Acetyl Coenzyme A:Arylamine Acetyltransferase

From arylamine N-acetyltransferase to folate-dependent acetyl CoA hydrolase: impact of folic acid on the activity of (HUMAN)NAT1 and its homologue (MOUSE)NAT2

Acetyl Coenzyme A-dependent N-, O- and N,O-acetylation of aromatic amines and hydrazines by arylamine N-acetyltransferases is well characterised. Here, we describe experiments demonstrating that human arylamine N-acetyltransferase Type 1 and its murine homologue (Type 2) can also catalyse the direct hydrolysis of acetyl Coenzyme A in the presence of folate. This folate-dependent activity is exclusive to these two isoforms; no acetyl Coenzyme A hydrolysis was found when murine arylamine N-acetyltransferase Type 1 or recombinant bacterial arylamine N-acetyltransferases were incubated with folate. Proton nuclear magnetic resonance spectroscopy allowed chemical modifications occurring during the catalytic reaction to be analysed in real time, revealing that the disappearance of acetyl CH₃ from acetyl Coenzyme A occurred concomitantly with the appearance of a CH₃ peak corresponding to that of free acetate and suggesting that folate is not acetylated during the reaction. We propose that folate is a cofactor for this reaction and suggest it as an endogenous function of this widespread enzyme. Furthermore, in silico docking of folate within the active site of human arylamine N-acetyltransferase Type 1 suggests that folate may bind at the enzyme’s active site, and facilitate acetyl Coenzyme A hydrolysis. The evidence presented in this paper adds to our growing understanding of the endogenous roles of human arylamine N-acetyltransferase Type 1 and its mouse homologue and expands the catalytic repertoire of these enzymes, demonstrating that they are by no means just xenobiotic metabolising enzymes but probably also play an important role in cellular metabolism. These data, together with the characterisation of a naphthoquinone inhibitor of folate-dependent acetyl Coenzyme A hydrolysis by human arylamine N-acetyltransferase Type 1/murine arylamine N-acetyltransferase Type 2, open up a range of future avenues of exploration, both for elucidating the developmental role of these enzymes and for improving chemotherapeutic approaches to pathological conditions including estrogen receptor-positive breast cancer.

Computational study of the three-dimensional structure of N-acetyltransferase 2-acetyl coenzyme a complex

N-Acetyltransferase 2 (NAT2) is one of the most important polymorphic drug-metabolizing enzymes and plays a significant role in individual differences of drug efficacies and/or side effects. Coenzyme A (CoA) is a cofactor in the experimentally determined crystal structure of NAT2, although the acetyl source of acetylation reactions catalyzed by NAT is not CoA, but rather acetyl CoA. In this study, the three-dimensional structure of NAT2, including acetyl CoA, was calculated using molecular dynamics simulation. By substituting acetyl CoA for CoA the amino acid residue Gly286, which is known to transform into a glutamate residue by NAT2*7A and NAT2*7B, comes close to the cofactor binding site. In addition, the binding pocket around the sulfur atom of acetyl CoA expanded in the NAT2-acetyl CoA complex.

Comparison of the Arylamine N-acetyltransferase from Mycobacterium marinum and Mycobacterium tuberculosis

Arylamine N-acetyltansferase (NAT) from Mycobacterium tuberculosis (TBNAT) is a potential drug target for anti-tubercular therapy. Recombinant TBNAT is much less soluble and is produced in lower yields than the closely related NAT from Mycobacterium marinum (MMNAT). In order to explore MMNAT as a model for TBNAT in drug discovery, we compare the two mycobacterial NAT enzymes. Two site-directed mutants of MMNAT have been prepared and characterised: MMNAT71, Tyr --> Phe and MMNAT209, Met --> Thr, in which residues within 6 A of the active-site cysteine have been replaced with the corresponding residue from TBNAT. Two chimeric proteins have also been produced in which the third domain of MMNAT has been replaced by the third domain of TBNAT and vice versa. The activity profile of the chimeric proteins suggests a role for the third domain in the evolutionary divergence of NAT between these closely related mycobacterial species.

