluorescence spectroscopy, and gel-exclusion chromatography. The three variant enzymes showed almost unaltered Km for MnADP but about a 20 000-fold decrease in Vmax for the PEP carboxylation reaction, as compared to wild-type PEP carboxykinase. The variant enzymes presented oxaloacetate decarboxylase activity at levels similar to those of the native protein; however, they lacked pyruvate kinase-like activity. The dissociation constant for the enzyme-MnATP complex was 1.3 +/- 0.3 microM for wild-type S. cerevisiae PEP carboxykinase, and the corresponding values for the Lys256Arg, Lys256Gln, and Lys256Ala mutants were 2.0 +/- 0.6 microM, 17 +/- 2 microM, and 20 +/- 6 microM, respectively. These results collectively show that a positively charged residue is required for proper binding of MnATP and that Lys256 plays an essential role in transition state stabilization during phosphoryl transfer for S. cerevisiae PEP carboxykinase.

Site-directed mutagenesis in basic amino acid residues of Saccharomyces cerevisiae phosphoenolpyruvate carboxykinase

Mutant Arg76Gln and Lys290Gln Saccharomyces cerevisiae phosphoenolpyruvate carboxykinases have been prepared and analyzed. No alteration in the apparent kinetic constants were detected for the Arg76Gln mutant enzyme, while the Lys290Gln mutant showed a 12-fold decrease in V(max)/K(m)ADP. These results indicate that Arg76 is not involved in CO2 binding, but support the hypothesis that the binding of this substrate induces a conformational change that protects the region around Arg76 from trypsin action [Herrera et al. (1993) J. Protein Chem. 12, 413-418]. These findings also indicate that Lys290, a highly reactive residue against pyrydoxal phosphate [Bazaes et al. (1995), FEBS Lett. 360, 207-210], does not perform an essential function for the enzyme activity.

Identification of reactive conserved histidines in phosphoenolpyruvate carboxykinases from Escherichia coli and Saccharomyces cerevisiae

Escherichia coli and Saccharomyces cerevisiae phospho enol pyruvate (PEP) carboxykinases are inactivated by diethylpyrocarbonate (DEP). Inactivation follows pseudo-first-order kinetics and exhibits a second order rate constant of 0.8 M-1 s-1 for the bacterial enzyme and of 3.3 M-1 s-1 for the yeast carboxykinase. A mixture of ADP + PEP + MnCl2 protects against inactivation by DEP, suggesting that residues within the active site are being modified. After digestion of the modified proteins with trypsin, the labeled peptides were isolated by reverse-phase high-performance liquid chromatography and sequenced by Edman degradation. His-271 of E. coli carboxykinase and His-273 of the yeast enzyme were identified as the reactive amino-acid residues. The modified histidine residues occupy equivalent positions in these enzymes, and they are located in a highly conserved region of all ATP-dependent phospho enol pyruvate carboxykinases described so far.

Circular dichroism and Fourier transform infrared spectroscopic studies on the secondary structure of Saccharomyces cerevisiae and Escherichia coli phospho enolpyruvate carboxykinases

The secondary structure of Saccharomyces cerevisiae and Escherichia coli phospho enolpyruvate (PEP) carboxykinases was quantitatively examined using circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopies. From CD analyses, values of 24% alpha-helix and 38% beta-sheet were obtained for the E. coli enzyme, while the corresponding values for the S. cerevisiae PEP carboxykinase were 20% and 36%. Analysis of the amide I' infrared band indicated 20% alpha-helix and 36% beta-sheet for the S. cerevisiae enzyme, while for the E. coli protein values of 40% beta-sheet and between 9 and 36% alpha-helix could be inferred. It is concluded that the bacterial enzyme has more secondary structure elements than the yeast protein. No alteration of the CD or FTIR spectra was detected upon substrate or metal ion binding to any enzyme.