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M T Mas

Publications and source records attributed to M T Mas.

30 records · Page 2Linked to original sources
PubMed1988

Structure-function relationships in 3-phosphoglycerate kinase: role of the carboxy-terminal peptide.

Yeast 3-phosphoglycerate kinase (PGK) is a monomeric enzyme (Mr approximately 45,000) composed of two globular domains. Each domain corresponds approximately to the amino- and carboxy-terminal halves of the polypeptide chain. The carboxy-terminal end extends over the interdomain "hinge" region and packs against the amino-terminal domain. It has been proposed that domain movement, resulting in closure of the active site cleft, is essential for the catalytic function of PGK. Large-scale conformational changes have also been postulated to explain activation of the enzyme by sulfate ions. Using site-specific mutagenesis, we have removed a 15-amino-acid carboxy-terminal fragment, in order to probe its role in the substrate- and sulfate-induced conformational changes. The truncated enzyme exhibited approximately 1% of the activity of native PGK and lost the ability to undergo sulfate-induced activation. The Km for ATP was essentially unchanged (Km = 0.23 mM) in comparison to the native enzyme (Km = 0.30 mM), whereas the Km value for 3-phosphoglycerate was increased about eightfold (Km = 3.85 mM and 0.50 mM, respectively). These results suggest that the carboxy-terminal segment is important for the mechanism of the substrate- and sulfate-induced conformational transitions. CD spectra and sedimentation velocity measurements indicate that the carboxy-terminal peptide is essential for structural integrity of PGK. The increased susceptibility of the truncated enzyme to thermal inactivation implies that the carboxy-terminal peptide also contributes to the stability of PGK.

Catalysis↗
PubMed1987

Site-directed mutagenesis of glutamate-190 in the hinge region of yeast 3-phosphoglycerate kinase: implications for the mechanism of domain movement.

In order to evaluate a possible contribution of glutamate-190, situated in the hinge region of yeast 3-phosphoglycerate kinase (PGK), to the mechanism of the substrate- and sulfate-induced domain movement, we have constructed two point mutants, Gln-190 and Asp-190, using oligonucleotide-directed in vitro mutagenesis. The Michaelis constants of the mutants for ATP and 3-phosphoglycerate were not significantly altered, whereas the catalytic activities were decreased, both in the absence and in the presence of sulfate ions. In the absence of sulfate, the Gln-190 and Asp-190 mutants exhibited 26% and 36% of the activity of the native enzyme. In the presence of 30 mM Na2SO4, a concentration at which native PGK exhibits maximum activation, the relative activities of the Gln-190 and Asp-190 mutants were 6% and 9%, respectively. In contrast to the native enzyme, which undergoes activation at low sulfate concentrations and inhibition at high concentrations, both mutants showed a complete loss of the salt activation effect. These results suggest that Glu-190 is not directly involved in the binding of substrates but might be important for conformational flexibility. We have also demonstrated that, similarly to native PGK, both mutants are completely inactivated by the incorporation of 1 mol of glycine ethyl ester/mol of enzyme. Appreciable protection against inactivation is observed in the presence of both substrates, MgATP and 3-phosphoglycerate. Only limited protection is observed in the presence of the individual substrates, suggesting that the modification does not occur at the substrate binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer Graphics↗
PubMed1986

Active human-yeast chimeric phosphoglycerate kinases engineered by domain interchange.

Phosphoglycerate kinase (PGK) is a monomeric protein composed of two domains of approximately equal size, connected by a hinge. Substrate-induced conformational change results in the closure of the active site cleft, which is situated between these two domains. In a study of the relations between structure and function of this enzyme, two interspecies hybrids were constructed, each composed of one domain from the human enzyme and one domain from the yeast enzyme. Despite a 35% difference in the amino acid composition between human and yeast PGK, catalytic properties of the hybrid enzymes are very similar to those of the parental proteins. This result demonstrates that the evolutionary substitutions within these two distantly related molecules do not significantly affect formation of the active site cleft, mechanism of domain closure, or enzyme activity itself.

Amino Acid Sequence↗
PubMed1985

Spectroscopic studies of the interactions of coenzymes and coenzyme fragments with pig heart, oxidized triphosphopyridine nucleotide specific isocitrate dehydrogenase.

