Structural characterization of sialic acid synthase by electrospray mass spectrometry--a tetrameric enzyme composed of dimeric dimers

Piperidinols that show anti-tubercular activity as inhibitors of arylamine N-acetyltransferase: an essential enzyme for mycobacterial survival inside macrophages

Latent M. tuberculosis infection presents one of the major obstacles in the global eradication of tuberculosis (TB). Cholesterol plays a critical role in the persistence of M. tuberculosis within the macrophage during latent infection. Catabolism of cholesterol contributes to the pool of propionyl-CoA, a precursor that is incorporated into cell-wall lipids. Arylamine N-acetyltransferase (NAT) is encoded within a gene cluster that is involved in the cholesterol sterol-ring degradation and is essential for intracellular survival. The ability of the NAT from M. tuberculosis (TBNAT) to utilise propionyl-CoA links it to the cholesterol-catabolism pathway. Deleting the nat gene or inhibiting the NAT enzyme prevents intracellular survival and results in depletion of cell-wall lipids. TBNAT has been investigated as a potential target for TB therapies. From a previous high-throughput screen, 3-benzoyl-4-phenyl-1-methylpiperidinol was identified as a selective inhibitor of prokaryotic NAT that exhibited antimycobacterial activity. The compound resulted in time-dependent irreversible inhibition of the NAT activity when tested against NAT from M. marinum (MMNAT). To further evaluate the antimycobacterial activity and the NAT inhibition of this compound, four piperidinol analogues were tested. All five compounds exert potent antimycobacterial activity against M. tuberculosis with MIC values of 2.3-16.9 µM. Treatment of the MMNAT enzyme with this set of inhibitors resulted in an irreversible time-dependent inhibition of NAT activity. Here we investigate the mechanism of NAT inhibition by studying protein-ligand interactions using mass spectrometry in combination with enzyme analysis and structure determination. We propose a covalent mechanism of NAT inhibition that involves the formation of a reactive intermediate and selective cysteine residue modification. These piperidinols present a unique class of antimycobacterial compounds that have a novel mode of action different from known anti-tubercular drugs.

Improvement of the expression and purification of Mycobacterium tuberculosis arylamine N-acetyltransferase (TBNAT) a potential target for novel anti-tubercular agents

Arylamine N-acetyltransferase from Mycobacterium tuberculosis (TBNAT) has been proposed as a drug target for latent tuberculosis treatment. The enzyme is essential for the survival of the mycobacterium in macrophages. However, TBNAT has been very difficult to generate as a soluble protein. In this work we describe production of soluble recombinant TBNAT at a reasonable yield achieved by subcloning the tbnat gene with a purification His-tag into the pVLT31 plasmid, and subsequent optimisation of the induction conditions. The expression system results in soluble protein optimised upon extended (60 h) low level isopropyl β-D-1-thiogalactopyranoside level induction (100 μM) at a temperature of 15 °C. The level of TBNAT expression obtained in E. coli has been significantly improved from ∼2 mg to a final yield of up to 16 mg per litre of culture at a purity level suitable for structural studies. The molecular mass of 31310 Da was confirmed using mass spectroscopy and the oligomerisation state was determined. The stability of TBNAT in different buffer systems was investigated by thermal shift assays and sufficient protein is now available for the screening of chemical libraries for inhibitors.

Evolution of an antifreeze protein by neofunctionalization under escape from adaptive conflict

The evolutionary model escape from adaptive conflict (EAC) posits that adaptive conflict between the old and an emerging new function within a single gene could drive the fixation of gene duplication, where each duplicate can freely optimize one of the functions. Although EAC has been suggested as a common process in functional evolution, definitive cases of neofunctionalization under EAC are lacking, and the molecular mechanisms leading to functional innovation are not well-understood. We report here clear experimental evidence for EAC-driven evolution of type III antifreeze protein gene from an old sialic acid synthase (SAS) gene in an Antarctic zoarcid fish. We found that an SAS gene, having both sialic acid synthase and rudimentary ice-binding activities, became duplicated. In one duplicate, the N-terminal SAS domain was deleted and replaced with a nascent signal peptide, removing pleiotropic structural conflict between SAS and ice-binding functions and allowing rapid optimization of the C-terminal domain to become a secreted protein capable of noncolligative freezing-point depression. This study reveals how minor functionalities in an old gene can be transformed into a distinct survival protein and provides insights into how gene duplicates facing presumed identical selection and mutation pressures at birth could take divergent evolutionary paths.

Cloning, expression, and characterization of sialic acid synthases

The most commonly occurring sialic acid, N-acetylneuraminic acid, is the repeating unit in polysialic acid chain of human neuronal cell adhesion molecule as well as in capsular polysialic acid of neuroinvasive bacteria, Escherichia coli K1 and Neisseria meningitidis. Sialic acid synthesis and polymerization occur in slightly different pathways in animals and bacteria. N-Acetylneuraminic acid (NeuNAc) is synthesized by the condensation of phosphoenolpyruvate and N-acetylmannosamine by NeuNAc synthase in bacteria. The mammalian homologue N-acetylneuraminic acid-9-phosphate (NeuNAc-9-P) synthase uses N-acetylmannosamine-6-phosphate in the condensation reaction to produce NeuNAc-9-P. Both subfamilies of sialic acid synthases possess N-terminal triosephosphate isomerase barrel domain and C-terminal antifreeze protein domain. We report cloning of the genes, expression, purification, and characterization of human NeuNAc-9-P synthase and N. meningitidis NeuNAc synthase. Stability of the purified enzymes and effects of pH and temperature on their activities were evaluated. Enzyme kinetics and preliminary mutagenesis experiments reveal the importance of C-terminal antifreeze protein domain and a conserved cysteine residue for the enzyme activities.