Spectral kinetics of glutamate-1-semialdehyde aminomutase of Synechococcus

Glutamate 1-semialdehyde aminotransferase is connected to GluTR by GluTR-binding protein and contributes to the rate-limiting step of 5-aminolevulinic acid synthesis

Tetrapyrroles play fundamental roles in crucial processes including photosynthesis, respiration, and catalysis. In plants, 5-aminolevulinic acid (ALA) is the common precursor of tetrapyrroles. ALA is synthesized from activated glutamate by the enzymes glutamyl-tRNA reductase (GluTR) and glutamate-1-semialdehyde aminotransferase (GSAAT). ALA synthesis is recognized as the rate-limiting step in this pathway. We aimed to explore the contribution of GSAAT to the control of ALA synthesis and the formation of a protein complex with GluTR. In Arabidopsis thaliana, two genes encode GSAAT isoforms: GSA1 and GSA2. A comparison of two GSA knockout mutants with the wild-type revealed the correlation of reduced GSAAT activity and ALA-synthesizing capacity in leaves with lower chlorophyll content. Growth and green pigmentation were more severely impaired in gsa2 than in gsa1, indicating the predominant role of GSAAT2 in ALA synthesis. Interestingly, GluTR accumulated to higher levels in gsa2 than in the wild-type and was mainly associated with the plastid membrane. We propose that the GSAAT content modulates the amount of soluble GluTR available for ALA synthesis. Several different biochemical approaches revealed the GSAAT-GluTR interaction through the assistance of GluTR-binding protein (GBP). A modeled structure of the tripartite protein complex indicated that GBP mediates the stable association of GluTR and GSAAT for adequate ALA synthesis.

Regulation and function of tetrapyrrole biosynthesis in plants and algae

Tetrapyrroles are macrocyclic molecules with various structural variants and multiple functions in Prokaryotes and Eukaryotes. Present knowledge about the metabolism of tetrapyrroles reflects the complex evolution of the pathway in different kingdoms of organisms, the complexity of structural and enzymatic variations of enzymatic steps, as well as a wide range of regulatory mechanisms, which ensure adequate synthesis of tetrapyrrole end-products at any time of development and environmental condition. This review intends to highlight new findings of research on tetrapyrrole biosynthesis in plants and algae. In the course of the heme and chlorophyll synthesis in these photosynthetic organisms, glutamate, one of the central and abundant metabolites, is converted into highly photoreactive tetrapyrrole intermediates. Thereby, several mechanisms of posttranslational control are thought to be essential for a tight regulation of each enzymatic step. Finally, we wish to discuss the potential role of tetrapyrroles in retrograde signaling and point out perspectives of the formation of macromolecular protein complexes in tetrapyrrole biosynthesis as an efficient mechanism to ensure a fine-tuned metabolic flow in the pathway. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Asymmetry of the active site loop conformation between subunits of glutamate-1-semialdehyde aminomutase in solution

Glutamate-1-semialdehyde aminomutase (GSAM) is a dimeric, pyridoxal 5′-phosphate (PLP)- dependent enzyme catalysing in plants and some bacteria the isomerization of L-glutamate-1-semialdehyde to 5-aminolevulinate, a common precursor of chlorophyll, haem, coenzyme B₁₂, and other tetrapyrrolic compounds. During the catalytic cycle, the coenzyme undergoes conversion from pyridoxamine 5′-phosphate (PMP) to PLP. The entrance of the catalytic site is protected by a loop that is believed to switch from an open to a closed conformation during catalysis. Crystallographic studies indicated that the structure of the mobile loop is related to the form of the cofactor bound to the active site, allowing for asymmetry within the dimer. Since no information on structural and functional asymmetry of the enzyme in solution is available in the literature, we investigated the active site accessibility by determining the cofactor fluorescence quenching of PMP- and PLP-GSAM forms. PLP-GSAM is partially quenched by potassium iodide, suggesting that at least one catalytic site is accessible to the anionic quencher and therefore confirming the asymmetry observed in the crystal structure. Iodide induces release of the cofactor from PMP-GSAM, apparently from only one catalytic site, therefore suggesting an asymmetry also in this form of the enzyme in solution, in contrast with the crystallographic data.

