The isolation and characterization of 3-phosphoglycerate dehydrogenase from peas

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.

Lysine, Lysine-Rich, Serine, and Serine-Rich Proteins: Link Between Metabolism, Development, and Abiotic Stress Tolerance and the Role of ncRNAs in Their Regulation

Lysine (Lys) is indispensable nutritionally, and its levels in plants are modulated by both transcriptional and post-transcriptional control during plant ontogeny. Animal glutamate receptor homologs have been detected in plants, which may participate in several plant processes through the Lys catabolic products. Interestingly, a connection between Lys and serotonin metabolism has been established recently in rice. 2-Aminoadipate, a catabolic product of Lys appears to play a critical role between serotonin accumulation and the color of rice endosperm/grain. It has also been shown that expression of some lysine-methylated proteins and genes encoding lysine-methyltransferases (KMTs) are regulated by cadmium even as it is known that Lys biosynthesis and its degradation are modulated by novel mechanisms. Three complex pathways co-exist in plants for serine (Ser) biosynthesis, and the relative preponderance of each pathway in relation to plant development or abiotic stress tolerance are being unfolded slowly. But the phosphorylated pathway of L-Ser biosynthesis (PPSB) appears to play critical roles and is essential in plant metabolism and development. Ser, which participates indirectly in purine and pyrimidine biosynthesis and plays a pivotal role in plant metabolism and signaling. Also, L-Ser has been implicated in plant responses to both biotic and abiotic stresses. A large body of information implicates Lys-rich and serine/arginine-rich (SR) proteins in a very wide array of abiotic stresses. Interestingly, a link exists between Lys-rich K-segment and stress tolerance levels. It is of interest to note that abiotic stresses largely influence the expression patterns of SR proteins and also the alternative splicing (AS) patterns. We have checked if any lncRNAs form a cohort of differentially expressed genes from the publicly available PPSB, sequence read archives of NCBI GenBank. Finally, we discuss the link between Lys and Ser synthesis, catabolism, Lys-proteins, and SR proteins during plant development and their myriad roles in response to abiotic stresses.

Deficiency in the Phosphorylated Pathway of Serine Biosynthesis Perturbs Sulfur Assimilation

Although the plant Phosphorylated Pathway of l-Ser Biosynthesis (PPSB) is essential for embryo and pollen development, and for root growth, its metabolic implications have not been fully investigated. A transcriptomics analysis of Arabidopsis (Arabidopsis thaliana) PPSB-deficient mutants at night, when PPSB activity is thought to be more important, suggested interaction with the sulfate assimilation process. Because sulfate assimilation occurs mainly in the light, we also investigated it in PPSB-deficient lines in the day. Key genes in the sulfate starvation response, such as the adenosine 5'phosphosulfate reductase genes, along with sulfate transporters, especially those involved in sulfate translocation in the plant, were induced in the PPSB-deficient lines. However, sulfate content was not reduced in these lines as compared with wild-type plants; besides the glutathione (GSH) steady-state levels in roots of PPSB-deficient lines were even higher than in wild type. This suggested that PPSB deficiency perturbs the sulfate assimilation process between tissues/organs. Alteration of thiol distribution in leaves from different developmental stages, and between aerial parts and roots in plants with reduced PPSB activity, provided evidence supporting this idea. Diminished PPSB activity caused an enhanced flux of ³⁵S into thiol biosynthesis, especially in roots. GSH turnover also accelerated in the PPSB-deficient lines, supporting the notion that not only biosynthesis, but also transport and allocation, of thiols were perturbed in the PPSB mutants. Our results suggest that PPSB is required for sulfide assimilation in specific heterotrophic tissues and that a lack of PPSB activity perturbs sulfur homeostasis between photosynthetic and nonphotosynthetic tissues.