Spectroscopic, ultrafiltration, and kinetic studies have been used to characterize interactions of reduced and oxidized triphosphopyridine nucleotides (TPNH and TPN), 2'-phosphoadenosine 5'-diphosphoribose (Rib-P2-Ado-P), and adenosine 2',5'-bisphosphate [Ado(2',5')P2] with with TPN-specific isocitrate dehydrogenase. Close similarity of the UV difference spectra and of the protein fluorescence changes accompanying the formation of the binary complexes provides evidence for the binding of these nucleotides to the same site on the enzyme. From the pH dependence of the dissociation constants for TPNH binding to TPN-specific isocitrate dehydrogenase in the absence and in the presence of Mn2+, over the pH range 5.8-7.6, it has been demonstrated that the nucleotide binds to the enzyme in its unprotonated, metal-free form. The involvement of positively charged residues, protonated over the pH range studied, has been postulated. One TPNH binding site per enzyme subunit has been measured by fluorescence and difference absorption titrations. A dramatic effect of ionic strength on binding has been demonstrated: about a 1000-fold decrease in the dissociation constant for TPNH has been observed at pH 7.6 upon decreasing ionic strength from 0.336 (Kd = 1.2 +/- 0.2 microM) to 0.036 M (Kd = 0.4 +/- 0.1 nM) in the presence and in the absence of 100 mM Na2SO4, respectively. Weak competition of sulfate ions for the nucleotide binding site has been observed (KI = 57 +/- 3 mM). The binding of TPN in the presence of 100 mM Na2SO4 at pH 7.6 is about 100-fold weaker (Kd = 110 +/- 22 microM) than the binding of the reduced coenzyme and is similarly affected by ionic strength. These results demonstrate the importance of electrostatic interactions in the binding of the coenzyme to TPN-specific isocitrate dehydrogenase. The large enhancement of protein fluorescence caused by binding of TPN and Rib-P2-Ado-P (delta Fmax = 50%) and of Ado(2',5')P2 (delta Fmax = 41%) has been ascribed to a local conformational change of the enzyme. An apparent stoichiometry of 0.5 nucleotide binding site per peptide chain was determined for TPN, Rib-P2-Ado-P, and Ado(2',5')P2 from fluorescence titrations, in contrast to one binding site per enzyme subunit determined from UV difference spectral titration and ultrafiltration experiments. Thus, the binding of one molecule of the nucleotide per dimeric enzyme molecule is responsible for the total increase in protein fluorescence, while binding to the second subunit does not cause further change.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Diphosphate↗
PubMed1984

Phosphorus-31 nuclear magnetic resonance studies of the binding of nucleotides to NADP+-specific isocitrate dehydrogenase.

The interaction of the 2'-phosphate-containing nucleotides (NADP+, NADPH, 2'-phosphoadenosine 5'-diphosphoribose, and adenosine 2',5'-bisphosphate) with NADP+ -specific isocitrate dehydrogenase was studied by using 31P NMR spectroscopy. The separate resonances corresponding to free and bound nucleotides, characteristic for slow exchange of nuclei on the NMR time scale, were observed in the spectra of the enzyme (obtained in the presence of excess ligand) with NADP+ and NADPH in the absence and presence of Mg2+ and with 2'-phosphoadenosine 5'-diphosphoribose in the absence of metal or in the presence of the substrate magnesium isocitrate. The position of the 31P resonance of the bound 2'-phosphate group in these spectra is invariant (delta = 6) in the pH range 5-8, indicating that the pK of this group is much lower in the complexes with the enzyme than that (pK = 6.13) in the free nucleotides. The additional downfield shift of this resonance by 1.8 ppm beyond that (delta = 4.22) of the dianionic form of the 2'-phosphate in free nucleotides suggests interaction with a positively charged group(s) and/or distortion of P-O-P angles as the result of binding to the enzyme. A single resonance of 2'-phosphate was observed in the spectrum of the enzyme complex with 2'-phosphoadenosine 5'-diphosphoribose in the presence of Mg2+, with the chemical shift dependent on the nucleotide to enzyme ratio, characteristic for the fast exchange situation. Addition of metal does not perturb the environment of the 2'-phosphate in the complexes of NADP+ and NADPH with isocitrate dehydrogenase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Highly phosphorylated region of chicken riboflavin-binding protein: chemical characterization and 31P NMR studies.

