Interaction ofN-acetyl-4-, 7-, 8- or 9-deoxyneuraminic acids andN-acetyl-4-, 7- or 8-mono-epi- and-7,8-di-epineuraminic acids withN-acetylneuraminate lyase

Reaction mechanism of N-acetylneuraminic acid lyase revealed by a combination of crystallography, QM/MM simulation, and mutagenesis

N-Acetylneuraminic acid lyase (NAL) is a Class I aldolase that catalyzes the reversible condensation of pyruvate with N-acetyl-d-mannosamine (ManNAc) to yield the sialic acid N-acetylneuraminic acid (Neu5Ac). Aldolases are finding increasing use as biocatalysts for the stereospecific synthesis of complex molecules. Incomplete understanding of the mechanism of catalysis in aldolases, however, can hamper development of new enzyme activities and specificities, including control over newly generated stereocenters. In the case of NAL, it is clear that the enzyme catalyzes a Bi-Uni ordered condensation reaction in which pyruvate binds first to the enzyme to form a catalytically important Schiff base. The identity of the residues required for catalysis of the condensation step and the nature of the transition state for this reaction, however, have been a matter of conjecture. In order to address, this we crystallized a Y137A variant of the E. coli NAL in the presence of Neu5Ac. The three-dimensional structure shows a full length sialic acid bound in the active site of subunits A, B, and D, while in subunit C, discontinuous electron density reveals the positions of enzyme-bound pyruvate and ManNAc. These 'snapshot' structures, representative of intermediates in the enzyme catalytic cycle, provided an ideal starting point for QM/MM modeling of the enzymic reaction of carbon-carbon bond formation. This revealed that Tyr137 acts as the proton donor to the aldehyde oxygen of ManNAc during the reaction, the activation barrier is dominated by carbon-carbon bond formation, and proton transfer from Tyr137 is required to obtain a stable Neu5Ac-Lys165 Schiff base complex. The results also suggested that a triad of residues, Tyr137, Ser47, and Tyr110 from a neighboring subunit, are required to correctly position Tyr137 for its function, and this was confirmed by site-directed mutagenesis. This understanding of the mechanism and geometry of the transition states along the C-C bond-forming pathway will allow further development of these enzymes for stereospecific synthesis of new enzyme products.

An unexpected elimination product leads to 4-alkyl-4-deoxy-4-epi-sialic acid derivatives

A useful, unexpected β,γ-unsaturated-α-keto ester (ethyl (E)-5-acetamido-3,4,5-trideoxy-6,7:8,9-di-O- isopropylidene-D-manno-non-3-en-2-ulosonate 5) was isolated in 91% yield following ozonolysis and chromatographic purification of its enoate ester precursor ethyl 5-acetamido-2,3,4,5-tetradeoxy-6,7:8,9-di-O-isopropylidene-2-methylene- 4-nitro-D-glycero-D-galacto-nononate (6). When the 4R enoate ester (ethyl 5-acetamido-2,3,4,5-tetradeoxy-6,7:8,9-di-O- isopropylidene-2-methylene-4-nitro-D-glycero-D-talo-nononate, 7) was subjected to the same conditions, enone 5 was a minor product (18%) while the major product did not eliminate HNO2 but instead cyclized to form a five-membered ring containing a hemiaminal linkage between C-2 and the amide nitrogen on C-5 (9, 70%). Conjugate addition to enone 5 opens up the potential to generate 4-substituted sialic acid derivatives, a general route to such compounds that has not been previously reported. In a preliminary investigation of such a route, diethylzinc and dimethylzinc were added to enone 5 resulting in generation of 4-alkyl-substituted cyclic hemiaminal structures 11 and 13, which could be deprotected to form 2,7-anhydrosialic acid analogues 14 and 15. These products could then be converted to peracetylated glycals 16 and 17, the 4-methyl-substituted compound 17 being finally deprotected to give a 4-methyl- substituted analogue of the glycal of sialic acid (5-acetamido-2,6-anhydro-3,4,5-trideoxy-4-methyl-D-glycero-D-talo-non-2-enonic acid 18).Key words: conjugate addition, dialkylzinc reagent, sialic acid, ozonolysis, inhibitors.

Synthesis and evaluation of C-9 modified N-acetylneuraminic acid derivatives as substrates for N-acetylneuraminic acid aldolase

Several C-9 modified N-acetylneuraminic acid derivatives have been synthesised and evaluated as substrates of N-acetylneuraminic acid aldolase. Simple C-9 acyl or ether modified derivatives of N-acetylneuraminic acid were found to be accepted as substrates by the enzyme, albeit being transformed more slowly than Neu5Ac itself. 1H NMR spectroscopy was used to evaluate the extent of the enzyme catalysed transformation of these compounds. Interestingly, the chain-extended Neu5Ac derivative 16 is not a substrate for N-acetylneuraminate lyase and behaves as an inhibitor of the enzyme.

Substrate-Assisted Catalysis in Sialic Acid Aldolase

Sialic acid aldolase catalyses the reversible aldol condensation of pyruvate and N-acetylmannosamine with an apparent lack of stereospecificity. Consistent with this, modeling of Schiff base and enamine intermediates in the active site of this enzyme yields two conformations, corresponding to si- and re-face attack in the aldol condensation reaction. The acceptor-aldehyde group is found on different sides of the enamine in the two conformations, but with the remainder of the substrate having very similar geometries in the protein. No histidine residue previously speculated to function as a general base in the mechanism is found near the enzyme active site. In the absence of functionally active groups in the active site, the carboxylate of the substrate is proposed to function as the general acid/base. Molecular orbital calculations indicate that the barrier to aldol cleavage via this mechanism in the gas phase of the related system, 4-hydroxy-2-methyiminopentanoic acid, is 74 kJ mol(-)(1).

