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A monofunctional prephenate dehydrogenase created by cleavage of the 5' 109 bp of the tyrA gene from Erwinia herbicola.

A cohesive phylogenetic cluster that is limited to enteric bacteria and a few closely related genera possesses a bifunctional protein that is known as the T-protein and is encoded by tyrA. The T-protein carries catalytic domains for chorismate mutase and for cyclohexadienyl dehydrogenase. Cyclohexadienyl dehydrogenase can utilize prephenate or L-arogenate as alternative substrates. A portion of the tyr A gene cloned from Erwinia herbicola was deleted in vitro with exonuclease III and fused in-frame with a 5' portion of lacZ to yield a new gene, denoted tyrA*, in which 37 N-terminal amino acids of the T-protein are replaced by 18 amino acids encoded by the polycloning site/5' portion of the lacZ alpha-peptide of pUC19. The TyrA* protein retained dehydrogenase activity but lacked mutase activity, thus demonstrating the separability of the two catalytic domains. While the Km of the TyrA* dehydrogenase for NAD+ remained unaltered, the Km for prephenate was fourfold greater and the Vmax was almost twofold greater than observed for the parental T-protein dehydrogenase. Activity with L-arogenate, normally a relatively poor substrate, was reduced to a negligible level. The prephenate dehydrogenase activity encoded by tyrA* was hypersensitive to feedback inhibition by L-tyrosine (a competitive inhibitor with respect to prephenate), partly because the affinity for prephenate was reduced and partly because the Ki value for L-tyrosine was decreased from 66 microM to 14 microM. Thus, excision of a portion of the chorismate mutase domain is shown to result in multiple extra-domain effects upon the cyclohexadienyl dehydrogenase domain of the bifunctional protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

Molecular characterization of a chromosomal region involved in the oxidation of acetyl-CoA to glyoxylate in the isocitrate-lyase-negative methylotroph Methylobacterium extorquens AM1.

A region on the Methylobacterium extorquens AM1 chromosome previously shown to complement a chemically induced mutant (PCT48) unable to convert acetyl-CoA into glyoxylate was characterized in detail in order to identify the gene(s) involved in the unknown pathway for acetyl-CoA oxidation. Six complete and two partial ORFs were identified by sequencing. Sequence comparisons suggested these might code for, respectively, a dehydrogenase of unknown specificity, a polypeptide of at least 15 kDa with unknown function, a coenzyme-B12-linked mutase, a catalase, an alcohol dehydrogenase (ADH) of unknown function, a polypeptide of 28 kDa, a ketol-acid reductoisomerase and a propionyl-CoA carboxylase (PCC). Insertion mutations were introduced into each ORF in order to determine their involvement in C1 and C2 metabolism. Mutations in three genes, encoding the mutase, ADH and PCC, resulted in a phenotype characteristic of mutants unable to oxidize acetyl-CoA, i.e. they were C1-and C2-negative and their growth on these compounds was restored by the addition of glycolate or glyoxylate. Mutants in the genes thought to encode catalase and PCC were found to be deficient in the corresponding enzyme activity, confirming the identity of these genes, while physiological substrates for the mutase and ADH remain unidentified. This work, in which three new genes necessary for conversion of acetyl-CoA into glyoxylate were identified, is an intermediary step on the way to the solution of the unknown pathway for acetyl-CoA oxidation in isocitrate-lyase-negative methylotrophs.

Acetyl Coenzyme A↗

Induction of Shikimic Acid Pathway Enzymes by Light in Suspension Cultured Cells of Parsley (Petroselinum crispum).

Light treatment of suspension cultured cells of parsley (Petroselinum crispum) was shown to increase the activity of the shikimic acid pathway enzyme, 3-deoxy-d-arabino-heptulosonic acid-7-phosphate (DAHP) synthase (EC 4.1.2.15). DAHP synthase activity was assayed for two isoforms, DS-Mn and DS-Co (RJ Ganson, TA d'Amato, RA Jensen [1986] Plant Physiol 82: 203-210). Light increased the enzymatic activity of the plastidic isoform DS-Mn as much as 2-fold, averaging 1.6-fold with >95% confidence. The cytosolic isoform DS-Co was unaffected. Cycloheximide and actinomycin D, translational and transcriptional inhibitors, respectively, both reversed induction of DS-Mn by light suggesting transcriptional regulation of the gene. Chorismate mutase activity was assayed for the two isoforms CM I and CM II (BK Singh, JA Connelly, EE Conn [1985] Arch Biochem Biophys 243: 374-384). Treatment by light did not significantly affect either chorismate mutase isoform. The ratio of the two chorismate mutase isoforms changed during the growth cycle, with an increase in the ratio of plastidic to cytosolic isoforms occurring towards the end of logarithmic growth.

