Substrate-mediated control of the conformation of an ancillary domain delivers a competent catalytic site for N-acetylneuraminic acid synthase

Kinetic characterization of two neuraminic acid synthases and evaluation of their application potential

Neuraminic acid synthases are an important yet underexplored group of enzymes. Thus, in this research, we performed a detailed kinetic and stability analysis and a comparison of previously known neuraminic acid synthase from Neisseria meningitidis, and a novel enzyme, PNH5, obtained from a metagenomic library. A systematic analysis revealed a high level of similarity of PNH5 to other known neuraminic acid synthases, except for its pH optimum, which was found to be at 5.5 for the novel enzyme. This is the first reported enzyme from this family that prefers an acidic pH value. The effect of different metal cofactors on enzyme activity, i.e. Co2+, Mn2+ and Mg2+, was studied systematically. The kinetics of neuraminic acid synthesis was completely elucidated, and an appropriate kinetic model was proposed. Enzyme stability study revealed that the purified enzyme exhibits changes in its structure during time as observed by differential light scattering, which cause a drop in its activity and protein concentration. The operational enzyme stability for the neuraminic acid synthase from N. meningitidis is excellent, where no activity drop was observed during the batch reactor experiments. In the case of PNH5, some activity drop was observed at higher concentration of substrates. The obtained results present a solid platform for the future application of these enzymes in the synthesis of sialic acids. KEY POINTS: • A novel neuraminic acid synthase was characterized. • The effect of cofactors on NeuS activity was elucidated. • Kinetic and stability characterization of two neuraminic acid synthases was performed.

Design, synthesis, and structural elucidation of novel NmeNANAS inhibitors for the treatment of meningococcal infection

Neisseria meningitidis is the primary cause of bacterial meningitis in many parts of the world, with considerable mortality rates among neonates and adults. In Saudi Arabia, serious outbreaks of N. meningitidis affecting several hundreds of pilgrims attending Hajj in Makkah were recorded in the 2000–2001 season. Evidence shows increased rates of bacterial resistance to penicillin and other antimicrobial agents that are used in the treatment of the meningococcal disease. The host’s immune system becomes unable to recognize the polysialic acid capsule of the resistant N. meningitidis that mimics the mammalian cell surface. The biosynthetic pathways of sialic acid (i.e., N-acetylneuraminic acid [NANA]) in bacteria, however, are somewhat different from those in mammals. The largest obstacle facing previously identified inhibitors of NANA synthase (NANAS) in N. meningitidis is that these inhibitors feature undesired chemical and pharmacological characteristics. To better comprehend the binding mechanism underlying these inhibitors at the catalytic site of NANAS, we performed molecular modeling studies to uncover essential structural aspects for the ultimate recognition at the catalytic site required for optimal inhibitory activity. Applying two virtual screening candidate molecules and one designed molecule showed promising structural scaffolds. Here, we report ethyl 3-benzoyl-2,7-dimethyl indolizine-1-carboxylate (INLZ) as a novel molecule with high energetic fitness scores at the catalytic site of the NmeNANAS enzyme. INLZ represents a promising scaffold for NmeNANAS enzyme inhibitors, with new prospects for further structural development and activity optimization.

Probing the determinants of phosphorylated sugar-substrate binding for human sialic acid synthase

N-acetylneuraminic acid (NeuNAc), the most naturally abundant sialic acid, is incorporated as the terminal residue of mammalian cell surface glycoconjugates and acts as a key facilitator of cellular recognition, adhesion and signalling. Several pathogenic bacteria similarly express NeuNAc on their cell surfaces, allowing evasion of their host's immune system. Prokaryotic NeuNAc biosynthesis proceeds via condensation of phosphoenolpyruvate (PEP) with N-acetylmannosamine (ManNAc), a reaction catalysed by the domain-swapped homodimeric enzyme, N-acetylneuraminic acid synthase (NeuNAcS). Conversely, the mammalian orthologue, N-acetylneuraminic acid 9-phosphate synthase (NeuNAc 9-PS) utilises the phosphorylated substrate N-acetylmannosamine 6-phosphate (ManNAc 6-P) in catalysis. Here we report an investigation into the determinants of substrate specificity of human NeuNAc 9-PS, using model-guided mutagenesis to delineate binding interactions with ManNAc 6-P. Modelling predicts the formation of a domain-swapped homodimer as observed for bacterial variants, which was supported by experimental small angle X-ray scattering. A number of conserved residues which may play key roles in the selection of ManNAc 6-P were identified and substituted for alanine to assess their function. Lys290 and Thr80 were identified as a putative phosphate binding pair, with the cationic lysine residue extending into the active site from the adjacent chain of the dimeric enzyme. Substitution of these residues results in a significant loss of activity and reduced affinity for ManNAc 6-P. These residues, along with the electropositive β2α2 loop, are likely to facilitate the PEP dependent binding and stabilisation of ManNAc 6-P. By utilising a phosphorylated sugar-substrate, the mammalian enzyme gains considerable catalytic affinity advantage over its bacterial counterpart.