Insights into how protein dynamics affects arylamine N-acetyltransferase catalysis

Arylamine N-acetyltransferases (NATs) detoxify arylamines and hydrazine xenobiotics by catalyzing their N-acetylation, which prevents their bioactivation. Here, we reveal how structural dynamics impact NAT protein function. Our data suggest that there are multiple conformations in the catalytic cavity of hamster NAT2 that exchange on the millisecond time scale and enable NATs to accommodate substrates of varying size. The regions spanning N177-L180 and D285-F288, which form unique structures in mammalian NATs, possess inherent motions on the nanosecond time scale. The latter segment becomes more restricted in its motions upon substrate binding according to our NMR XNOE data. This greater rigidity appears to stem from interactions with the substrate. Finally, NAT acetylation has been suggested to protect these enzymes from ubiquitination. Our NMR data on a catalytically active state of hamster NAT2 suggest that structural rearrangements caused by its acetylation might contribute to this protection.

ArylamineN-Acetyltransferases: What We Learn from Genes and Genomes

Arylamine N-acetyltransferases (NATs) are phase II xenobiotic metabolizing enzymes, catalyzing acetyl-CoA-dependent N- and O-acetylation reactions. All NATs have a conserved cysteine protease-like Cys-His-Asp catalytic triad inside their active site cleft. Other residues determine substrate specificity, while the C-terminus may control hydrolysis of acetyl-CoA during acetyltransfer. Prokaryotic NAT-like coding sequences are found in >30 bacterial genomes, including representatives of Actinobacteria, Firmicutes and Proteobacteria. Of special interest are the nat genes of TB-causing Mycobacteria, since their protein products inactivate the anti-tubercular drug isoniazid. Targeted inactivation of mycobacterial nat leads to impaired mycolic acid synthesis, cell wall damage and growth retardation. In eukaryotes, genes for NAT are found in the genomes of certain fungi and all examined vertebrates, with the exception of canids. Humans have two NAT isoenzymes, encoded by highly polymorphic genes on chromosome 8p22. Syntenic regions in rodent genomes harbour two Nat loci, which are functionally equivalent to the human NAT genes, as well as an adjacent third locus with no known function. Vertebrate genes for NAT invariably have a complex structure, with one or more non-coding exons located upstream of a single, intronless coding region. Ubiquitously expressed transcripts of human NAT1 and its orthologue, murine Nat2, are initiated from promoters with conserved Sp1 elements. However, in humans, additional tissue-specific NAT transcripts may be expressed from alternative promoters and subjected to differential splicing. Laboratory animals have been widely used as models to study the effects of NAT polymorphism. Recently generated knockout mice have normal phenotypes, suggesting no crucial endogenous role for NAT. However, these strains will be useful for understanding the involvement of NAT in carcinogenesis, an area extensively investigated by epidemiologists, often with ambiguous results.

Divergence of cofactor recognition across evolution: coenzyme A binding in a prokaryotic arylamine N-acetyltransferase

Arylamine N-acetyltransferase (NAT) enzymes are widespread in nature. They serve to acetylate xenobiotics and/or endogenous substrates using acetyl coenzyme A (CoA) as a cofactor. Conservation of the architecture of the NAT enzyme family from mammals to bacteria has been demonstrated by a series of prokaryotic NAT structures, together with the recently reported structure of human NAT1. We report here the cloning, purification, kinetic characterisation and crystallographic structure determination of NAT from Mycobacterium marinum, a close relative of the pathogenic Mycobacterium tuberculosis. We have also determined the structure of M. marinum NAT in complex with CoA, shedding the first light on cofactor recognition in prokaryotic NATs. Surprisingly, the principal CoA recognition site in M. marinum NAT is located some 30 A from the site of CoA recognition in the recently deposited structure of human NAT2 bound to CoA. The structure explains the Ping-Pong Bi-Bi reaction mechanism of NAT enzymes and suggests mechanisms by which the acetylated enzyme intermediate may be protected. Recognition of CoA in a much wider groove in prokaryotic NATs suggests that this subfamily may accommodate larger substrates than is the case for human NATs and may assist in the identification of potential endogenous substrates. It also suggests the cofactor-binding site as a unique subsite to target in drug design directed against NAT in mycobacteria.