Journal Article↗

Enzyme alterations in tyrosine and phenylalanine auxotrophs of Salmonella typhimurium.

The enzyme activities specified by the tyrA and pheA genes were studied in wildtype strain Salmonella typhimurium and in phenylalanine and tyrosine auxotrophs. As in Aerobacter aerogenes and Escherichia coli, the wild-type enzymes of Salmonella catalyze two consecutive reactions: chorismate --> prephenate --> 4-hydroxy-phenylpyruvate (tyrA), and chorismate --> prephenate --> phenylpyruvate (pheA). A group of tyrA mutants capable of interallelic complementation had altered enzymes which retained chorismate mutase T activity but lacked prephenate dehydrogenase. Similarly, pheA mutants (in which interallelic complementation does not occur) had one group with altered enzymes which retained chorismate mutase P but lacked prephenate dehydratase. Tyrosine and phenylalanine auxotrophs outside of these categories showed loss of both activities of their respective bifunctional enzyme. TyrA mutants which had mutase T were considerably derepressed in this activity by tyrosine starvation and consequently excreted prephenate. A new and specific procedure was developed for assaying prephenate dehydrogenase activity.

Cell-Free System↗

A novel alternate anaplerotic pathway to the glyoxylate cycle in streptomycetes.

ccr encoding crotonyl coenzyme A (CoA) reductase (CCR), which catalyzes the conversion of crotonyl-CoA to butyryl-CoA in the presence of NADPH, was previously cloned from Streptomyces collinus. We now report that a complete open reading frame, designated meaA, is located downstream from ccr. The predicted gene product showed 35% identity with methylmalonyl-CoA mutases from various sources. In addition, the predicted amino acid sequences of S. collinus ccr and meaA exhibit strong similarity to that of adhA (43% identity), a putative alcohol dehydrogenase gene, and meaA (62% identity) of Methylobacterium extorquens, respectively. Both adhA and meaA are involved in the assimilation of C1 and C2 compounds in an unknown pathway in the isocitrate lyase (ICL)-negative Methylobacterium. We have demonstrated that S. collinus can grow with acetate as its sole carbon source even though there is no detectable ICL, suggesting that in this organism ccr and meaA may also be involved in a pathway for the assimilation of C2 compounds. Previous studies with streptomycetes provided a precedent for a pathway that initiates with the condensation of two acetyl-CoA molecules to form butyryl-CoA, which is then transformed to succinyl-CoA with two separate CoB12-mediated rearrangements and a series of oxidations. The biological functions of ccr and meaA in this process were investigated by gene disruption. A ccr-blocked mutant showed no detectable crotonyl-CoA reductase activity and, compared to the wild-type strain, exhibited dramatically reduced growth when acetate was the sole carbon source. An meaA-blocked mutant also exhibited reduced growth on acetate. However, both methylmalonyl-CoA mutase and isobutyryl-CoA mutase, which catalyze the two CoB12-dependent rearrangements in this proposed pathway, were shown to be present in the meaA-blocked mutant. These results suggested that both ccr and meaA are involved in a novel pathway for the growth of S. collinus when acetate is its sole carbon source.

Acetates↗

Action of growth hormone on erythropoiesis: changes in red blood cell enzyme activities in growth-retarded patients with and without growth hormone deficiency.