Structure and function of enzymes in heme biosynthesis

Tetrapyrroles like hemes, chlorophylls, and cobalamin are complex macrocycles which play essential roles in almost all living organisms. Heme serves as prosthetic group of many proteins involved in fundamental biological processes like respiration, photosynthesis, and the metabolism and transport of oxygen. Further, enzymes such as catalases, peroxidases, or cytochromes P450 rely on heme as essential cofactors. Heme is synthesized in most organisms via a highly conserved biosynthetic route. In humans, defects in heme biosynthesis lead to severe metabolic disorders called porphyrias. The elucidation of the 3D structures for all heme biosynthetic enzymes over the last decade provided new insights into their function and elucidated the structural basis of many known diseases. In terms of structure and function several rather unique proteins were revealed such as the V-shaped glutamyl-tRNA reductase, the dipyrromethane cofactor containing porphobilinogen deaminase, or the "Radical SAM enzyme" coproporphyrinogen III dehydrogenase. This review summarizes the current understanding of the structure-function relationship for all heme biosynthetic enzymes and their potential interactions in the cell.

Enzymic and mechanistic studies on the conversion of glutamate to 5-aminolaevulinate

Higher plants, algae, cyanobacteria and several other photosynthetic and non-photosynthetic bacteria synthesize 5-aminolaevulinate by a tRNA(Glu)-mediated pathway. Glutamate is activated at the alpha-carboxyl by ligation to tRNA(Glu) with an aminoacyl-tRNA synthetase. An NADPH-dependent reductase converts glutamyl-tRNA(Glu) to glutamate 1-semialdehyde, which is finally converted to 5-aminolaevulinate by an aminotransferase. These components are soluble and in plants and algae are located in the chloroplast stroma. In plants and algae the tRNA(Glu) is encoded in chloroplast DNA whereas the enzymes are encoded in nuclear DNA. The tRNA(Glu) has a hypermodified 5-methylaminomethyl-2-thiouridine-pseudouridine-C anticodon and probably plays a role in the light-dark regulation of 5-aminolaevulinate synthesis. Ligation of glutamate to tRNA(Glu) requires ATP and Mg2+ and proceeds via a ternary intermediate. Glutamyl-tRNA(Glu) reduction appears to involve formation of a complex. Glutamate 1-semialdehyde non-enzymically synthesized by reductive ozonolysis from gamma-vinyl GABA is used as substrate by the last enzyme. Glutamate-1-semialdehyde aminotransferase contains pyridoxal phosphate as a prosthetic group. The enzyme is converted to spectrally different forms by treatment with 4,5-diaminovalerate or 4,5-dioxovalerate. The pyridoxamine 5'-phosphate form of the enzyme converts (S)-glutamate 1-semialdehyde to 5-aminolaevulinate via 4,5-diaminovalerate through a bi-bi ping-pong mechanism.

Expression of a Brassic napus glutamate 1-semialdehyde aminotransferase in Escherichia coli and characterization of the recombinant protein

Glutamate 1-semialdehyde aminotransferase (GSA-AT) is a key regulatory enzyme, which converts glutamate 1-semialdehyde (GSA) to 5-aminolevulinic acid (ALA) in chlorophyll biosynthesis. ALA is the universal precursor for the synthesis of chlorophyll, heme, and other tetrapyrroles. To study the regulation of chlorophyll biosynthesis in Brassica napus, two cDNA clones of GSA-AT were isolated for genetic manipulation. A SalI-XbaI fragment from one of the two cDNA clones of GSA-AT was used for recombinant protein expression by inserting it at the 3' end of a calmodulin-binding-peptide (CBP) tag of the pCaln vector. The CBP tagged recombinant protein, expressed in Escherichia coli, was purified to apparent homogeneity in a one step purification process using a calmodulin affinity column. The purified CBP tagged GSA-AT is biologically active and has a specific activity of 16.6 nmol/min/mg. Cleavage of the CBP tag from the recombinant protein with thrombin resulted in 9.2% loss of specific activity. However, removal of the cleaved CBP tag from the recombinant protein solution resulted in 60% loss of specific activity, suggesting possible interactions between the recombinant protein and the CBP tag. The enzyme activity of the CBP tagless recombinant protein, referred as TR-GSA-AT hereafter, was not affected by the addition of pyridoxamine 5' phosphate (PMP). Addition of glutamate and pyridoxal 5' phosphate (PLP) to the TR-GSA-AT enhanced the enzyme activity by 3-fold and 3.6-fold, respectively. Addition of both glutamate and PLP increased the enzyme activity by 4.6-fold. Similar to the GSA-AT of B. napus, the active TR-GSA-AT is a dimeric protein of 88 kDa with 45.5 kDa subunits. As the SalI-XbaI fragment encodes a biologically active GSA-AT that has the same molecular mass as the native GSA-AT, it is concluded that the SalI-XbaI fragment is the coding sequence of GSA-AT. The highly active polyclonal antibodies generated from TR-GSA-AT were used for the detection of GSA-AT of B. napus.

