The Roles of Tyr70 and Tyr225 in Aspartate Aminotransferase Assessed by Analysing the Effects of Mutations on the Multiple Reactions of the Substrate Analogue Serine O-Sulphate

Crystal structure of an oxidized mutant of human mitochondrial branched-chain aminotransferase

This study presents the crystal structure of a thiol variant of the human mitochondrial branched-chain aminotransferase protein. Human branched-chain aminotransferase (hBCAT) catalyzes the transamination of the branched-chain amino acids leucine, valine and isoleucine and α-ketoglutarate to their respective α-keto acids and glutamate. hBCAT activity is regulated by a CXXC center located approximately 10 Å from the active site. This redox-active center facilitates recycling between the reduced and oxidized states, representing hBCAT in its active and inactive forms, respectively. Site-directed mutagenesis of the redox sensor (Cys315) results in a significant loss of activity, with no loss of activity reported on the mutation of the resolving cysteine (Cys318), which allows the reversible formation of a disulfide bond between Cys315 and Cys318. The crystal structure of the oxidized form of the C318A variant was used to better understand the contributions of the individual cysteines and their oxidation states. The structure reveals the modified CXXC center in a conformation similar to that in the oxidized wild type, supporting the notion that its regulatory mechanism depends on switching the Cys315 side chain between active and inactive conformations. Moreover, the structure reveals conformational differences in the N-terminal and inter-domain region that may correlate with the inactivated state of the CXXC center.

Cofactor-Directed Reversible Denaturation Pathways: The Cofactor-Stabilized Escherichia coli Aspartate Aminotransferase Homodimer Unfolds through a Pathway That Differs from That of the Apoenzyme

While the urea-mediated unfolding pathway of the Escherichia coli aspartate aminotransferase (eAATase) homodimer proceeds through a reversible three-state process with a partially folded dimeric intermediate, D D* 2U (E. Deu and J. F. Kirsch, accompanying paper), that of a cofactor-stabilized form differs. Pyridoxal phosphate, which binds at the intersubunit active sites, stabilizes the native form by 6 kcal mol-1 and dissociates during the D <==> D* transition. Reductive trapping of the cofactor to a nondissociable derivative (PPL-eAATase) precludes the formation of D*. A novel monomeric intermediate (M'-PPL) with 70% of the native secondary structure (circular dichroism) was identified in the unfolding pathway of PPL-eAATase: D-PPL2 <==> 2M'-PPL <==> 2U-PPL. The combined results define two structural regions with distinct stabilities: the active site region (ASR) and the generally more stable, dimerization region (DMR). The DMR includes the key intersubunit contacts. It is responsible for the multimeric nature of D*, and its disorder leads to dimer dissociation. Selective strengthening of the ASR-cofactor interactions by cofactor trapping reverses the relative stabilities of the two regions (from DMR > ASR in the apoenzyme to ASR > DMR in PPL-eAATase) and results in a reordering of the eAATase denaturation pathway.

An NMR study of alterations in [1-13C]glucose metabolism in C6 glioma cells by gliotoxic amino acids

A series of glutamate analogues, known as gliotoxins, are toxic to astrocytes in culture, and are inhibitors or substrates of high affinity sodium-dependent glutamate transporters. The mechanisms by which these gliotoxins cause toxicity are not fully understood. The effects of a series of gliotoxic amino acids (L-alpha-aminoadipate, L-serine-O-sulphate, D-aspartate and L-cysteate) on metabolism of [1-13C]glucose were examined in C6 glioma cells using 13C nuclear magnetic resonance (NMR) spectroscopy. The cells were preincubated in the presence of sub toxic concentrations of each gliotoxin (400 micromol/l) for 20 h. This was followed by incubation (4 h) with [1-13C]glucose (5.5 mmol/l) in the presence and absence of each gliotoxin. The incorporation of 13C label into the observed metabolites was analysed. Following preincubation with L-alpha-aminoadipate, D-aspartate, and L-serine-O-sulphate there was a significant decrease in the incorporation of 13C label into glutamate, alanine and lactate from [1-13C]glucose. In the presence of L-cysteate production of labelled glutamate was decreased, while there was no significant effect on the concentrations of labelled lactate and alanine. There was no change in the quantity of LDH released into the medium after incubation of the cells with any of the gliotoxins. Overall these results indicate that the presence of gliotoxins profoundly alters the flux of glucose to lactate, alanine, aspartate and glutamate.