Fifteen red cell enzyme activities of growth-retarded patients with and without growth hormone (GH) deficiency were investigated before and after GH administration. The 15 enzymes were Hexokinase, phosphoglucomutase, glucose phosphate, isomerase, phosphofructokinase, fructose diphosphate aldolase, glyceraldehyde-3-phosphae dehydrogenase, triosephosphate isomerase, 2,3-diphosphoglycerate mutase, 3-phosphoglycerate kinase, 3-phosphoglycerate mutase, enolase, pyruvate kinase, glycose-6-phosphate dehydrogenase, 6-phosphogluconic dehydrogenase, glutathione reducase. Sixty-six subjects were studied: 30 normal control subjects (group N) and 36 patients (aged 5-23 years) with short stature. Complete endocrine evaluation showed 21 (group I) to have GH deficiency (10 patients with isolated GH deficiency) and 15 (group II) to have normal hypothalamic and pituitary function except for two patients with a moderate hypothyroidism. Both had been receiving thyroid hormone treatment for a long time before our studies. All 36 patients were treated with 2 mg human growth hormone intramuscularly for 7 days. Before GH treatment no significant difference was observed between hematologic data in group I (GH deficiency) and group II (no GH deficiency). After GH therapy there was a significant increase in reticulocyte count in both groups of patients with short stature. The mean pretreatment value in group I was 1.294% +/- 0.084 (SEM); the mean post-treatment value was 2.081% +/- 0.287 (SEM)< P less than 0.005. The mean pretreatment value in group II was 1.0% 0.184 (SEM); the mean post-treatment value was 1.407% +/- 0.193 (SEM), P less than 0.01. In group II (no GH deficiency) mean pretreatment erythrocyte enzyme activities were not significantly different from those activities observed in normal control subjects (group N). However, in patients who lacked GH, the pretreatment activities of five red cell enzymes (glucose phosphate isomerase, triosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, 2,3-diphosphoglycerate mutase, 3-phosphoglycerate kinase) were significantly decreased before GH administration compared with the values in normal control subjects...

Adolescent↗

Recognition, isolation, and characterization of rat liver D-methylmalonyl coenzyme A hydrolase.

Certain amino acids and other compounds are metabolized via: Propionyl-CoA carboxylase in equilibrium D-methylmalonyl-CoA racemase in equilibrium L-methylmalonyl-CoA (adenosylcobalamin) mutase in equilibrium succinyl-CoA in equilibrium tricarboxylic acid cycle. In cobalamin deficiency and in genetic disorders involving adenosylcobalamin or the mutase, large amounts of methylmalonic acid are excreted in the urine. Its origin is unknown, however, since nonesterified methylmalonic acid is not present in the above or other known pathways. To investigate the origin of methylmalonic acid, we fractionated rat liver by gel filtration and found a single peak (Mr = 35,000) of activity for the hydrolysis of DL-methylmalonyl-CoA to methylmalonic acid and CoA. The enzyme has been purified 3,100-fold with a yield of 2.1% from 1.6 kg of rat liver using a purification scheme consisting of ammonium sulfate fractionation, ion exchange chromatography on CM (carboxymethyl)-cellulose and DEAE-cellulose, affinity chromatography on CoA-agarose, and gel filtration on Sephadex G-100. The final preparation gave a single band on polyacrylamide gel electrophoresis. The molecular weight of the purified enzyme is 35,000 based on gel filtration, and sodium dodecyl sulfate-polyacrylamide electrophoresis in the presence and absence of 1% 2-mercaptoethanol. The enzyme was shown to be active on the D-isomer, but not on the L-isomer, of methylmalonyl-CoA by CD spectropolarimetry. The Km for D-methylmalonyl-CoA is 0.7 mM and the molar activity is 1,400 molecules of substrate/min/molecule of enzyme. At substrate concentrations of 0.5 mM, the relative rate of hydrolysis of CoA esters was as follows: D-methylmalonyl-CoA (100%), malonyl-CoA (16%), propionyl-CoA (3%), acetyl-CoA (1%), succinyl-CoA (less than 1%), and palmitoyl-CoA (less than 1%). This enzyme appears to account for the markedly increased amounts of methylmalonic acid that are excreted in the urine in cobalamin deficiency and in genetic disorders involving adenosylcobalamin or L-methylmalonyl-CoA mutase.

Acyl Coenzyme A↗

Phosphocreatine-dependent protein phosphorylation in rat skeletal muscle.