D-3-Phosphoglycerate Dehydrogenase

l-Serine is the immediate precursor of d-serine, a major agonist of the N-methyl-d-aspartate (NMDA) receptor. l-Serine is a pivotal amino acid since it serves as a precursor to a large number of essential metabolites besides d-serine. In all non-photosynthetic organisms, including mammals, a major source of l-serine is the phosphorylated pathway of l-serine biosynthesis. The pathway consists of three enzymes, d-3-phosphoglycerate dehydrogenase (PGDH), phosphoserine amino transferase (PSAT), and l-phosphoserine phosphatase (PSP). PGDH catalyzes the first step in the pathway by converting d-3-phosphoglycerate (PGA), an intermediate in glycolysis, to phosphohydroxypyruvate (PHP) concomitant with the reduction of NAD⁺. In some, but not all organisms, the catalytic activity of PGDH can be regulated by feedback inhibition by l-serine. Three types of PGDH can be distinguished based on their domain structure. Type III PGDHs contain only a nucleotide binding and substrate binding domain. Type II PGDHs contain an additional regulatory domain (ACT domain), and Type I PGDHs contain a fourth domain, termed the ASB domain. There is no consistent pattern of domain content that correlates with organism type, and even when additional domains are present, they are not always functional. PGDH deficiency results in metabolic defects of the nervous system whose systems range from microcephaly at birth, seizures, and psychomotor retardation. Although deficiency of any of the pathway enzymes have similar outcomes, PGDH deficiency is predominant. Dietary or intravenous supplementation with l-serine is effective in controlling seizures but has little effect on psychomotor development. An increase in PGDH levels, due to overexpression, is also associated with a wide array of cancers. In culture, PGDH is required for tumor cell proliferation, but extracellular l-serine is not able to support cell proliferation. This has led to the hypothesis that the pathway is performing some function related to tumor growth other than supplying l-serine. The most well-studied PGDHs are bacterial, primarily from Escherichia coli and Mycobacterium tuberculosis, perhaps because they have been of most interest mechanistically. However, the relatively recent association of PGDH with neuronal defects and human cancers has provoked renewed interest in human PGDH.

Overexpression of PSP1 enhances growth of transgenic Arabidopsis plants under ambient air conditions

The importance of the phosphorylated pathway (PPSB) of L-serine (Ser) biosynthesis in plant growth and development has been demonstrated, but its specific role in leaves and interaction with photorespiration, the main leaf Ser biosynthetic pathway at daytime, are still unclear. To investigate whether changes in biosynthesis of Ser by the PPSB in leaves could have an impact on photorespiration and plant growth, we overexpressed PSP1, the last enzyme of this pathway, under control of the Cauliflower Mosaic Virus 35S promoter in Arabidopsis thaliana. Overexpressor plants grown in normal air displayed larger rosette diameter and leaf area as well as higher fresh and dry weight than the wild type. By contrast, no statistically significant differences to the wild type were observed when the overexpressor seedlings were transferred to elevated CO₂, indicating a relationship between PSP1 overexpression and photorespiration. Additionally, the transgenic plants displayed higher photorespiration, an increase in CO₂ net-uptake and stronger expression in the light of genes encoding enzymes involved in photorespiration. We further demonstrated that expression of many genes involved in nitrogen assimilation was also promoted in leaves of transgenic plants and that leaf nitrate reductase activity increased in the light, too, although not in the dark. Our results suggest a close correlation between the function of PPSB and photorespiration, and also nitrogen metabolism in leaves.

Serine in plants: biosynthesis, metabolism, and functions

Serine (Ser) has a fundamental role in metabolism and signaling in living organisms. In plants, the existence of different pathways of Ser biosynthesis has complicated our understanding of this amino acid homeostasis. The photorespiratory glycolate pathway has been considered to be of major importance, whereas the nonphotorespiratory phosphorylated pathway has been relatively neglected. Recent advances indicate that the phosphorylated pathway has an important function in plant metabolism and development. Plants deficient in this pathway display developmental defects in embryos, male gametophytes, and roots. We propose that the phosphorylated pathway is more important than was initially thought because it is the only Ser source for specific cell types involved in developmental events. Here, we discuss its importance as a link between metabolism and development in plants.

Arabidopsis phosphoglycerate dehydrogenase1 of the phosphoserine pathway is essential for development and required for ammonium assimilation and tryptophan biosynthesis

In plants, two independent serine biosynthetic pathways, the photorespiratory and glycolytic phosphoserine (PS) pathways, have been postulated. Although the photorespiratory pathway is well characterized, little information is available on the function of the PS pathway in plants. Here, we present a detailed characterization of phosphoglycerate dehydrogenases (PGDHs) as components of the PS pathway in Arabidopsis thaliana. All PGDHs localize to plastids and possess similar kinetic properties, but they differ with respect to their sensitivity to serine feedback inhibition. Furthermore, analysis of pgdh1 and phosphoserine phosphatase mutants revealed an embryo-lethal phenotype and PGDH1-silenced lines were inhibited in growth. Metabolic analyses of PGDH1-silenced lines grown under ambient and high CO2 conditions indicate a direct link between PS biosynthesis and ammonium assimilation. In addition, we obtained several lines of evidence for an interconnection between PS and tryptophan biosynthesis, because the expression of PGDH1 and phosphoserine aminotransferase1 is regulated by MYB51 and MYB34, two activators of tryptophan biosynthesis. Moreover, the concentration of tryptophan-derived glucosinolates and auxin were reduced in PGDH1-silenced plants. In essence, our results provide evidence for a vital function of PS biosynthesis for plant development and metabolism.

