Purification and characterization of serine-glyoxylate aminotransferase from a serine-producing methylotroph, Hyphomicrobium methylovorum GM2

Novel archaeal alanine:glyoxylate aminotransferase from Thermococcus litoralis

A novel alanine:glyoxylate aminotransferase was found in a hyperthermophilic archaeon, Thermococcus litoralis. The amino acid sequence of the enzyme did not show a similarity to any alanine:glyoxylate aminotransferases reported so far. Homologues of the enzyme appear to be present in almost all hyperthermophilic archaea whose whole genomes have been sequenced.

Initial velocity, spectral, and pH studies of the serine-glyoxylate aminotransferase from Hyphomicrobiuim methylovorum

Serine-glyoxylate aminotransferase (SGAT) from Hyphomicrobium methylovorum is a pyridoxal 5'-phosphate (PLP) enzyme that catalyzes the interconversion of L-serine and glyoxylate to hydroxypyruvate and glycine. The initial velocity and dead-end inhibition patterns are consistent with a ping-pong kinetic mechanism. The Km values for L-serine and the alternative substrate ketomalonate are 0.28 +/- 0.02 and 1.13 +/- 0.08 mM, respectively. The spectrum of SGAT at pH 7.5 shows an absorbance maximum at 413 nm and a shoulder centered at 330 nm corresponding to the ketoenamine and enolimine forms of the protonated Schiff's base with the enolimine tautomer predominating. As determined by the changes in the enzyme absorbance spectrum the enzyme can be converted from the E-PLP to the E-pyridoxamine 5'-phosphate (E-PMP) form on addition of L-serine. The enzyme can subsequently be converted back to E-PLP by addition of glyoxylate or hydroxypyruvate. The enzyme displays a pH-dependent spectral change with a pK of about 8.2 which is ascribed to the ionization of an enzymatic residue that effects the tautomeric equilibrium between the ketoenamine and enolimine tautomers of the protonated aldimine. The V/K(L-serine) pH profile displays two pK values at pH 7.5 and 8.5 with limiting slopes of 1 and -1. The V/K(ketomalonate) pH profile displays one pK at 8.2 on the basic side with a limiting slope of 1 and the log K(I oxalate) pH profile shows one pK on the basic side at pH 7.2. The data suggest the active enzyme is the protonated aldimine and an enzymatic base with a pK of 7.5 accepts a proton from the alpha-amine of substrate to initiate catalysis.

Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is a photorespiratory enzyme with multiple substrates in Arabidopsis thaliana

At least two glyoxylate aminotransferases are hypothesized to participate in the steps of photorespiration located in peroxisomes. Until recently, however, genes encoding these enzymes had not been identified. We describe the isolation and characterization of an alanine : glyoxylate aminotransferase (AGT1, formerly AGT) cDNA from Arabidopsis thaliana. Southern blot analysis confirmed that Arabidopsis AGT1 is encoded by a single gene. Homologs of this class IV aminotransferase are also known in other plants, animals, and methylotrophic bacteria, suggesting an ancient evolutionary origin of this enzyme. AGT1 transcripts were present in all tissues of Arabidopsis, but were most abundant in green, leafy tissues. Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. Sequence analysis of the photorespiratory sat mutants revealed a single nucleotide substitution in the AGT1 gene from these plants. This transition mutation is predicted to result in a proline-to-leucine substitution at residue 251 of AGT1. When this mutation was engineered into the recombinant AGT1 protein, enzymatic activity using all three donor : acceptor pairs was abolished. We conclude that Arabidopsis AGT1 is a peroxisomal photorespiratory enzyme that catalyzes transamination reactions with multiple substrates.