Phosphocreatine (PCr) was found to alter the phosphorylation state of two proteins of apparent molecular masses 18 and 29 kDa in dialysed cell-free extracts of rat skeletal muscle in the presence of [gamma-32P]ATP. The 29 kDa protein was identified as phosphoglycerate mutase (PGM), phosphorylated at the active-site histidine residue by 2,3-bisphosphoglycerate (2,3-biPG). 2,3-biPG labelling from [gamma-32P]ATP occurred through the concerted action of phosphoglycerate kinase and 2,3-bisphosphoglycerate mutase. PCr-dependent labelling, which required creatine kinase, resulted from a shift in the phosphoglycerate kinase equilibrium towards 1,3-bisphosphoglycerate (1,3-biPG) synthesis, ultimately resulting in an increase in available [2-32P]2,3-biPG. The maximal catalytic activity of PGM was unaffected by PCr. The 18 kDa protein was transiently phosphorylated at a histidine residue, probably by 1,3-biPG. No proteins of this monomeric molecular mass are known to bind 1,3-biPG, suggesting that the 18 kDa protein is an undescribed phosphoenzyme intermediate. Previous observations of 2- and 3-phosphoglycerate-dependent protein phosphorylation in cytosolic extracts [Ueda & Plagens (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1229-1233; Pek, Usami, Bilir, Fischer-Bovenkerk & Ueda (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 4294-4298], attributed to the action of novel kinases, are likely to represent phosphoenzyme intermediates labelled by bisphosphorylated metabolites in a similar manner.

2,3-Diphosphoglycerate↗

Seven novel mutations in mut methylmalonic aciduria.

Methylmalonic aciduria (MMA) is an autosomal recessive inborn error of metabolism that results from functional defects in methylmalonyl CoA mutase (MCM), a nuclear-encoded, mitochondrial enzyme that uses the vitamin B12 derivative, adenosylcobalamin (AdoCbl) as a cofactor. To date, 23 mutations have been identified at the MUT locus on the short arm of chromosome 6, causing the mut forms of MMA (mut complementation group; mut MMA, McKusick #251000). We now report seven novel mutations. Three were found inmut0 patients: R228Q (c759G-->A) was found as a heterozygous change; G312V (c1011G-->T) and 346delL (c1112delCTT) were both found as homozygous changes. Four mutations were found in mut patients: A191E (c648C-->A) and V633G (c1974T-->G) were found in the same patient; 684insL (c2128insCTC) and L685R (c2130T-->G) were both found as homozygous changes. The recent modelling of the human methylmalonyl CoA mutase allowed for an interpretation of the identified mutations.

Alleles↗

Cross-species identification of novel Candida albicans immunogenic proteins by combination of two-dimensional polyacrylamide gel electrophoresis and mass spectrometry.

We have previously reported the usefulness of two-dimensional gel electrophoresis followed by Western blotting with sera from patients with systemic candidiasis in the detection of the major Candida albicans antigens (Pitarch et al., Electrophoresis 1999, 20, 1001-1010). The identification of these antigens would be useful for the characterization of good markers for the disease, and for the development of efficient diagnostic strategies. In this work we have used nanoelectrospray tandem mass spectrometry to obtain amino acid sequence information from the immunogenic proteins previously detected. We report here the cross-species identification of these antigens by matching of tandem mass spectrometry data to Saccharomyces cerevisiae proteins. Using this approach, we unambiguously identified the four C. albicans immunogenic proteins analyzed, namely aconitase, pyruvate kinase, phosphoglycerate mutase and methionine synthase. Furthermore, we report for the first time that aconitase, methionine synthase and phosphoglycerate mutase have antigenic properties in C. albicans.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗

Changes in enzyme activities involved in the degradation of 1,3-bisphosphoglycerate during erythropoiesis in rat bone marrow.

Rat bone marrow cells have been fractionated by density gradient in Percoll. Differential counting of erythroid cells, haemoglobin concentration and bisphosphoglycerate mutase and phosphoglycerate kinase activities have been determined in cellular fractions. As shown by means of a statistical approach, an increase in bisphosphoglycerate mutase activity and a slight decrease in phosphoglycerate kinase activity is found in erythroid cells as their haemoglobin content increases. Our results suggest that there is a synthesis of 2,3-bisphosphoglycerate during the erythropoietic process which parallels the synthesis of haemoglobin.