Functional characterization of the plastidial 3-phosphoglycerate dehydrogenase family in Arabidopsis

This work contributes to unraveling the role of the phosphorylated pathway of serine (Ser) biosynthesis in Arabidopsis (Arabidopsis thaliana) by functionally characterizing genes coding for the first enzyme of this pathway, 3-phosphoglycerate dehydrogenase (PGDH). We identified two Arabidopsis plastid-localized PGDH genes (3-PGDH and EMBRYO SAC DEVELOPMENT ARREST9 [EDA9]) with a high percentage of amino acid identity with a previously identified PGDH. All three genes displayed a different expression pattern indicating that they are not functionally redundant. pgdh and 3-pgdh mutants presented no drastic visual phenotypes, but eda9 displayed delayed embryo development, leading to aborted embryos that could be classified as early curled cotyledons. The embryo-lethal phenotype of eda9 was complemented with an EDA9 complementary DNA under the control of a 35S promoter (Pro-35S:EDA9). However, this construct, which is poorly expressed in the anther tapetum, did not complement mutant fertility. Microspore development in eda9.1eda9.1 Pro-35S:EDA9 was arrested at the polarized stage. Pollen from these lines lacked tryphine in the interstices of the exine layer, displayed shrunken and collapsed forms, and were unable to germinate when cultured in vitro. A metabolomic analysis of PGDH mutant and overexpressing plants revealed that all three PGDH family genes can regulate Ser homeostasis, with PGDH being quantitatively the most important in the process of Ser biosynthesis at the whole-plant level. By contrast, the essential role of EDA9 could be related to its expression in very specific cell types. We demonstrate the crucial role of EDA9 in embryo and pollen development, suggesting that the phosphorylated pathway of Ser biosynthesis is an important link connecting primary metabolism with development.

Serine biosynthesis by photorespiratory and non-photorespiratory pathways: an interesting interplay with unknown regulatory networks

Photorespiration is a primary metabolic pathway, which, given its energy costs, has often been viewed as a wasteful process. Despite having reached the consensus that one important function of photorespiration is the removal of toxic metabolite intermediates, other possible functions have emerged, and others could well emerge in the future. As a primary metabolic pathway, photorespiration interacts with other routes; however the nature of these interactions is not well known. One of these interacting pathways could be the biosynthesis of serine, since this amino acid is synthesised through photorespiratory and non-photorespiratory routes. At present, the exact contribution of each route to serine supply in different tissues and organs, their biological significance and how pathways are integrated and/or regulated remain unknown. Here, we review the non-photorespiratory serine biosynthetic pathways, their interactions with the photorespiratory pathway, their putative role in plants and their biotechnological interest.

The phosphorylated pathway of serine biosynthesis is essential both for male gametophyte and embryo development and for root growth in Arabidopsis

This study characterizes the phosphorylated pathway of Ser biosynthesis (PPSB) in Arabidopsis thaliana by targeting phosphoserine phosphatase (PSP1), the last enzyme of the pathway. Lack of PSP1 activity delayed embryo development, leading to aborted embryos that could be classified as early curled cotyledons. The embryo-lethal phenotype of psp1 mutants could be complemented with PSP1 cDNA under the control of Pro35S (Pro35S:PSP1). However, this construct, which was poorly expressed in the anther tapetum, did not complement mutant fertility. Microspore development in psp1.1/psp1.1 Pro35S:PSP1 arrested at the polarized stage. The tapetum from these lines displayed delayed and irregular development. The expression of PSP1 in the tapetum at critical stages of microspore development suggests that PSP1 activity in this cell layer is essential in pollen development. In addition to embryo death and male sterility, conditional psp1 mutants displayed a short-root phenotype, which was reverted in the presence of Ser. A metabolomic study demonstrated that the PPSB plays a crucial role in plant metabolism by affecting glycolysis, the tricarboxylic acid cycle, and the biosynthesis of amino acids. We provide evidence of the crucial role of the PPSB in embryo, pollen, and root development and suggest that this pathway is an important link connecting primary metabolism with development.