Investigation of the catalytic triad of arylamine N-acetyltransferases: essential residues required for acetyl transfer to arylamines

The NATs (arylamine N-acetyltransferases) are a well documented family of enzymes found in both prokaryotes and eukaryotes. NATs are responsible for the acetylation of a range of arylamine, arylhydrazine and hydrazine compounds. We present here an investigation into the catalytic triad of residues (Cys-His-Asp) and other structural features of NATs using a variety of methods, including site-directed mutagenesis, X-ray crystallography and bioinformatics analysis, in order to investigate whether each of the residues of the catalytic triad is essential for catalytic activity. The catalytic triad of residues, Cys-His-Asp, is a well defined motif present in several families of enzymes. We mutated each of the catalytic residues in turn to investigate the role they play in catalysis. We also mutated a key residue, Gly126, implicated in acetyl-CoA binding, to examine the effects on acetylation activity. In addition, we have solved the structure of a C70Q mutant of Mycobacterium smegmatis NAT to a resolution of 1.45 A (where 1 A=0.1 nm). This structure confirms that the mutated protein is correctly folded, and provides a structural model for an acetylated NAT intermediate. Our bioinformatics investigation analysed the extent of sequence conservation between all eukaryotic and prokaryotic NAT enzymes for which sequence data are available. This revealed several new sequences, not yet reported, of NAT paralogues. Together, these studies have provided insight into the fundamental core of NAT enzymes, and the regions where sequence differences account for the functional diversity of this family. We have confirmed that each of the three residues of the triad is essential for acetylation activity.

Probing the mechanism of hamster arylamine N-acetyltransferase 2 acetylation by active site modification, site-directed mutagenesis, and pre-steady state and steady state kinetic studies

Arylamine N-acetyltransferases (NATs) catalyze an acetyl group transfer from acetyl coenzyme A (AcCoA) to arylamines, hydrazines, and their N-hydroxylated arylamine metabolites. The recently determined three-dimensional structures of prokaryotic NATs have revealed a cysteine protease-like Cys-His-Asp catalytic triad, which resides in a deep and hydrophobic pocket. This catalytic triad is strictly conserved across all known NATs, including hamster NAT2 (Cys-68, His-107, and Asp-122). Treatment of NAT2 with either iodoacetamide (IAM) or bromoacetamide (BAM) at neutral pH rapidly inactivated the enzyme with second-order rate constants of 802.7 +/- 4.0 and 426.9 +/- 21.0 M(-1) s(-1), respectively. MALDI-TOF and ESI mass spectral analysis established that Cys-68 is the only site of alkylation by IAM. Unlike the case for cysteine proteases, no significant inactivation was observed with either iodoacetic acid (IAA) or bromoacetic acid (BAA). Pre-steady state and steady state kinetic analysis with p-nitrophenyl acetate (PNPA) and NAT2 revealed a single-exponential curve for the acetylation step with a second-order rate constant of (1.4 +/- 0.05) x 10(5) M(-1) s(-1), followed by a slow linear rate of (7.85 +/- 0.65) x 10(-3) s(-1) for the deacetylation step. Studies of the pH dependence of the rate of inactivation with IAM and the rate of acetylation with PNPA revealed similar pK(a)(1) values of 5.23 +/- 0.09 and 5.16 +/- 0.04, respectively, and pK(a)(2) values of 6.95 +/- 0.27 and 6.79 +/- 0.25, respectively. Both rates reached their maximum values at pH 6.4 and decreased by only 30% at pH 9.0. Kinetic studies in the presence of D(2)O revealed a large inverse solvent isotope effect on both inactivation and acetylation of NAT2 [k(H)(inact)/k(D)(inact) = 0.65 +/- 0.02 and (k(2)/K(m)(acetyl))(H)/(k(2)/K(m)(acetyl))(D) = 0.60 +/- 0.03], which were found to be identical to the fractionation factors (Phi) derived from proton inventory studies of the rate of acetylation at pL 6.4 and 8.0. Substitution of the catalytic triad Asp-122 with either alanine or asparagine resulted in the complete loss of protein structural integrity and catalytic activity. From these results, it can be concluded that the catalytic mechanism of NAT2 depends on the formation of a thiolate-imidazolium ion pair (Cys-S(-)-His-ImH(+)). However, in contrast to the case with cysteine proteases, a pH-dependent protein conformational change is likely responsible for the second pK(a), and not deprotonation of the thiolate-imidazolium ion. In addition, substitutions of the triad aspartate are not tolerated. The enzyme appears, therefore, to be engineered to rapidly form a stable acetylated species poised to react with an arylamine substrate.

