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At least 361 records · Page 20Linked to original sources
PubMed2002

Structure of the Plasmodium falciparum triosephosphate isomerase-phosphoglycolate complex in two crystal forms: characterization of catalytic loop open and closed conformations in the ligand-bound state.

Triosephosphate isomerase (TIM) has been the subject of many structural and mechanistic studies. At position 96, there is a highly conserved Ser residue, which is proximal to the catalytic site. Thus far, no specific role has been ascribed to this residue. Plasmodium falciparum TIM (PfTIM), a fully catalytically active enzyme, is unique in possessing a Phe residue at position 96. The structure of PfTIM complexed to phosphoglycolate (PG), a transition state analogue, has been determined in an effort to probe the effects of the mutation at residue 96 on the nature of inhibitor-enzyme interactions and the orientation of the critical catalytic loop (loop 6, residues 166-176) in TIM. Crystal structures of PfTIM complexed to phosphoglycolate in orthorhombic (P2(1)2(1)2(1)) and monoclinic (C2) forms were determined and refined at resolutions of 2.8 and 1.9 A, respectively. The P2(1)2(1)2(1) form contains two dimers in the asymmetric unit. In the C2 form, the molecular and crystal 2-fold axes are coincident, leading to a monomer in the asymmetric unit. The catalytic loop adopts the open state in the P2(1)2(1)2(1) form and the closed conformation in the C2 crystal. The open conformation of the loop in the P2(1)2(1)2(1) form appears to be a consequence of the Ser to Phe mutation at residue 96. The steric clash between Phe96 and Ile172 probably impedes loop closure in PfTIM-ligand complexes. The PfTIM-PG complex is the first example of a TIM-ligand complex being observed in both loop open and closed forms. In the C2 form (loop closed), Phe96 and Leu167 adopt alternative conformations that are different from the ones observed in the open form, permitting loop closure. These structures provide strong support for the view that loop closure is not essential for ligand binding and that dynamic loop movement may occur in both free and ligand-bound forms of the enzyme.

Amino Acid Substitution↗
PubMed1999

cDNA cloning and characterization of maize phosphoenolpyruvate carboxykinase, a bundle sheath cell-specific enzyme.

We isolated a full-length cDNA that encodes ATP-dependent phosphoenolpyruvate carboxykinase (EC 4.1.1.49, PCK) from leaves of maize, an NADP-malic enzyme type C4 plant. The mRNA was specifically and rather abundantly expressed in bundle sheath cells in accordance with the recent finding of cell-type-specific localization of PCK protein in maize, which has been detected with antibodies against cucumber PCK protein. The predicted protein had an N-terminal extension, which is characteristic of plant PCKs. The transcript level was much higher in the daytime than at night in 14-day old seedlings. However, in 42-day old plants the extent of diurnal change decreased. The maize PCK was expressed in Escherichia coli with the pET32 plasmid and purified to homogeneity. Through digestion with enterokinase, two types of enzyme were prepared; one with an intact N-terminus and the other lacking its N-terminal 77 amino acid residues due to over-digestion. The truncated protein had about 2-fold higher specific activity than the intact one, and was inhibited by 3-phosphoglycerate (3-PGA) with an I0.5 of 17.5 mM. In contrast, the intact protein was almost insensitive to 3-PGA. These results strongly suggest that the intact N-terminal extension may be involved in the regulation of PCK activity in vivo through some modification such as reversible phosphorylation.

Amino Acid Sequence↗
PubMed1996

A novel alpha-ketoglutarate reductase activity of the serA-encoded 3-phosphoglycerate dehydrogenase of Escherichia coli K-12 and its possible implications for human 2-hydroxyglutaric aciduria.