The proline regulatory axis and cancer

Studies in metabolism and cancer have characterized changes in core pathways involving glucose and glutamine, emphasizing the provision of substrates for building cell mass. But recent findings suggest that pathways previously considered peripheral may play a critical role providing mechanisms for cell regulation. Several of these mechanisms involve the metabolism of non-essential amino acids, for example, the channeling of glycolytic intermediates into the serine pathway for one-carbon transfers. Historically, we proposed that the proline biosynthetic pathway participated in a metabolic interlock with glucose metabolism. The discovery that proline degradation is activated by p53 directed our attention to the initiation of apoptosis by proline oxidase/dehydrogenase. Now, however, we find that the biosynthetic mechanisms and the metabolic interlock may depend on the pathway from glutamine to proline, and it is markedly activated by the oncogene MYC. These findings add a new dimension to the proline regulatory axis in cancer and present attractive potential targets for cancer treatment.

Contrasting catalytic and allosteric mechanisms for phosphoglycerate dehydrogenases

D-3-Phosphoglycerate dehydrogenases (PGDH) exist with at least three different structural motifs and the enzymes from different species display distinctly different mechanisms. In many species, particularly bacteria, the catalytic activity is regulated allosterically through binding of l-serine to a distinct structural domain, termed the ACT domain. Some species, such as Mycobacterium tuberculosis, contain an additional domain, called the "allosteric substrate binding" or ASB domain, that functions as a co-domain in the regulation of catalytic activity. That is, both substrate and effector function synergistically in the regulation of activity to give the enzyme some interesting properties that may have physiological relevance for the persistent state of tuberculosis. Both enzymes function through a V-type regulatory mechanism and, in the Escherichia coli enzyme, it has been demonstrated that this results from a dead-end complex that decreases the concentration of active species rather than a decrease in the velocity of the active species. This review compares and contrasts what we know about these enzymes and provides additional insight into their mechanism of allosteric regulation.

L-serine synthesis in the central nervous system: a review on serine deficiency disorders

The de novo synthesis of the amino acid L-serine plays an essential role in the development and functioning of the central nervous system (CNS). L-serine displays many metabolic functions during different developmental stages; among its functions providing precursors for amino acids, protein synthesis, nucleotide synthesis, neurotransmitter synthesis and L-serine derived lipids. Patients with congenital defects in the L-serine synthesizing enzymes present with severe neurological abnormalities and underscore the importance of this synthetic pathway. In this review, we will discuss the cellular functions of the L-serine pathway, structure and enzymatic properties of the enzymes involved and genetic defects associated with this pathway.

Molecular cloning, sequence and characterization of a novel streptococcal phosphoglycerate dehydrogenase gene

The nucleotide sequence of a Streptococcus mutans serA gene that encodes D-3-phosphoglycerate dehydrogenase has been determined. The gene consisted of 1308-bp nucleotides coding for a 436-amino-acid polypeptide (48,546 Da). The deduced amino acid sequence showed a 66% identity with SerA from Bacillus subtilis and possessed specific residues (G-R-P-N-V-G) in the coenzyme-binding domain, alpha B helix. Recombinant streptococcal SerA was expressed using pMAL-c2 expression vector and purified by amylose resin affinity chromatography and DEAE-Sephacel column chromatography. This SerA enzyme catalyzed detectable reduction of alpha-ketoglutarate to 2-hydroxyglutaric acid. These findings indicate that the novel streptococcal phosphoglycerate dehydrogenase, SerA, is a member of a D-isomer-specific family of 2-hydroxyacid dehydrogenases.

Regulation of serine biosynthesis in Arabidopsis. Crucial role of plastidic 3-phosphoglycerate dehydrogenase in non-photosynthetic tissues

In plants, Ser is synthesized through a couple of pathways. 3-Phosphoglycerate dehydrogenase (PGDH), the first enzyme that is involved in the phosphorylated pathway of Ser biosynthesis, is responsible for the oxidation of 3-phosphoglycerate to phosphohydroxypyruvate. Here we report the first molecular cloning and characterization of PGDH from Arabidopsis thaliana. Sequence analysis of cDNA and a genomic clone revealed that the PGDH gene is composed of three exons, encoding a 623-amino acid polypeptide (66, 453 Da). The deduced protein, containing three of the most conserved regions in the NAD-dependent 2-hydroxyacid dehydrogenase family, has 38-39% identity to its animal and bacterial counterparts. The presence of an N-terminal signal sequence for translocation into plastids was confirmed by particle-gun bombardment experiments using green fluorescence protein as a reporter protein for subcellular localization. Southern hybridization analysis and restriction fragment length polymorphism mapping indicated that PGDH is a single-copy gene that is mapped to the upper arm of chromosome 1. Northern hybridization analysis indicated preferential expression of PGDH mRNA in root tissues of light-grown plants, suggesting that the phosphorylated pathway of Ser biosynthesis plays an important role in supplying Ser to non-photosynthetic tissues. The recombinant enzyme overproduced in Escherichia coli displayed hyperbolic kinetics with respect to 3-phosphoglycerate and NAD+.