The COOH terminus of arylamine N-acetyltransferase from Salmonella typhimurium controls enzymic activity

Arylamine N-acetyltransferases (NATs) are a homologous family of enzymes, which acetylate arylamines, arylhydroxylamines, and arylhydrazines by acetyl transfer from acetyl-coenzyme A (Ac-CoA) and are found in many organisms. NAT was first identified as the enzyme responsible for the inactivation of the anti-tubercular drug isoniazid in humans. The three-dimensional structure of NAT from Salmonella typhimurium has been resolved and shown to have three distinct domains and an active site catalytic triad composed of "Cys(69)-His(107)-Asp(122)," which is typical of hydrolytic enzymes such as the cysteine proteases. The crystal unit cell consists of a dimer of tetramers, with the C terminus of individual monomers juxtaposed. To investigate the function of the first two domains of full-length NAT from S. typhimurium and to investigate the role of the C terminus of NAT, truncation mutants were made with either the C-terminal undecapeptide or the entire third domain (85 amino acids) missing. Unlike the full-length NAT protein (281 amino acids), the truncation mutants of NAT from S. typhimurium are toxic when overexpressed intracellularly in Escherichia coli. Full-length NAT hydrolyses Ac-CoA but only in the presence of an arylamine substrate. Both truncation mutants, however, hydrolyze Ac-CoA even in the absence of arylamine substrate, illustrating that the C-terminal undecapeptide controls hydrolysis of Ac-CoA by NAT from S. typhimurium.

Kinetic analysis of the catalytic mechanism of serotonin N-acetyltransferase (EC 2.3.1.87)

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is the penultimate enzyme in melatonin biosynthesis. This enzyme is of special biological interest because large changes in its activity drive the large night/day rhythm in circulating melatonin in vertebrates. In this study the kinetic mechanism of AANAT action was studied using bacterially expressed glutathione S-transferase (GST)-AANAT fusion protein. The enzymologic behavior of GST-AANAT and cleaved AANAT was essentially identical. Two-substrate kinetic analysis generated an intersecting line pattern characteristic of a ternary complex mechanism. The dead end inhibitor analog desulfo-CoA was competitive versus acetyl-CoA and noncompetitive versus tryptamine. Tryptophol was not an alternative substrate but was a dead end competitive inhibitor versus tryptamine and an uncompetitive inhibitor versus acetyl-CoA, indicative of an ordered binding mechanism requiring binding of acetyl-CoA first. N-Acetyltryptamine, a reaction product, was a noncompetitive inhibitor versus tryptamine and uncompetitive with respect to acetyl-CoA. Taken together these results support an ordered BiBi ternary complex (sequential) kinetic mechanism for AANAT and provide a framework for inhibitor design.

Purification and characterization of an acetyl-CoA hydrolase from Saccharomyces cerevisiae

Acetyl-CoA hydrolase, which hydrolyzes acetyl-CoA to acetate and CoASH, was isolated from Saccharomyces cerevisiae and demonstrated by protein sequence analysis to be NH2-terminally blocked. The enzyme was purified 1080-fold to apparent homogeneity by successive purification steps using DEAE-Sepharose, gel filtration and hydroxylapatite. The molecular mass of the native yeast acetyl-CoA hydrolase was estimated to be 64 +/- 5 kDa by gel-filtration chromatography. SDS/PAGE analysis revealed that the denatured molecular mass was 65 +/- 2 kDa and together with that for the native enzyme indicates that yeast acetyl-CoA hydrolase was monomeric. The enzyme had a pH optimum near 8.0 and its pI was approximately 5.8. Several acyl-CoA derivatives of varying chain length were tested as substrates for yeast acetyl-CoA hydrolase. Although acetyl-CoA hydrolase was relatively specific for acetyl-CoA, longer acyl-chain CoAs were also hydrolyzed and were capable of functioning as inhibitors during the hydrolysis of acetyl-CoA. Among a series of divalent cations, Zn2+ was demonstrated to be the most potent inhibitor. The enzyme was inactivated by chemical modification with diethyl pyrocarbonate, a histidine-modifying reagent.

Characterization of the active site, substrate specificity and kinetic properties of acetyl-CoA:arylamine N-acetyltransferase from pigeon liver