A new strategy to decrease N-methyl-D-aspartate (NMDA) receptor coactivation: inhibition of D-serine synthesis by converting serine racemase into an eliminase

Serine racemase is a brain-enriched enzyme that synthesizes d-serine, an endogenous modulator of the glycine site of N-methyl-d-aspartate (NMDA) receptors. We now report that serine racemase catalyzes an elimination reaction toward a nonphysiological substrate that provides a powerful tool to study its neurobiological role and will be useful to develop selective enzyme inhibitors. Serine racemase catalyzes robust elimination of l-serine O-sulfate that is 500 times faster than the physiological racemization reaction, generating sulfate, ammonia, and pyruvate. This reaction provides the most simple and sensitive assay to detect the enzyme activity so far. We establish stable cell lines expressing serine racemase and show that serine racemase can also be converted into a powerful eliminase in cultured cells, while the racemization of l-serine is inhibited. Likewise, l-serine O-sulfate inhibits the synthesis of d-serine in primary astrocyte cultures. We conclude that the synthetic compound l-serine O-sulfate is a better substrate than l-serine as well as an inhibitor of d-serine synthesis. Inhibition of serine racemase provides a new strategy to selectively decrease NMDA receptor coactivation and may be useful in conditions in which overstimulation of NMDA receptors plays a pathological role.

Aminotransferase activity and bioinformatic analysis of 1-aminocyclopropane-1-carboxylate synthase

The mechanistic fate of pyridoxal phosphate (PLP)-dependent enzymes diverges after the quinonoid intermediate. 1-Aminocyclopropane-1-carboxylate (ACC) synthase, a member of the alpha family of PLP-dependent enzymes, is optimized to direct electrons from the quinonoid intermediate to the gamma-carbon of its substrate, S-adenosyl-L-methionine (SAM), to yield ACC and 5'-methylthioadenosine. The data presented show that this quinonoid may also accept a proton at C(4)' of the cofactor to yield alpha-keto acids and the pyridoxamine phosphate (PMP) form of the enzyme when other amino acids are presented as alternative substrates. Addition of excess pyruvate converts the PMP form of the enzyme back to the PLP form. C(alpha)-deprotonation from L-Ala is shown by NMR-monitored solvent exchange to be reversible with a rate that is less than 25-fold slower than that of deprotonation of SAM. The rate-determining step for transamination follows the formation of the quinonoid intermediate. The rate-determining step for alpha, gamma-elimination from enzyme-bound SAM is likewise shown to occur after C(alpha)-deprotonation, and the quinonoid intermediate accumulates during this reaction. BLAST searches, sequence alignments, and structural comparisons indicate that ACC synthases are evolutionarily related to the aminotransferases. In agreement with previously published reports, an absence of homology was found between the alpha and beta families of the PLP-dependent enzyme superfamily.

13C and (15)N kinetic isotope effects on the reaction of aspartate aminotransferase and the tyrosine-225 to phenylalanine mutant

Heavy atom isotope effects at C-2, C-3, and the amino nitrogen of aspartate were determined for the reaction of porcine heart cytosolic aspartate aminotransferase and the tyrosine-225 to phenylalanine mutant of Escherichia coli aspartate aminotransferase. The effects of deuteration at C-2 of aspartate and of D(2)O on the observed heavy atom isotope effects were determined. The multiple isotope effects support the contribution of C(alpha)-H cleavage, ketimine hydrolysis, and oxaloacetate dissociation to the rate limitation with the wild-type enzyme. The existence of a quinonoid intermediate could not be determined due to the kinetic complexity of the enzyme. For the tyrosine-225 to phenylalanine mutant, we are able to conclude that ketimine hydrolysis is the major rate-determining step.

Aspartate aminotransferase from the Antarctic bacterium Pseudoalteromonas haloplanktis TAC 125. Cloning, expression, properties, and molecular modelling

The gene encoding aspartate aminotransferase from the psychrophilic bacterium Pseudoalteromonas haloplanktis TAC 125 was cloned, sequenced and overexpressed in Escherichia coli. The recombinant protein (PhAspAT) was characterized both at the structural and functional level in comparison with the E. coli enzyme (EcAspAT), which is the most closely related (52% sequence identity) bacterial counterpart. PhAspAT is rapidly inactivated at 50 degrees C (half-life = 6.8 min), whereas at this temperature EcAspAT is stable for at least 3 h. The optimal temperature for PhAspAT activity is approximately 64 degrees C, which is some 11 degrees C below that of EcAspAT. The protein thermal stability was investigated by following changes in both tryptophan fluorescence and amide ellipticity; this clearly suggested that a first structural transition occurs at approximately 50 degrees C for PhAspAT. These results agree with the expected thermolability of a psychrophilic enzyme, although the observed stability is much higher than generally found for enzymes isolated from cold-loving organisms. Furthermore, in contrast with the higher efficiency exhibited by several extracellular psychrophilic enzymes, both kcat and kcat/Km of PhAspAT are significantly lower than those of EcAspAT over the whole temperature range. This behaviour possibly suggests that the adaptation of this class of endocellular enzymes to a cold environment may have only made them less stable and not more efficient. The affinity of PhAspAT for both amino-acid and 2-oxo-acid substrates decreases with increasing temperature. However, binding of maleate and 2-methyl-L-aspartate, which both inhibit the initial steps of catalysis, does not change over the temperature range tested. Therefore, the observed temperature effect may occur at any of the steps of the catalytic mechanism after the formation of the external aldimine. A molecular model of PhAspAT was constructed on the basis of sequence homology with other AspATs. Interestingly, it shows no insertion or extension of loops, but some cavities and a decrease in side chain packing can be observed.