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.

Stereospecificity for nicotinamide nucleotides in enzymatic and chemical hydride transfer reactions

The pyridine nucleotide (NAD and NADP)-linked enzymes are a large class of enzymes constituting approximately 17% of all classified enzymes. When these enzymes catalyze their reactions, the hydride transfer between the substrate and the reaction site (i.e., C-4 of the nicotinamide/dihydronicotinamide ring) of the coenzyme takes place in a stereospecific manner. Thus, in the reaction of oxidation of the reduced coenzyme, one group of enzymes catalyzes the extraction of only the hydrogen having the R configuration at the No. 4 carbon, while the other group catalyzes the removal of only that with the S configuration. Because this aspect of enzyme stereospecificity provides essential information for a given enzyme's reaction mechanism, active site structure, and evolutionary relationship with other enzymes, intensive effort has been made to establish the stereospecificities of as many enzymes as possible. This review presents the compilation of the stereospecificities of these enzymes. Some empirical rules, which are useful but not definitive, in predicting a given enzyme's stereospecificity are also described. In addition, the stereospecificity in enzymatic reactions is compared to the stereo-preference in chemical oxidoreduction of the coenzyme. In order to elucidate the mechanism for the enzyme stereospecificity, the conformations of the coenzyme in free-state and enzyme-bound state are extensively discussed here.

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

Phosphoglycerate dehydrogenase (EC 1.1.1.95), an enzyme believed to be involved in the synthesis of serine, an intermediate in ureide biosynthesis, has been purified about 200-fold from nodules of soybean (Glycine max L. Merr. cv Amsoy 71). The reaction was reversible and exhibited a strong pH dependence with optima of 9.4 and 6.1 for the forward and reverse reactions. The K(m) values for the forward reaction were 0.25 millimolar for NAD(+) and 0.29 millimolar for d-3-phosphoglycerate at pH 9.4, while those for the reverse reaction were 12 mum for NADH and 0.15 millimolar for 3-phosphohydroxypyruvate at pH 7.5. NADPH functioned as an alternate reductant with a K(m) of 0.15 millimolar. Product inhibition for the reverse reaction was competitive for NAD(+) with respect to NADH and noncompetitive for phosphoglycerate with respect to phosphohydroxypyruvate. Phosphoglycerate dehydrogenase activity was dependent on inorganic ions and was not inhibited by serine.

The specificity of dehydrogenases

The specificity of dehydrogenases for coenzyme (and coenzyme analogues), and substrate (and substrate analogues) is discussed in relation to structure, function, and evolution. Examples concern compounds that have very different structures, reactions that play widely differing roles in the life of the organism, and organisms of greatly differing types. The examples illustrate general points of interest and importance.

Determination of the hydride transfer stereospecificity of nicotinamide adenine dinucleotide linked oxidoreductases by proton magnetic resonance

A facile proton magnetic resonance technique is described for the determination of the coenzyme stereospecificity during hydride transfer reactions catalyzed by pyridine nucleotide dependent oxidoreductases. The reliability of this technique was demonstrated by examining the coenzyme stereospecificity of lactate, malate, and 3-phosphoglycerate dehydrogenases, which are known to be A-stereospecific enzymes, as well as triosephosphate and octopine dehydrogenases, which are known to be B-stereospecific enzymes. Furthermore, by applying this technique, it was shown that the previously unstudied enzymes D-beta-hydroxybutyrate and 4-aminobutanal dehydrogenases are B- and A-stereospecific enzymes, respectively. In addition, the nicotinamide adenine dinucleotide linked reaction of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides was found to be B stereospecific, like the reaction of the nicotinamide adenine dinucleotide phosphate linked yeast enzyme.

