title: Enzymatic characterization of the target for isoniazid in Mycobacterium tuberculosis.
abstract: The inhA gene has been recently shown to encode a common protein target for isoniazid and ethionamide action in Mycobacterium tuberculosis. In this paper, we demonstrate that the M. tuberculosis InhA protein catalyzes the NADH-specific reduction of 2-trans-enoyl-ACP, essential for fatty acid elongation. This enzyme preferentially reduces long-chain substrates (12-24 carbons), consistent with its involvement in mycolic acid biosynthesis. Steady-state kinetic studies showed that the two substrates bind to InhA via a sequential kinetic mechanism, with the preferred ordered addition of NADH and the enoyl substrate. The chemical mechanism involves stereospecific hydride transfer of the 4S hydrogen of NADH to the C3 position of the 2-trans-enoyl substrate, followed by protonation at C2 of an enzyme-stabilized enolate intermediate. Kinetic and microcalorimetric analysis demonstrates that the binding of NADH to the S94A mutant InhA, known to confer resistance to both isoniazid and ethionamide, is altered. This difference can account for the isoniazid-resistance phenotype, with the formation of a binary InhA-NADH complex required for drug binding. Isoniazid binding to either the wild-type or S94A mutant InhA could not be detected by titration microcalorimetry, suggesting that this compound is a prodrug, which must be converted to its active form.
authors.affiliation: Department of Biochemistry, Howard Hughes Medical Institute, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
authors.forename: A
authors.initials: A
authors.lastname: Quémard
authors.forename: J C
authors.initials: JC
authors.lastname: Sacchettini
authors.forename: A
authors.initials: A
authors.lastname: Dessen
authors.forename: C
authors.initials: C
authors.lastname: Vilcheze
authors.forename: R
authors.initials: R
authors.lastname: Bittman
authors.forename: W R
authors.initials: WR
authors.lastname: Jacobs
authors.forename: J S
authors.initials: JS
authors.lastname: Blanchard
mesh_terms: D000214:Acyl Coenzyme A / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D001665:Binding Sites; D002151:Calorimetry; D003001:Cloning, Molecular; D015870:Gene Expression; D005798:Genes, Bacterial; D007538:Isoniazid / Q000494:pharmacology*; D007700:Kinetics; D013058:Mass Spectrometry; D008958:Models, Molecular; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics* / Q000378:metabolism; D009243:NAD / Q000378:metabolism; D010088:Oxidoreductases; D016133:Polymerase Chain Reaction; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D000214:Acyl Coenzyme A; D001426:Bacterial Proteins; D011994:Recombinant Proteins; D009243:NAD; D010088:Oxidoreductases; C085257:InhA protein, Mycobacterium; D007538:Isoniazid