Functional and structural analysis of cis-proline mutants of Escherichia coli aspartate aminotransferase

To elucidate the role of the two conserved cis-proline residues of aspartate aminotransferase (AspAT), one double and two single mutants of the enzyme from Escherichia coli (EcAspAT) were prepared: P138A, P195A and P138A/P195A in which the two prolines were replaced by alanine. The crystal structures of P195A and P138A/P195A have been determined at 2.3-2.1 A resolution. The wild-type geometry, including the cis conformation of the 194-195 peptide bond is retained upon substitution of proline 195 by alanine, whereas the trans conformation is adopted at the 137-138 peptide bond. Quite surprisingly, the replacement of each of the two prolines by alanine does not significantly affect either the activity or the stability of the protein. All the three mutants follow the same pathway as the wild type for unfolding equilibrium induced by guanidine hydrochloride [Herold, M., and Kirschner, K. (1990) Biochemistry 29, 1907-1913]. The kinetics of renaturation of P195A, where the alanine retains the wild-type cis conformation, is faster than wild type, whereas renaturation of P138A, which adopts the trans conformation, is slower. We conclude that cis-prolines seem to have been retained throughout the evolution of aspartate aminotransferase to possibly play a subtle role in directing the traffic of intermediates toward the unique structure of the native state, rather than to respond to the needs for a specific catalytic or functional role.

Kinetic properties and thermal stabilities of mutant forms of mitochondrial aspartate aminotransferase

Kinetic properties and thermal stabilities of the precursor form of mitochondrial aspartate aminotransferase, the mature form lacking 9 amino acids from the N-terminus, and forms of the mature protein in which cysteine-166 had been mutated to serine or alanine were compared with those of the mature enzyme. The precursor and the cysteine mutants showed moderately impaired catalytic properties consistent with decreased ability to undergo transition from the open to the closed conformation which is an integral part of the mechanism of action of the enzyme. The deletion mutant had a kcat only 2% of that of the mature enzyme but also much reduced Km values for both substrates. In addition it showed enhanced reactivity of cysteine-166 with 5,5'-dithiobis(2-nitrobenzoate), which is characteristic of the closed form of the enzyme, with no enhancement of reactivity in the presence of substrates. This is taken to show that the deletion mutant adopts a conformation that is significantly different from that of the mature enzyme particularly in respect of the small domain. The deletion mutant was found to be more resistant to thermal inactivation over a range of temperatures than were the other forms of the enzyme consistent with its having a more tightly packed small domain.

The mechanism of high-yielding chiral syntheses catalysed by wild-type and mutant forms of aspartate aminotransferase

The ability of aspartate aminotransferase to catalyse beta-elimination of alpha-amino acids that have a good leaving group at C beta has been exploited in the synthesis of novel amino acids by the inclusion of appropriate nucleophiles as co-substrates. Two compounds, L-serine O-sulphate and 3-chloro-L-alanine, were used as beta-elimination substrates. Nucleophiles used successfully as co-substrates were thiosulphate, 2-mercaptoethanol, mercaptoacetate and aminoethylthiopseudourea. The synthesis achieved using serine O-sulphate and thiosulphate was found to produce sulphocysteine with a yield of 70%. Circular dichroism demonstrated that the compound was a single enantiomer and, therefore, that nucleophilic addition had taken place on the enzyme. The initial rate of synthesis was 10% of the rate at which the enzyme catalyses its normal transamination reaction. The synthetic reaction was accompanied by minor side reactions that led to small amounts of additional amino acid and oxo acid products through partitions of the main reaction at two stages in the mechanism. By mutating Arg292, which is the residue that binds the distal carboxyl group of natural substrates, the wild-type enzyme was converted to a form that could discriminate completely between serine O-sulphate and chloroalanine as beta-eliminating substrate. Similar alterations in nucleophile cosubstrate specificity were also observed. Whereas, for example, the wild-type enzyme catalysed syntheses between 3-chloroalanine and either mercaptoethanol or mercaptoacetate with equal facility, the Arg292Asp enzyme showed complete preference for mercaptoethanol. The system should be of general use in the synthesis of novel amino acids as single enantiomers with potentially interesting biological activities.