Phosphate residues on chicken riboflavin-binding protein (RBP) have been implicated in the recognition and deposition of the protein-vitamin complex in egg yolk by the ovary. We demonstrate that all of the phosphate of RBP is linked to serine and that most if not all of the phosphoserine residues are contained in a single tryptic peptide having a composition SerP8, Glx6, His2, Leu2, Ala, Met, Lys. Despite differences in the tissue of synthesis, RBPs isolated from egg yolk and egg white yield phosphopeptides virtually identical in their amino acid composition and 101-MHz 31P NMR spectra. This implies that posttranslational phosphorylation of RBP is the same in liver and oviduct. The 31P NMR spectrum of the phosphopeptide is quite different from that of the phosphoprotein. The 31P NMR spectra of egg yolk and egg white RBP are quite similar but not identical and are unaffected by the binding of riboflavin. Optimal resolution of seven phosphorus resonances was obtained near pH 7.0. The titration behavior of all the phosphoserine residues is similar. We propose that this highly anionic peptide, exposed at the surface of RBP, is the principal determinant for the uptake of RBP by the vitelline membrane of the ovarian follicle.

Amino Acids↗
PubMed1983

Modification of TPN-dependent isocitrate dehydrogenase by the 2', 3'-dialdehyde derivatives of TPNH and TPN.

Catalytic reaction of the 2', 3'-dialdehyde analog of TPN (oTPN) with pig heart TPN-dependent isocitrate dehydrogenase in the presence of the substrate manganous isocitrate results in the formation of the dialdehyde derivative of TPNH (oTPNH). In the absence of the substrate, modification by oTPN leads to a progressive inactivation of the enzyme. The dependence of the pseudo-first order rate constants on the reagent concentration indicates the formation of a reversible complex with the enzyme prior to covalent modification (kmax = 5.5 X 10(-2) min-1; K1 = 290 microM). Reaction of [14C]oTPN with the enzyme results in the incorporation of 2 mol of oTPN/mol of peptide chain. No appreciable protection against either inactivation or incorporation by the natural ligands TPN and TPNH was obtained, suggesting different modes of binding of the analog in the presence and absence of the substrate isocitrate. Enzymatically synthesized oTPNH has been isolated and demonstrated to act as an affinity label for a TPNH-binding site of isocitrate dehydrogenase. The inactivation process exhibits saturation kinetics (kmax = 2.67 X 10(-3) min-1; K1 = 33 microM). Protection against activity loss, as well as a decrease in incorporation from 2 to 1 eq of [14C]oTPNH bound/peptide chain was observed in the presence of 1 mM TPNH. From the TPNH concentration dependence of the inactivation rate by oTPNH, a dissociation constant of 3.4 microM is calculated for TPNH, indicating binding of the analog to a specific TPNH-binding site on the enzyme. Although dialdehyde derivatives are frequently assumed to form Schiff bases with proteins, the evidence presented suggests the formation of morpholino derivatives as the products of the covalent reaction of isocitrate dehydrogenase with the dialdehyde derivatives of TPN and TPNH. The new reagent, oTPNH, may serve as an affinity label for other dehydrogenases.

Adenosine Diphosphate↗
PubMed1981

Spectral evidence for distinct mode of interaction of nucleotides with rabbit muscle and rabbit liver aldolase.

Ultraviolet difference spectra produced by the binding of mononucleotides and phosphates to rabbit aldolase A and B were analyzed. Both isozymes exhibit a distinct mode of interaction with the ligands. The binding seems to be based on multipoint interaction of nucleotides with each aldolase, indicating the existence of specific nucleotide binding domains in both proteins.

Animals↗
PubMed1980

Topography of rhodopsin in rod outer segment disk membranes. Photochemical labeling with N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonate.

Rod cell disk membranes have been photochemically reacted with the water-soluble, membrane-impermeable nitrene precursor N-(4-azido-2-nitrophenyl)-2-aminoethane-sulfonate [NAP-taurine, NAPT]. Rhodopsin, minor membrane proteins, and lipids all incorporate the (nitrophenyl)[35S]taurine (NPT) label. We also find that rhodopsin may be labeled in the dark using prephotolyzed NAPT. This reaction is presumably due to long-lived photoproducts of NAPT. NAPT modifies rhodopsin in the membrane in a selective manner; some cyanogen bromide peptides of NPT-rhodopsin contain appreciable NPT label and some contain essentially none. Precise specific radioactivities could not be determined for the [35S]NPT-peptides since loss of label from some of the peptides was observed during purification procedures. Rhodopsin's carboxyl-terminal cyanogen bromide peptides are well labeled when the protein is modified in disk membranes but the amino-terminal peptide is poorly labeled. When rhodopsin is labeled in reconstituted membranes in which both surfaces of rhodopsin are accessible to reagent, labeling of rhodopsin's amino-terminal peptide is enhanced. These results are consistent with a model in which rhodopsin's carboxyl-terminal region is located at the cytoplasmic (external) surface of disk membranes and its amino terminus is located at the intradiskal membrane surface.

Affinity Labels↗