Isolation and characterization of sialate lyase from pig kidney

Sialate lyase (sialate aldolase; systematic name N-acetylneuraminate pyruvate-lyase, EC 4.1.3.3) was isolated as soluble enzyme from pig kidney and purified 630-fold using a heating step, gel filtration, and chromatography on immobilized neuraminic acid beta-methyl glycoside in 14% yield to apparent homogeneity as tested by SDS-gel electrophoresis. The molecular mass is 58 kDa and the pH-optimum is at pH 7.2. Kinetic parameters were determined with N-acetyl-neuraminic acid as substrate: Km 3.7 mM and Vmax 37.1 mU. The lyase cleaves only free sialic acids with relative rates of 100% for N-acetylneuraminic acid, 55% for N-glycolylneuraminic acid and 32% for N-acetyl-9-O-acetylneuraminic acid, whereas N-acetyl-4-O-acetylneuraminic acid or 2-deoxy-2,3-didehydro-N-acetylneuraminic acid are not substrates. Enzyme activity was inhibited with p-chloromercuribenzoate, o-phenanthroline, cyanide, 5-diazonium-1-H-tetrazole, 5,5'-dithiobis(2-nitrobenzoic acid), diethylpyro-carbonate, and Rose Bengal in the presence of light and O2. Reduction with sodium borohydride in the presence of N-acetylneuraminic acid or pyruvate resulted in irreversible inhibition of enzyme activity. The inhibition experiments suggest the involvement of histidine, lysine and SH-residues in enzyme catalysis. Thus, this mammalian lyase most probably belongs to the Class I aldolases, and has properties similar to the same enzyme from Clostridium perfringens and is active with the alpha-form of N-acetylneuraminic acid.

The three-dimensional structure of N-acetylneuraminate lyase from Escherichia coli

BACKGROUND

N-acetylneuraminate lyase catalyzes the cleavage of N-acetylneuraminic acid (sialic acid) to form pyruvate and N-acetyl-D-mannosamine. The enzyme plays an important role in the regulation of sialic acid metabolism in bacteria. The reverse reaction can be exploited for the synthesis of sialic acid and some of its derivatives.

RESULTS

The structure of the enzyme from Escherichia coli has been determined to 2.2 A resolution by X-ray crystallography. The enzyme is shown to be a tetramer, in which each subunit consists of an alpha/beta-barrel domain followed by a carboxy-terminal extension of three alpha-helices.

CONCLUSIONS

The active site of the enzyme is tentatively identified as a pocket at the carboxy-terminal end of the eight-stranded beta-barrel. Lys165 lies within this pocket and is probably the reactive residue which forms a Schiff base intermediate with the substrate. The sequence of N-acetylneuraminate lyase has similarities to those of dihydrodipicolinate synthase and MosA (an enzyme implicated in rhizopine synthesis) suggesting that these last two enzymes share a similar structure to N-acetylneuraminate lyase.

Sialate O-acetylesterases: key enzymes in sialic acid catabolism

Sialate 9(4)-O-acetylesterases (EC 3.1.1.53) have been isolated from equine liver, bovine brain and influenza C virus. In this latter case, the esterase represents the receptor-destroying enzyme of the virus. The kinetic properties of these enzymes were determined with Neu5,9Ac2 and in part with 4-methylumbelliferyl acetate and Neu5,9Ac2-lactose. The Km values vary between 0.13 and 24 mM and the Vmax values from 0.55 to 11 U/mg of protein. The pH optima are in the range of 7.4-8.5, the molecular masses at 56,500 and 88,000 Da. In addition to a fast hydrolysis found for aromatic acetates, such as 4-methylumbelliferyl acetate or 4-nitrophenyl acetate, N-acetyl-9-O-acetylneuraminic acid is de-O-acetylated at the highest relative rate. Other substituents at the 9-position, such as lactoyl residues, or acetyl groups at other positions within the side chain are not hydrolyzed. Neu4,5Ac2, however, is a substrate for all 3 enzymes. The hydrolysis rates of this ester function, which renders sialic acids resistant to the action of sialidases, vary from 3 to 100% relative to Neu5,9Ac2. Whereas Neu5,9Ac2-lactose is hydrolyzed by the bovine and viral esterases, other O-acetylated sialic acids in glycoconjugates are only attacked by the enzyme from influenza C virus and not by that from bovine brain. The esterase from horse liver also releases 4-O-acetyl groups from equine submandibular gland mucin. By incubation with appropriate substrates and inhibition studies, carboxylesterase, amidase and choline esterase activities were excluded, as well as the cleavage of other acyls, e.g., butyryl groups. Thus, the enzymes investigated belong to the acetylesterases.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of N-acetylneuraminate lyase by N-acetyl-4-oxo-D-neuraminic acid

We show that the 4-oxo analogue of N-acetyl-D-neuraminic acid strongly inhibits N-acetylneuraminate lyase (NeuAc aldolase, EC 4.1.3.3) from Clostridum perfringens (Ki = 0.025 mM) and Escherichia coli (Ki = 0.15 mM). In each case the inhibition was competitive. N-Acetyl-D-neuraminic acid; N-Acetylneuraminate lyase; N-Acetyl-D-neuraminic acid analog; 5-Acetamido-3,5-dideoxy-beta-D-manno-non-2,4-diulosonic acid; 2-Deoxy-2,3-didehydro-N-acetyl-4-oxo-neuraminic acid; Competitive inhibitor.