Glutamate-1-semialdehyde aminotransferase from Sulfolobus solfataricus

Glutamate-1-semialdehyde aminotransferase (GSA-AT) from the extremely thermophilic bacterium Sulfolobus solfataricus has been purified to homogeneity and characterized. GSA-AT is the last enzyme in the C5 pathway for the conversion of glutamate into the tetrapyrrole precursor delta-aminolaevulinate (ALA) in plants, algae and several bacteria. The active form of GSA-AT from S. solfataricus seems to be a homodimer with a molecular mass of 87 kDa. The absorption spectrum of the purified aminotransferase is indicative of the presence of pyridoxamine 5'-phosphate (PMP) cofactor, and the catalytic activity of the enzyme is further stimulated by addition of PMP. 3-Amino-2,3-dihydrobenzoic acid is an inhibitor of the aminotransferase activity. The N-terminal amino acid sequence of GSA-AT from S. solfataricus was found to share significant similarity with the eukaryotic and eubacterial enzymes. Evidence is provided that ALA synthesis in S. solfataricus follows the C5 pathway characteristic of plants, algae, cyanobacteria and many other bacteria.

[Unusual pathways and environmentally regulated genes of bacterial heme biosynthesis]

The majority of bacteria, all investigated archaea and plants form the general precursor molecule of all tetrapyrroles 5-aminolevulinic acid by a unique transformation of transfer RNA bound glutamate. Only the alpha-group of the proteobacteria, mammals and yeast synthesize 5-aminolevulinic acid via the well known condensation of succinyl-CoA and glycine. The late steps in tetrapyrrole biosynthesis also contain alternative biosynthetic pathways for the formation and oxidative decarboxylation of coproporphyrinogen III. Unusual enzymatic reactions including the utilization of two substrate molecules as cofactor by the porphobilinogen deaminase and the formation of a spiro intermediate are involved in the formation of uroporphyrinogen III. The biosynthesis of hemes in bacteria is strictly regulated at the formation of 5-aminolevulinic acid and the oxidative decarboxylation of coproporphyrinogen III. The involved heme biosynthetic genes are regulated by the environmental concentrations of oxygen, iron, nitrate, growth phase and intracellular levels of heme. The current knowledge on the various enzymatic reactions and gene regulatory mechanisms is reviewed.

The regulation of enzymes involved in chlorophyll biosynthesis

All living organisms contain tetrapyrroles. In plants, chlorophyll (chlorophyll a plus chlorophyll b) is the most abundant and probably most important tetrapyrrole. It is involved in light absorption and energy transduction during photosynthesis. Chlorophyll is synthesized from the intact carbon skeleton of glutamate via the C5 pathway. This pathway takes place in the chloroplast. It is the aim of this review to summarize the current knowledge on the biochemistry and molecular biology of the C5-pathway enzymes, their regulated expression in response to light, and the impact of chlorophyll biosynthesis on chloroplast development. Particular emphasis will be placed on the key regulatory steps of chlorophyll biosynthesis in higher plants, such as 5-aminolevulinic acid formation, the production of Mg(2+)-protoporphyrin IX, and light-dependent protochlorophyllide reduction.

A visible marker for antisense mRNA expression in plants: inhibition of chlorophyll synthesis with a glutamate-1-semialdehyde aminotransferase antisense gene

Glutamate 1-semialdehyde aminotransferase [(S)-4-amino-5-oxopentanoate 4,5-aminomutase, EC 5.4.3.8] catalyzes the last step in the conversion of glutamate to delta-aminolevulinate of which eight molecules are needed to synthesize a chlorophyll molecule. Two full-length cDNA clones that probably represent the homeologous Gsa genes of the two tobacco (Nicotiana tabacum) genomes have been isolated. The deduced amino acid sequences of the 468-residue-long precursor polypeptides differ by 10 amino acids. The cDNA sequence of isoenzyme 2 was inserted in reverse orientation under the control of a cauliflower mosaic virus 35S promoter derivative in an expression vector and was introduced by Agrobacterium-mediated transformation into tobacco plants. Antisense gene expression decreased the steady-state mRNA level of glutamate 1-semialdehyde aminotransferase, the translation of the enzyme, and chlorophyll synthesis. Remarkably, partial or complete suppression of the aminotransferase mimics in tobacco a wide variety of chlorophyll variegation patterns caused by nuclear or organelle gene mutations in different higher plants. The antisense gene is inherited as a dominant marker.

