Substrate Report for: N-acetylanthranilate

Enzymatic catalysis of oxygenation reactions in the absence of metal or organic cofactors is a considerable biochemical challenge. The CO-forming 1-H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HOD) from Arthrobacter nitroguajacolicus Ru61a and 1-H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase (QDO) from Pseudomonas putida 33/1 are homologous cofactor-independent dioxygenases involved in the breakdown of N-heteroaromatic compounds. To date, they are the only dioxygenases suggested to belong to the alpha/beta-hydrolase fold superfamily. Members of this family typically catalyze hydrolytic processes rather than oxygenation reactions. We present here the crystal structures of both HOD and QDO in their native state as well as the structure of HOD in complex with its natural 1-H-3-hydroxy-4-oxoquinaldine substrate, its N-acetylanthranilate reaction product, and chloride as dioxygen mimic. HOD and QDO are structurally very similar. They possess a classical alpha/beta-hydrolase fold core domain additionally equipped with a cap domain. Organic substrates bind in a preorganized active site with an orientation ideally suited for selective deprotonation of their hydroxyl group by a His/Asp charge-relay system affording the generation of electron-donating species. The "oxyanion hole" of the alpha/beta-hydrolase fold, typically employed to stabilize the tetrahedral intermediate in ester hydrolysis reactions, is utilized here to host and control oxygen chemistry, which is proposed to involve a peroxide anion intermediate. Product release by proton back transfer from the catalytic histidine is driven by minimization of intramolecular charge repulsion. Structural and kinetic data suggest a nonnucleophilic general-base mechanism. Our analysis provides a framework to explain cofactor-independent dioxygenation within a protein architecture generally employed to catalyze hydrolytic reactions.

N-acetylanthranilate amidase (Amq), a 32.8-kDa monomeric amide hydrolase, is involved in quinaldine degradation by Arthrobacter nitroguajacolicus Ru61a. Sequence analysis and secondary structure predictions indicated that Amq is related to carboxylesterases and belongs to the alpha/beta-hydrolase-fold superfamily of enzymes; inactivation of (His(6)-tagged) Amq by phenylmethanesulfonyl fluoride and diethyl pyrocarbonate and replacement of conserved residues suggested a catalytic triad consisting of S155, E235, and H266. Amq is most active towards aryl-acetylamides and aryl-acetylesters. Remarkably, its preference for ring-substituted analogues was different for amides and esters. Among the esters tested, phenylacetate was hydrolyzed with highest catalytic efficiency (k(cat)/K(m) = 208 mM(-1) s(-1)), while among the aryl-acetylamides, o-carboxy- or o-nitro-substituted analogues were preferred over p-substituted or unsubstituted compounds. Hydrolysis by His(6)Amq of primary amides, lactams, N-acetylated amino acids, azocoll, tributyrin, and the acylanilide and urethane pesticides propachlor, propham, carbaryl, and isocarb was not observed; propanil was hydrolyzed with 1% N-acetylanthranilate amidase activity. The catalytic properties of the cysteine-deficient variant His(6)AmqC22A/C63A markedly differed from those of His(6)Amq. The replacements effected some changes in K(m)s of the enzyme and increased k(cat)s for most aryl-acetylesters and some aryl-acetylamides by factors of about three to eight while decreasing k(cat) for the formyl analogue N-formylanthranilate by several orders of magnitude. Circular dichroism studies indicated that the cysteine-to-alanine replacements resulted in significant change of the overall fold, especially an increase in alpha-helicity of the cysteine-deficient protein. The conformational changes may also affect the active site and may account for the observed changes in kinetic properties.

1H-3-Hydroxy-4-oxoquinaldine 2,4-dioxygenase (MeQDO) was purified from quinaldine-grown Arthrobacter sp. Ru61a. It was enriched 59-fold in a yield of 22%, and its properties were compared with 1H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase (QDO) purified from Pseudomonas putida 33/1. The enzyme-catalyzed conversions were performed in an (18O)O2/(16O)O2 atmosphere. Two oxygen atoms of either (18O)O2 or (16O)O2 were incorporated at C2 and C4 of the respective substrates, indicating that these unusual enzymes, which catalyze the cleavage of two carbon-carbon bonds concomitant with CO formation, indeed are 2,4-dioxygenases. Both enzymes are small monomeric proteins of 32 kDa (MeQDO) and 30 kDa (QDO). The apparent K(m) values of MeQDO for 1H-3-hydroxy-4-oxoquinaldine and QDO for 1H-3-hydroxy-4-oxoquinoline were 30 microM and 24 microM, respectively. In both 2,4-dioxygenases, there was no spectral evidence for the presence of a chromophoric cofactor. EPR analyses of MeQDO did not reveal any signal that could be assigned to an organic radical species or to a metal, and X-ray fluorescence spectrometry of both enzymes did not show any metal present in stoichiometric amounts. Ethylxanthate, metal-chelating agents (tiron, alpha, alpha'-bipyridyl, 8-hydroxyquinoline, o-phenanthroline, EDTA, diphenylthiocarbazone, diethyldithiocarbamate), reagents that modify sulfhydryl groups (iodoacetamide, N-ethylmaleimide, p-hydroxymercuribenzoate), and reducing agents (sodium dithionite, dithiothreitol, mercaptoethanol) either did not affect 2,4-dioxygenolytic activities at all or inhibited at high concentrations only. With respect to the supposed lack of any cofactor and with respect to the inhibitors of dioxygenolytic activities, MeQDO and QDO resemble aci-reductone oxidase (CO-forming) from Klebsiella pneumoniae, which catalyzes 1,3-dioxygenolytic cleavage of 1,2-dihydroxy-3-keto-S-methylthiopentene anion (Wray, J. W. & Abeles, R. H. (1993) J. Biol. Chem. 268, 21466-21469; Wray, J. W. & Abeles, R. H. (1995) J. Biol. Chem. 270, 3147-3153). 1H-3-Hydroxy-4-oxoquinaldine and 1H-3-hydroxy-4-oxoquinoline were reactive towards molecular oxygen in the presence of the base catalyst potassium-tert.-butoxide in the aprotic solvent N,N-dimethylformamide. Base-catalyzed oxidation, yielding the same products as the enzyme-catalyzed conversions, provides a non-enzymic model reaction for 2,4-dioxygenolytic release of CO from 1H-3-hydroxy-4-oxoquinaldine and 1H-3-hydroxy-4-oxoquinoline.