Cloning and expression of the gene for serine-glyoxylate aminotransferase from an obligate methylotroph Hyphomicrobium methylovorum GM2

The gene encoding serine-glyoxylate aminotransferase, one of key enzymes for the assimilation of one-carbon compounds in methylotrophs, and its flanking regions were isolated from an obligate methylotrophic bacterium, Hyphomicrobium methylovorum GM2. Nucleotide sequencing of the recombinant plasmids revealed that the serine-glyoxylate aminotransferase gene encodes a 405-amino-acid protein with a calculated molecular mass of 43880 Da. The amino acid sequence of the enzyme showed identity to the sequences of serine-glyoxylate aminotransferase of Methylobacterium extorquens AM1 (57%), aspartate aminotransferase of Methanobacterium thermoformicicum (31%), human peroxisomal alanine-glyoxylate aminotransferase (27%), and serine-pyruvate aminotransferase of rat liver mitochondria (33%). The recombinant plasmid, which was constructed by ligation of the cloned gene and an expression vector pKK223-3, was introduced into Escherichia coli HB101. The recombinant enzyme was purified from transformed E. coli cells and analyzed by immunological and enzymological methods. The overexpressed enzyme was indistinguishable from the wild-type enzyme isolated from H. methylovorum GM2.

Immunological characterization of serine-glyoxylate aminotransferase and hydroxypyruvate reductase from a methylotrophic bacterium, Hyphomicrobium methylovorum GM2

Immunological characterization of serine-glyoxylate aminotransferase and hydroxypyruvate reductase, key enzymes for the assimilation of one-carbon compounds in methylotrophs, was performed using antibodies raised against these enzymes purified from Hyphomicrobium methylovorum GM2. Immunodiffusion studies indicated that serine-glyoxylate aminotransferase and hydroxypyruvate reductase of all seven Hyphomicrobium strains tested were immunochemically similar. In immunotitration experiments and Western blot analyses of both enzymes in the genera Hyphomicrobium and Methylobacterium, the serine-glyoxylate aminotransferase of the genus Methylobacterium exhibited low similarity to that of the genus Hyphomicrobium. For hydroxypyruvate reductase, no immunological relationship was observed between the genera Hyphomicrobium and Methylobacterium, which was in agreement with the differences in primary structure and enzymological properties.

L-serine production by a methylotroph and its related enzymes

The production process of L-serine from methanol and glycine has been developed using a methylotroph with the serine pathway. Consecutive reactions of two enzymes, methanol dehydrogenase (MDH) and serine hydroxymethyltransferase (SHMT) are involved in the production. We screened a high producer, Hyphomicrobium methylovorum, which is an obligate methylotroph. With resting cells of the bacterium, 24 mg/ml of L-serine was produced from 100 mg/ml of glycine and 48 mg/ml of methanol in 3 days under optimal conditions. Next, a glycine-resistant mutant GM2 showed improved serine production (32-34 mg/ml). The mutant GM2 was found to have elevated activities of MDH and SHMT. Since there has so far been little report on the systematic characterization of enzymes of the serine pathway in methylotrophs, not only the above two enzymes but also the other three enzymes in H. methylovorum were purified and characterized: MDH, SHMT and hydroxypyruvate reductase (HPR) were crystallized; serine-glyoxylate aminotransferase (SGAT) and glycerate kinase (GK) were purified to homogeneity. As a result, all these enzymes were found to be stable against preservation and to exist abundantly in the bacterium. The gene of SHMT was cloned and its deduced amino acid sequence had homology to those of Escherichia coli (55%) and rabbit liver (44%), whereas the enzyme of the bacterium was immunochemically distinguishable from those of microorganisms other than Hyphomicrobium strains and mammalian livers.

Purification and characterization of glycerate kinase from a serine-producing methylotroph, Hyphomicrobium methylovorum GM2

The glycerate kinase of a serine-producing methylotroph, Hyphomicrobium methylovorum GM2, was purified to complete homogeneity and characterized, the first time for an enzyme from a methylotroph. The enzyme was a monomer with a molecular mass about 41-52 kDa. The enzyme was stable against heating at 35 degrees C for 30 min at pH values over 6-10. Maximum activity was observed at pH 8.0 and around 50 degrees C. The Km values for D-glycerate and ATP were 0.13 mM and 0.13 mM, respectively. The enzyme showed high specificity for D-glycerate, and was activated by potassium and ammonium ions. The reaction product of the enzyme was identified as 2-phosphoglycerate.