Sandbox Reserved 321: Difference between revisions

The inhA enzyme <scene name='Sandbox_Reserved_321/Structural_progresion/1'>(go to original scene)</scene> of ''M. tuberculosis'' is a homotetramer composed of a repeating subunit comprised of a single domain with a [http://en.wikipedia.org/wiki/Rossmann_fold Rossmann Fold] in the core that provides a NADH binding site<ref name ="crystallographic studies"/>.  The single domain can be broken down into two substructures that are connected by short peptide loop<ref name ="making drugs for inhA"/><ref name ="crystallographic studies">PMID:17588773</ref>.  The overall structure exhibits α/β folding of a series of α strands flanking a central β sheet of multiple parallel β strands<ref name ="crystallographic studies"/>.

The reaction takes place as follows.  Initially NADH binds to the active site mediated by [http://en.wikipedia.org/wiki/Van_der_Waals_force van der Waal] interactions with phenylalanine 41 (F41) and interactions with lysine 165 <scene name='Sandbox_Reserved_321/Lys165/1'>(K165)</scene><ref name ="Roles of T158"/><ref name ="crystallographic studies"/>.  Binding of NADH causes a conformational change in the Aspartate 42 and Arginine 43 <scene name='Sandbox_Reserved_321/Asp_42_and_arg_43/1'>(E42 and R43)</scene> side chains and an over all conformational change in InhA<ref name ="crystallographic studies"/>.  In addition tyrosine 158 (Y158) plays an important role in aligning the carbonyl substrate, in fact; rotation about its Cα-Cβ bond by 60° brings it into a position where it can hydrogen bond to the carbonyl of the 2-trans enoyl-ACP and provide it with electrophilic stabilization<ref name ="Roles of T158"/>.  The substrate binds in a U-shaped conformation with its trans double bond adjacent to the nicotinamide ring of NAD+<ref name ="Fatty acyl in InhA"/>.  Inha then reduces the 2-trans double bond of the substrate by forming a enoyl intermediate through the transfer of a hydride ion from NADH to the third carbon of the substrate, followed by protonation of the second carbon<ref name ="crystallographic studies"/>.  The binding of both the substrate and the cofactor induces another conformational change in InhA that allows for the release of the meromycolic acid product<ref name ="crystallographic studies"/>.  The meromycolic acids undergo [http://en.wikipedia.org/wiki/Claisen_condensation claisen condensation] with a C26 fatty acid followed by reduction to a mature mycolic acid<ref name ="Fatty Acid Synthesis"/><ref name ="crystallographic studies"/>.

The primary target of the thioamide drugs PTH and ETH has been shown to be InhA in both gentic and molecular experiments<ref name ="mech of thioamide drug action"/>. Both PTH and ETH require activation by various cellular componets to form the NAD adduct that acts to inhibit InhA, and therefore connot be studied in vivo]<ref name ="mech of thioamide drug action"/>.  The ETH-NAD adduct (EAD), and the PTH-NAD adducts (PAD) have been found to occupy the same hydrophobic pocket of InhA as NADH<ref name ="mech of thioamide drug action"/>.

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