Escherichia coli serA-encoded 3-phosphoglycerate (3PG) dehydrogenase catalyzes the first step of the major phosphorylated pathway of L-serine (Ser) biosynthesis. The SerA enzyme is evolutionarily related to the pdxB gene product, 4-phosphoerythronate dehydrogenase, which catalyzes the second step in one branch of pyridoxal 5'-phosphate coenzyme biosynthesis. Both the Ser and pyridoxal 5'-phosphate biosynthetic pathways use the serC(pdxF)-encoded transaminase in their next steps. In an analysis of these parallel pathways, we attempted to couple the transaminase and dehydrogenase reactions in the reverse direction. Unexpectedly, we found that the SerA enzyme catalyzes a previously undetected reduction of alpha-ketoglutarate (alpha KG) to 2-hydroxyglutaric acid (HGA). Numerous criteria ruled out the possibility that this SerA alpha KG reductase activity was caused by contamination in the substrate or purified enzyme preparations. HGA was confirmed as the product of the SerA alpha KG reductase reaction by thin-layer chromatography and by enzyme assays showing that both the D- and L-isomers of HGA were substrates for the reverse (dehydrogenase) reaction. Detailed steady-state kinetic analyses showed that alpha KG reduction (apparent Michaelis-Menten constant [Km(app)] = 88 microM; apparent catalytic constant [kcat(app)] = 33.3 s-1) and 3-phosphohydroxypyruvate reduction (Km(app) = 3.2 microM; kcatapp = 27.8 s-1), which is the reverse reaction of 3PG oxidation, were the major in vitro activities of the SerA enzyme. The SerA alpha KG reductase was inhibited by Ser, D-HGA, 3PG, and glycine (Gly), whereas the D-HGA dehydrogenase was inhibited by Ser, alpha KG, 3-phosphohydroxypyruvate, and Gly. The implications of these findings for the regulation of Ser biosynthesis, the recycling of NADH, and the enzymology of 2-hydroxyacid dehydrogenases are discussed. Since the same pathway of Ser biosynthesis seems to be present in all organisms, these results suggest that a mutation in the human SerA homolog may contribute to the neurometabolic diseases D- and L-2-hydroxyglutaric aciduria, which lead to the accumulation of D-HGA and L-HGA, respectively.

Alcohol Oxidoreductases↗
PubMed1980

Metabolic regulation by pH gradients. Inhibition of photosynthesis by indirect proton transfer across the chloroplast envelope.

Anions of several weak acids inhibited photosynthesis in isolated spinach chloroplasts. Inhibition was drastic at low pH and weak or absent at high pH. Glyoxylate was particularly effective and inhibition decreased in the order: glyoxylate, nitrite, glycerate, formate, hydroxypyruvate, glycolate, propionate, acetate, pyruvate. These anions operated as indirect proton shuttles across the chloroplast envelope. They compensated active proton fluxes into the medium, minimized gradients in proton activity across the chloroplast envelope, and so prevented light-dependent stroma alkalization. This caused inhibition of sugar bisphosphatases which are known to be pH-regulated. At concentrations that caused potosynthesis inhibition, the proton shuttles were not effective in decreasing the proton gradient across the thylakoids. Some anions also inhibited fructose-bisphosphatase directly, when present at concentratins higher than needed for photosynthesis inhibition.

Anions↗
PubMed2006

Domain swapping between a cyanobacterial and a plant subunit ADP-glucose pyrophosphorylase.

ADP-glucose pyrophosphorylase (ADP-Glc PPase) catalyzes the regulatory step in the pathway for synthesis of bacterial glycogen and starch in plants. ADP-Glc PPases from cyanobacteria (homotetramer) and from potato (Solanum tuberosum) tuber (heterotetramer) are activated by 3-phosphoglycerate and inhibited by inorganic orthophosphate. To study the function of two putative domains, chimeric enzymes were constructed. PSSANA contained the N-terminus (292 amino acids) of the potato tuber ADP-Glc PPase small subunit (PSS) and the C-terminus (159 residues) of the Anabaena PCC 7120 enzyme. ANAPSS was the inverse chimera. These constructs were expressed separately or together with the large subunit of the potato tuber ADP-Glc PPase (PLS), to obtain homo- and heterotetrameric chimeric proteins. Characterization of these forms showed that the N-terminus determines stability and regulatory redox-dependent properties. The chimeric forms exhibited intermediate 3-phosphoglycerate activation properties with respect to the wild-type homotetrameric enzymes, indicating that the interaction between the putative N- and C-domains determines the affinity for the activator. Characterization of the chimeric heterotetramers showed the functionality of the large subunit, mainly in modulating regulation of the enzyme by the coordinate action of 3-phosphoglycerate and inorganic orthophosphate.

Amino Acid Sequence↗