Phosphoglycerate dehydrogenase from soybean nodules : partial purification and some kinetic properties

Novel regulatory mechanism of serine biosynthesis associated with 3-phosphoglycerate dehydrogenase in Arabidopsis thaliana

The proteinogenic amino acid l-serine is a precursor for various essential biomolecules in all organisms. 3-Phosphoglycerate dehydrogenase (PGDH) is the first committed enzyme of the phosphorylated pathway of l-serine biosynthesis, and is regulated by negative feedback from l-serine in bacteria and plants. In the present study, two Arabidopsis PGDH isoforms were inhibited by l-serine but were activated by l-amino acids such as l-homocysteine in vitro. Activation and inhibition by these amino acids was cooperative, suggesting an allosteric mechanism. Moreover, the half maximal effective concentration of l-homocysteine was 2 orders of magnitude lower than that of l-serine, suggesting greater regulatory potency. These are the first data to show that PGDH is activated by various biomolecules and indicate that serine biosynthesis is regulated by multiple pathways.

Molecular and functional characterization of D-3-phosphoglycerate dehydrogenase in the serine biosynthetic pathway of the hyperthermophilic archaeon Sulfolobus tokodaii

A gene (ST1218) encoding a D-3-phosphoglycerate dehydrogenase (PGDH; EC 1.1.1.95) homolog was found in the genome of Sulfolobus tokodaii strain 7 by screening a database of enzymes likely to contribute to l-serine biosynthesis in hyperthermophilic archaea. After expressing the gene in Escherichia coli, the PGDH activity of the recombinant enzyme was assessed. Homogeneous PGDH was obtained using conventional chromatography steps, though during the purification an unexpected decline in enzyme activity was observed if the enzyme was stored in plastic tubes, but not in glass ones. The purified enzyme was a homodimer with a subunit molecular mass of about 35kDa and was highly thermostable. It preferably acted as an NAD-dependent D-3-phosphoglycerate (3PGA) dehydrogenase. Although NADP had no activity as the electron acceptor, both NADPH and NADH acted as electron donors. Kinetic analyses indicated that the enzyme reaction proceeds via a Theorell-Chance Bi-Bi mechanism. Unlike E. coli PGDH, the S. tokodaii enzyme was not inhibited by l-serine. In addition, both the NAD-dependent 3PGA oxidation and the reverse reaction were enhanced by phosphate and sulfate ions, while NADPH-dependent 3-phosphohydroxypyruvate (PHP) reduction was inhibited. Thus S. tokodaii PGDH appears to be subject to a novel regulatory mechanism not seen elsewhere. A database analysis showed that ST1218 gene forms a cluster with ST1217 gene, and a functional analysis of the ST1217 product expressed in E. coli revealed that it possesses l-glutamate-PHP aminotransferase activity. Taken together, our findings represent the first example of a phosphorylated serine pathway in a hyperthermophilic archaeon.

Purification, cloning and functional expression of hydroxyphenylpyruvate reductase involved in rosmarinic acid biosynthesis in cell cultures of Coleus blumei

Hydroxyphenylpyruvate reductase (HPPR) is an enzyme involved in the biosynthesis of rosmarinic acid in Lamiaceae reducing hydroxyphenylpyruvates in dependence of NAD(P)H to the corresponding hydroxyphenyllactates. The HPPR protein was purified from suspension cells of Coleus blumei accumulating high levels of rosmarinic acid by ammonium sulfate precipitation, anion exchange chromatography, hydroxylapatite chromatography, chromatography on 2',5'-ADP-Sepharose 4B and SDS-polyacrylamide gel electrophoresis. The protein was tryptically digested and the peptides sequenced. Sequence information was used to isolate a full-length cDNA-clone for HPPR (EMBL accession number AJ507733) by RT-PCR, screening of a C. blumei cDNA-library and 5'-RACE-PCR. The open reading frame of the HPPR-cDNA consists of 939 nucleotides encoding a protein of 313 amino acid residues. The sequence showed that HPPR belongs to the family of D-isomer-specific 2-hydroxyacid dehydrogenases. The HPPR-cDNA was heterologously expressed in Escherichia coli and the protein was shown to catalyse the NAD(P)H-dependent reduction of 4-hydroxyphenylpyruvate to 4-hydroxyphenyllactate and 3,4-dihydroxyphenylpyruvate to 3,4-dihydroxyphenyllactate.

Location of two isoenzymes of NADH-dependent glutamate synthase in root nodules of Phaseolus vulgaris L

The two isoenzymes of NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14), previously identified in root nodules of Phaseolus vulgaris L., have both been shown to be located in root-nodule plastids. The nodule specific NADH-GOGAT II accounts for the majority of the activity in root nodules, and is present almost exclusively in the central tissue of the nodule. However about 20% of NADH-GOGAT I activity is present in the nodule cortex, at about the same specific activity as this isoenzyme is found in the central tissue. Glutamine synthetase (GS; EC 6.3.1.2) occurs predominantly as the γ polypeptide in the central tissue, whereas in the cortex, the enzyme is represented mainly by the β polypeptide. Over 90% of both GS and NADH-GOGAT activities are located in the central tissue of the nodule and GS activity exceeds NADH-GOGAT activity by about twofold in this region. Using the above information, a model for the subcellular location and stoichiometry of nitrogen metabolism in the central tissue of P. vulgaris root nodules is presented.

Nucleoside diphosphatase and 5'-nucleotidase activities of soybean root nodules and other tissues

Nucleoside diphosphatase and 5'-nucleotidase activities were both found to be very high in extracts of soybean (Glycine max L.) root nodules. Both activities increased early in soybean nodule development, prior to the rise in leghemoglobin, and both were found at equivalent levels in nitrogenfixing and nonfixing nodules. Based on a survey of other tissues, these activities were both highest in soybean nodules (1300 nanomoles per milligram protein per minute, nucleoside diphosphatase and 500 nanomoles per milligram protein per minute, 5'-nucleotidase), but they were not always associated with each other; in some tissues one was high and the other low. Neither activity correlated well with ureide production; both seem, rather, to be primarily involved in some other metabolic function. Both the nucleoside diphosphatase and 5'-nucleotidase of soybean nodules were soluble proteins, and neither appeared to be associated with plastids, mitochondria, or bacteroids.

Phosphoserine aminotransferase in soybean root nodules : demonstration and localization

Phosphoserine aminotransferase activity was detected in the plant and bacteroid fractions from soybean (Glycine max) root nodules. Both total and specific activities increased in the plant fraction during nodule development. Serine-pyruvate aminotransferase activity was not detectable in the plant or bacteroid fractions of these nodules. Sucrose density gradient fractionation indicated a proplastid localization for phosphoserine aminotransferase. The data presented support a role for this enzyme in carbon supply to purine biosynthesis in the pathway of ureide biogenesis in soybean nodules.