Cloning and characterization of a putative human d-2-hydroxyacid dehydrogenase in chromosome 9q

Phosphoserine aminotransferase has conserved active site from microbes to higher eukaryotes with minor deviations

Serine is ubiquitously synthesized in all living organisms from the glycolysis intermediate 3-phosphoglycerate (PGA) by phosphoserine biosynthetic pathway, consisting of three different enzymes, namely: 3-phosphoglycerate dehydrogenase (PGDH), phosphoserine aminotransferase (PSAT), and phosphoserine phosphatase (PSP). Any functional defect or mutation in these enzymes may cause deliberating conditions, such as colon cancer progression and chemoresistance in humans. Phosphoserine aminotransferase (PSAT) is the second enzyme in this pathway that converts phosphohydroxypyruvate (PHP) to O-phospho-L-serine (OPLS). Humans encode two isoforms of this enzyme: PSAT1 and PSAT2. PSAT1 exists as a functional dimer, where each protomer has a large and a small domain; each large domain contains a Lys residue that covalently binds PLP. The PLP-binding site of human PSAT1 and most of its active site residues are highly conserved in all known PSAT structures except for Cys-80. Interestingly, Two PSAT structures from different organisms show halide binding near their active site. While the human PSAT1 shows a water molecule at this site with different interacting residues, suggesting the inability of halide binding in the human enzyme. Analysis of the human PSAT1 structure showed a big patch of positive charge around the active site, in contrast to the bacterial PSATs. Compared to human PSAT1, the PSAT2 isoform lacks 46 residues at its C-terminal tail. This tail region is present at the opening of the active site as observed in the other PSAT structures. Further structural work on human PSAT2 may reveal the functional importance of these 46 residues.

Promoter rearrangements cause species-specific hepatic regulation of the glyoxylate reductase/hydroxypyruvate reductase gene by the peroxisome proliferator-activated receptor alpha

In liver, the glyoxylate cycle contributes to two metabolic functions, urea and glucose synthesis. One of the key enzymes in this pathway is glyoxylate reductase/hydroxypyruvate reductase (GRHPR) whose dysfunction in human causes primary hyperoxaluria type 2, a disease resulting in oxalate accumulation and formation of kidney stones. In this study, we provide evidence for a transcriptional regulation by the peroxisome proliferator-activated receptor alpha (PPARalpha) of the mouse GRHPR gene in liver. Mice fed with a PPARalpha ligand or in which PPARalpha activity is enhanced by fasting increase their GRHPR gene expression via a peroxisome proliferator response element located in the promoter region of the gene. Consistent with these observations, mice deficient in PPARalpha present higher plasma levels of oxalate in comparison with their wild type counterparts. As expected, the administration of a PPARalpha ligand (Wy-14,643) reduces the plasma oxalate levels. Surprisingly, this effect is also observed in null mice, suggesting a PPARalpha-independent action of the compound. Despite a high degree of similarity between the transcribed region of the human and mouse GRHPR gene, the human promoter has been dramatically reorganized, which has resulted in a loss of PPARalpha regulation. Overall, these data indicate a species-specific regulation by PPARalpha of GRHPR, a key gene of the glyoxylate cycle.

Molecular and structural characterization of NADPH-dependent d-glycerate dehydrogenase from the enteric parasitic protist Entamoeba histolytica

Putative NADPH-dependent GDH (L-glycerate dehydrogenase) of the protozoan parasite Entamoeba histolytica (EhGDH) has been characterized. The EhGDH gene encodes a protein of 318 amino acids with a calculated isoelectric point of 6.29 and a molecular mass of 35.8 kDa. EhGDH showed highest identities with GDH from epsilon-proteobacteria. This close kinship was also supported by phylogenetic analyses, suggesting possible lateral transfer of the gene from epsilon-proteobacteria to E. histolytica. In contrast with the implications from protein alignment and phylogenetic analysis, kinetic studies revealed that the amoebic GDH showed biochemical properties similar to those of mammalian GDH, i.e. a preference for NADPH as cofactor and higher affinities towards NADPH and beta-hydroxypyruvate than towards NADP+ and L-glycerate. Whereas the amino acids involved in nucleotide binding and catalysis are totally conserved in EhGDH, substitution of a negatively charged amino acid with a non-charged hydroxy-group-containing amino acid is probably responsible for the observed high affinity of EhGDH for NADP+/NADPH. In addition, the amoebic GDH, dissimilar to the bacterial and mammalian GDHs, lacks glyoxylate reductase activity. Native and recombinant EhGDH showed comparable subunit structure, kinetic parameters and elution profiles on anion-exchange chromatography. We propose that the GDH enzyme is likely to be involved in regulation of the intracellular concentration of serine, and, thus, also in controlling cysteine biosynthesis located downstream of serine metabolic pathways in this protist.