Serine biosynthesis in human hair follicles by the phosphorylated pathway: follicular 3-phosphoglycerate dehydrogenase

The phosphorylated pathway of serine biosynthesis was demonstrated in human hair bulbs and sheaths by the formation of phosphoserine and serine from (14C)3-phosphoglyceric acid. The initial and rate limiting enzyme of the pathway, 3-phosphoglycerate dehydrogenase (3-PGDH) was demonstrated by enzyme determinations in human and rat hair follicles, human epidermis, and chicken epidermis. Follicular 3-PGDH was characterized using a sensitive fluorometric assay with NADH as a co-substrate. Monovalent cations (Na+, K+, Li+, or NH4+) were necessary for full enzyme activity. p-Hydroxymercuribenzoate inhibited activity, and activity was 3 times higher with NADH as a co-substrate than with NADPH. The apparent Km for the substrate hydroxyphosphopyruvic acid was 32.8 muM, and the apparent Km for NADH 4.8 muM similar to the Kms for other mammalian 3-PGDHs. Enzyme activity was not altered by parenteral corticosteroids, a high carbohydrate diet, low protein diet, or starvation. Enzyme activity decreased over the first 12 days of life in newborn rats. The phosphorylated pathway of serine synthesis provides a potential nondietary and nonhepatic source of serine, glycine, and their products in keratinizing tissues.

Inhibition of 3-phosphoglycerate dehydrogenase from Pisum sativum by purine nucleotides

The inhibition of 3-phosphoglycerate dehydrogenase from etiolated pea epicotyls by purine nucleoside di- and tri-phosphates is linear, competitive with regard to NADH, and the nucleotides are mutually exclusive in their binding. Free ATP and ADP are more effective inhibitors than are the respective magnesium complexes.

Nonreversible d-Glyceraldehyde 3-Phosphate Dehydrogenase of Plant Tissues

Preparations of TPN-linked nonreversible d-glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.9), free of TPN-linked reversible d-glyceraldehyde 3-phosphate dehydrogenase, have been obtained from green shoots, etiolated shoots, and cotyledons of pea (Pisum sativum), cotyledons of peanut (Arachis hypogea), and leaves of maize (Zea mays). The properties of the enzyme were similar from each of these sources: the Km values for d-glyceraldehyde 3-phosphate and TPN were about 20 mum and 3 mum, respectively. The enzyme activity was inhibited by l-glyceraldehyde 3-phosphate, d-erythrose 4-phosphate, and phosphohydroxypyruvate. Activity was found predominantly in photosynthetic and gluconeogenic tissues of higher plants. A light-induced, phytochrome-mediated increase of enzyme activity in a photosynthetic tissue (pea shoots) was demonstrated. Appearance of enzyme activity in a gluconeogenic tissue (endosperm of castor bean, Ricinus communis) coincided with the conversion of fat to carbohydrate during germination. In photosynthetic tissue, the enzyme is located outside the chloroplast, and at in vivo levels of triose-phosphates and pyridine nucleotides, the activity is probably greater than that of DPN-linked reversible d-glyceraldehyde 3-phosphate dehydrogenase. Several possible roles for the enzyme in plant carbohydrate metabolism are considered.

The stereospecificity of nicotinamide-adenine dinucleotide-dependent oxidoreductases from plants

1. The stereospecificity of 20 enzymes from plants is reported. 2. The stereospecificity of all known forms of malate dehydrogenase in plants and animals has been shown to be A-specific. 3. The generalization that ;the stereospecificity of a particular reaction is independent of the source of the enzyme' is confirmed for a total of 12 plant enzymes. 4. A new generalization is proposed: ;When a metabolic sequence involves consecutive nicotinamide-adenine dinucleotide-dependent reactions, the dehydrogenases have the same stereospecificity.'

Inhibition of 3-phosphoglycerate dehydrogenase by l-serine

1. l-Serine was shown to be a highly specific inhibitor of 3-phosphoglycerate dehydrogenase. 2. 3-Phosphoglycerate dehydrogenase is cold-labile with respect to its catalytic activity and to sensitivity to serine. 3. l-Serine protects the catalytic site as well as the inhibitor site. 4. Glycerol protects the catalytic site as well as the inhibitor site. 5. Serine acts as a ;classical' non-competitive inhibitor of fresh preparations of 3-phosphoglycerate dehydrogenase. 6. ;Aged' preparations when assayed at pH6.5 show sigmoid inhibition curves at saturating substrate concentrations. 7. A generalized model is advanced to account for the variation of the catalytic activity and the inhibitory effect of l-serine with time and conditions. 8. The possibility that the sigmoid kinetics of inhibition observed are an artifact of isolation is discussed.