It could be demonstrated that a sulfhydryl group is involved in the catalysis of acetyl-CoA:arylamine N-acetyltransferase from pigeon liver (EC 2.3.1.5). From ping-pong kinetics it was concluded that there is a covalent acetyl-enzyme intermediate. The respective intermediate could be isolated and chemically characterized as a cysteinyl thioester. Electrophoretically homogeneous acetyl-CoA:acylamine N-acetyltransferase from pigeon liver was able to acetylate a broad variety of aromatic and aliphatic amines from different acetyldonors such as acetyl-CoA, p-nitroacetanilide and p-nitrophenylacetate. Apparent Km values were determined for a number of acetyl donors and acetyl acceptors. Additionally, Ki values were evaluated for CoA, 3',5'-ADP and AMP. Correlation studies of basicity of acceptor amines and acetylation rate demonstrated that there is a limit of the pKa value (about pKa = 1) where the covalently-bound acetyl-enzyme intermediate can still be saponified. Testing crude liver homogenates of several animals including turkey, duck, chicken, cow, pig, horse, sheep, carp, trout and herring the outstanding nature of the pigeon liver enzyme in acetylating very weakly basic amines could be demonstrated. It is shown that the enzyme is quite flexible concerning sterically different acceptor amines, because arylamines whose amino group was effected by large o-substituents could be quantitatively acetylated. After enzymatic acetylation of the first amino group, 1,2-phenylendiamine formed the heterocyclic compound 2-methylbenzimidazole by a spontaneous condensation reaction. There is evidence that with distinct amines formation of heterocyclic compounds may also occur in vivo.

The metabolism of O-acyl-N-acylneuraminic acids. Biosynthesis of O-acylated sialic acids in bovine and equine submandibular glands

1. The enzymic synthesis of 4-O-acetylneuraminic acid, 4-O-acetyl-N-glycolyneuraminic acid, 4-O-glycolyl-N-acetylneuraminic acid, 9-O-acetyl-N-acetylneuraminic acid and 9-O-acetyl-N-glycolyneuraminic acid is shown using radioactive precursors with surviving slices, membrane fractions or particle-free homogenates from bovine and equine submandibular glands. 2. Acetyl-CoA: N-acetylneuraminate-9(or 7)-O-acetyltransferase activity was found in a microsome fraction and in the cytosol of bovine submandibular glands. The properties of the membrane-bound enzyme acting on endogenous, glycoprotein-bound N-acetyl- and N-glycolylneuraminic acids were compared with those of the soluble enzyme, O-acetylating exogenous, non-glycosidically bound N-acetyl- and N-glycolyneuraminic acids. 3. A rapid, radioactive assay for the membrane-bound enzyme activity is described. The enzyme activity shows an optimum at pH 7 and has a Km for acetyl-CoA of 0.1 mM. The enzyme is inhibited by p-chloromercuribenzoate and iodoacetate. Divalent cations, EDTA and glutathione have no influence on its activity while CoA proved to be a competitive inhibitor with a Ki of 0.56 mM. 4. The soluble enzyme activity, assayed using a radioactive procedure, shows Km values of 0.01 mM, 0.5 mM and 0.39 mM for acetyl-CoA, N-acetylneuraminic acid and N-glycolylneuraminic acid respectively. The general properties are similar to those found for the membrane-bound enzyme, except that membrane-bound activity is stable for longer on storage at 4 degrees C. 5. Acetyl-CoA, acyl-CoA and CoA concentrations of 33 nmol, 65 nmol and 106 nmol/g wet tissue respectively are found in fresh bovine submandibular glands. 6. The occurrence of the CMP-glycosides of N-acetylneuraminic acid, 9-O-acetyl-N-acetyl-neuraminic acid and N-glycolylneuraminic acid in bovine submandibular glands is demonstrated. 7. The results are discussed in relation to the general metabolism of acylneuraminic acids.

Human brain and placental choline acetyltransferase: purification and properties

Choline acetyltransferase (EC 2.3.1.6) catalyzes the biosynthesis of acetylcholine according to the following chemical equation: acetyl-CoA + choline in equilibrium to acetylcholine + CoA. In addition to nervous tissue, primate placenta is the only other animal source which contains appreciable acetylcholine and its biosynthetic enzyme. Human brain caudate nucleus and human placental choline acetyltransferase were purified to electrophoretic homogeneity using ion-exchange and blue dextran-Sepharose affinity chromatography. The molecular weights determined by Sephadex G-150 gel filtration and sodium dodecyl sulfate gel electrophoresis are 67000 plus or minus 3000. N-Ethylmaleimide, p-chloromercuribenzoate, and dithiobis(2-nitrobenzoic acid) inhibit the enzyme. Dithiothreitol reverses the inhibition produced by the latter two reagents. The pKa of the group associated with N-ethylmaleimide inhibition is 8.6 plus or minus 0.3. A chemically competent acetyl-thioenzyme is isolable by Sephadex gel filtration. The enzymes from the brain and placenta are thus far physically and biochemically indistinguishable.