Intermolecular nitrogen transfer in the enzymic conversion of glutamate to delta-aminolevulinic acid by extracts of Chlorella vulgaris

delta-Aminolevulinic acid (ALA), the universal biosynthetic precursor of tetrapyrrole pigments, is synthesized from glutamate in plants, algae, and many bacteria via a three-step process that begins with activation by ligation of glutamate to tRNA(Glu), followed by reduction to glutamate-1-semialdehyde (GSA) and conversion of GSA to ALA. The GSA aminotransferase step requires no substrate other than GSA. A previous study examined whether the aminotransferase reaction proceeds via intramolecular or intermolecular N transfer and concluded that the reaction catalyzed by Chlamydomonas extracts occurs via intermolecular N transfer (Y.-H.L. Mau and W.-Y. Wang [1988] Plant Physiol 86: 793-797). However, in that study the possibility was not excluded that the result was a consequence of N exchange among product ALA molecules during the incubation, rather than intermolecular N transfer during the conversion of GSA to ALA. Therefore, this question was reexamined in another species and with additional controls. A gel-filtered extract of Chlorella vulgaris cells was incubated with ATP, Mg2+, NADPH, tRNA, and a mixture of L-glutamate molecules, one-half of which were labeled with 15N and the other half with 13C at C-1. The ALA product was purified, derivatized, and analyzed by gas chromatography-mass spectrometry. A significant fraction of the ALA molecules was heavy by two mass units, indicating incorporation of both 15N and 13C. These results show that the N and C atoms of each ALA molecule were derived from different glutamate molecules. Control experiments indicated that the results could not be attributed to exchange of N atoms between glutamate or ALA molecules during the incubation. These results confirm the earlier conclusion that GSA is converted to ALA via intermolecular N transfer and extend the results to another species. The labeling results, combined with the results of kinetic and inhibitor studies, support a model for the GSA aminotransferase reaction in which a single molecule of GSA is converted to ALA via an enzyme-bound 4,5-diaminovaleric acid intermediate.

Purification and Characterization of Glutamate 1-Semialdehyde Aminotransferase from Barley Expressed in Escherichia coli

The immediate precursor in the synthesis of tetrapyrroles is Delta-aminolevulinate (ALA). ALA is synthesized from glutamate in higher plants, algae, and certain bacteria. Glutamate 1-semialdehyde aminotransferase (EC 5.4.3.8) (GSA-AT), the third enzyme involved in this metabolic pathway, catalyzes the transamination of GSA to form ALA. The gene encoding this aminotransferase has previously been isolated from barley (Hordeum vulgare) and inserted into an Escherichia coli expression vector. We describe herein the purification of this recombinant barley GSA-AT expressed in Escherichia coli. Coexpression of GroEL and GroES is required for isolation of active aminotransferase from the soluble protein fraction of Escherichia coli. Purified GSA-AT exhibits absorption maxima characteristic of vitamin B(6)-containing enzymes. GSA-AT is primarily in the pyridoxamine form when isolated and can be interconverted between this and the pyridoxal form by addition of 4,5-dioxovalerate and 4,5-diaminovalerate. The conversion of GSA to ALA under steady-state conditions exhibited typical Michaelis-Menten kinetics. Values for K(m) (d,l-GSA) and k(cat) were determined to be 25 micromolar and 0.11 per second, respectively, by nonlinear regression analysis. Stimulation of ALA synthesis by increasing concentrations of d,l-GSA at various fixed concentrations of 4,5-diaminovalerate supports the hypothesis that 4,5-diaminovalerate is the intermediate in the synthesis of ALA.

Gabaculine resistance of Synechococcus glutamate 1-semialdehyde aminotransferase

Glutamate 1-semialdehyde aminotransferase (GSA-AT) catalyzes the transfer of the C2 amino group of glutamate 1-semialdehyde (GSA) to the C1 position. Nucleic acid sequences encoding this enzyme from wild type and a gabaculine (GAB) resistant strain of Synechococcus have been cloned and overexpressed in Escherichia coli. Tolerance to GAB of the mutant GSA-AT resulted from a point mutation, Met-248-Ile, in the middle of the polypeptide chain accompanied by a deletion of three amino acids close to the NH2 terminus but can also be effected by the point mutation alone. Purified enzymes from these two strains contain vitamin B6 and use a typical ping-pong Bi-Bi mechanism, in which 4,5-diaminovalerate (DAVA) is a likely intermediate. The catalytic efficiency (Kcat/Km) of wild-type GSA-AT for GSA is about 3 times larger than that of the mutant enzyme. Comparison of substrate specificities (kmax/Km) for GSA and various analogues reveals that wild-type GSA-AT has values that are about 2-20 times larger than those of the mutant enzyme, except in the case of GAB for which the specificity is 2-3 orders of magnitude larger. These differences are attributed to impaired prototropic rearrangement and transaldimination by mutant GSA-AT. They lead to accumulation of quinonoid and other intermediates upon addition of various substrates such as ALA and DOVA, as well as to instability of their aldimines (418 nm) upon Sephadex gel filtration.