Animals↗

Influence of mouse trisomy 16 on expression of specific genes.

We examined developmental changes in the relative activities of three different isozyme systems: aldolase, enolase and phosphoglycerate mutase, in tissues of fetal mice with trisomy 16 and of fetal euploid littermates. We wanted to determine whether morphological abnormalities such as reduced weight and size, which are generally observed in murine trisomy, are reflected at the molecular level. Following electrophoretic separation and subsequent measurement of relative activities of enolase isozymes in brain and phosphoglycerate mutase isozymes in heart, we found no significant differences between trisomy 16 fetuses and their euploid littermates. Synthesis of liver-specific aldolase was, however, delayed in trisomy 16 fetuses.

Animals↗

Phosphorylation of glycolytic and gluconeogenic enzymes by the insulin receptor kinase.

Various glycolytic and gluconeogenic enzymes were tested as substrates for the insulin receptor kinase. Phosphofructokinase and phosphoglycerate mutase were found to be the best substrates. Phosphorylation of these enzymes was rapid, stimulated 2- to 6-fold by 10(-7) M insulin and occurred exclusively on tyrosine residues. Enolase, fructose 1,6-bisphosphatase, lactate dehydrogenases in decreasing order, were also subject to insulin-stimulated phosphorylation but to a smaller extent than that for phosphofructokinase or phosphoglycerate mutase. The phosphorylation of phosphofructokinase was studied most extensively since phosphofructokinase is known to catalyze a rate-limiting step in glycolysis. The apparent Km of the insulin receptor for phosphofructokinase was 0.1 microM, which is within the physiologic range of concentration of this enzyme in most cells. Tyrosine phosphorylation of phosphofructokinase paralleled autophosphorylation of the beta-subunit of the insulin receptor with respect to time course, insulin dose response (half maximal effect between 10(-9) and 10(-8) M insulin), and cation requirement (Mn2+ greater than Mg2+ much greater than Ca2+). Further study will be required to determine whether the tyrosine phosphorylation of phosphofructokinase plays a role in insulin-stimulated increases in glycolytic flux.

Adenosine Triphosphate↗

Relative tolerance of mesostable and thermostable protein homologs to extensive mutation.

Evolvability, designability, and plasticity of a protein are properties that are important to protein engineers, but difficult to quantify. Here, we directly compare homologous AroQ chorismate mutases from the thermophile Methanococcus jannaschii and the mesophile Escherichia coli with respect to their capacity to accommodate extensive mutation. The N-terminal helix comprising about 40% of these proteins was randomized at the genetic level using a binary pattern of hydrophobic and hydrophilic residues based on the respective wild-type sequences. Catalytically active library members were identified by a survival-selection assay in a chorismate mutase-deficient E. coli strain. Functional variants were found approximately approximately 10-times more frequently with the thermostable protein compared to its mesostable counterpart. Moreover, detailed sequence analysis revealed that functional M. jannaschii enzyme variants contained a smaller number of conserved residues and tolerated greater variability at individual sequence positions. Our results thus highlight the greater robustness of the thermostable protein with respect to amino acid substitution, while identifying specific sites important for constructing active enzymes. Overall, they support the notion that redesign projects will benefit from using a thermostable starting structure, even at very high mutational loads.

Amino Acid Sequence↗

Evolution of L-phenylalanine biosynthesis in rRNA homology group I of Pseudomonas.

Group I pseudomonads exhibit diversity for L-phenylalanine biosynthesis that is a basis for separation of two subgroups. Subgroup Ib (fluorescent species such as Pseudomonas aeruginosa, P. fluorescens, or P. putida) possesses an unregulated overflow pathway to L-phenylalanine, together with a second, regulated pathway. Subgroup Ia (non-fluorescent species such as P. stutzeri, P. mendocina, or P. alcaligenes) possess only the regulated pathway to L-phenylalanine. Thus, subgroup Ia species lack an unregulated isozyme of chorismate mutase and arogenate dehydratase, enzymes which are thought to divert chorismate to L-phenylalanine under conditions of high carbon input into aromatic biosynthesis. A priori the overflow pathway could have been either lost in subgroup Ia or gained in subgroup Ib. Since Group V pseudomonads (mainly Xanthomonas) are known to branch off from the Group I lineage at a deeper phylogenetic level than the point of divergence for subgroups Ia and Ib, the presence of the overflow pathway in Group V pseudomonads reveals that the overflow pathway must have been lost in the evolution of subgroup Ia. All Group I species possess a bifunctional protein (P-protein) which catalyzes both chorismate mutase and prephenate dehydratase reactions. In subgroup Ia species this highly conserved protein must be the sole source of prephenate to be used for tyrosine biosynthesis. Thus, the channeling action of the P-protein whereby chorismate is committed towards L-phenylalanine formation can be negated by selective feedback inhibition exerted by L-phenylalanine upon the prephenate dehydratase component of the P-protein. Diversion of prephenate molecules under the latter conditions towards L-tyrosine comprises a channel-shuttle mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological Evolution↗

Clues from Xanthomonas campestris about the evolution of aromatic biosynthesis and its regulation.

The recent placement of major Gram-negative prokaryotes (Superfamily B) on a phylogenetic tree (including, e.g., lineages leading to Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter calcoaceticus) has allowed initial insights into the evolution of the biochemical pathway for aromatic amino acid biosynthesis and its regulation to be obtained. Within this prokaryote grouping, Xanthomonas campestris ATCC 12612 (a representative of the Group V pseudomonads) has played a key role in facilitating deductions about the major evolutionary events that shaped the character of aromatic biosynthesis within this grouping. X. campestris is like P. aeruginosa (and unlike E. coli) in its possession of dual flow routes to both L-phenylalanine and L-tyrosine from prephenate. Like all other members of Superfamily B, X. campestris possesses a bifunctional P-protein bearing the activities of both chorismate mutase and prephenate dehydratase. We have found an unregulated arogenate dehydratase similar to that of P. aeruginosa in X. campestris. We separated the two tyrosine-branch dehydrogenase activities (prephenate dehydrogenase and arogenate dehydrogenase); this marks the first time this has been accomplished in an organism in which these two activities coexist. Superfamily B organisms possess 3-deoxy-D-arabino-heptulosonate 7-P (DAHP) synthase as three isozymes (e.g., in E. coli), as two isozymes (e.g., in P. aeruginosa), or as one enzyme (in X. campestris). The two-isozyme system has been deduced to correspond to the ancestral state of Superfamily B. Thus, E. coli has gained an isozyme, whereas X. campestris has lost one. We conclude that the single, chorismate-sensitive DAHP synthase enzyme of X. campestris is evolutionarily related to the tryptophan-sensitive DAHP synthase present throughout the rest of Superfamily B. In X. campestris, arogenate dehydrogenase, prephenate dehydrogenase, the P-protein, chorismate mutase-F, anthranilate synthase, and DAHP synthase are all allosteric proteins; we compared their regulatory properties with those of enzymes of other Superfamily B members with respect to the evolution of regulatory properties. The network of sequentially operating circuits of allosteric control that exists for feedback regulation of overall carbon flow through the aromatic pathway in X. campestris is thus far unique in nature.

3-Deoxy-7-Phosphoheptulonate Synthase↗

The absence of 2,3-diphosphoglycerate from myocytes, hepatocytes and adipocytes.

Myocytes, hepatocytes and adipocytes were prepared from heart, liver and epididymal fat pad of the rat. No detectable level of 2,3-diphosphoglycerate was found. Evidence is also presented which indicates the absence from these cells of 2,3-diphosphoglycerate mutase and 2,3-diphosphoglycerate phosphatase. Previous findings by others of the presence of 2,3-diphosphoglycerate and 2,3-diphosphoglycerate mutase probably resulted from erythrocytes sequestered in the tissue.

Adipose Tissue↗

Growth of Penicillium janthinellum on glycine as sole carbon and nitrogen source.

Penicillium janthinellum is able to grow on glycine as the sole carbon and nitrogen source. The amino acid is transaminated to glyoxylate which is further metabolised to pyruvate by the glycerate pathway. The reaction product of partially purified glycerate kinase from this fungus is 2-phosphoglycerate. Phosphoglycerate mutase initiates gluconeogenesis from glycine. Partially purified phosphoglycerate mutase is inhibited by fructose 6-phosphate. The possible significance of this regulation is discussed.

Carbon↗