Crystal structure of the Mycobacterium tuberculosis enoyl-ACP reductase, InhA, in complex with NAD(+) and a C16 fatty acyl substrate

Enoyl-ACP reductases participate in fatty acid biosynthesis by utilizing NA DH to reduce the trans double bond between positions C2 and C3 of a fatty a cyl chain linked to the acyl carrier protein. The enoyl-ACP reductase from My. cobacterium tuberculosis, known as InhA, is a member of an unusual FAS- II system that prefers longer chain fatty acyl substrates for the purpose o f synthesizing mycolic acids, a major component of mycobacterial cell walls . The crystal structure of InhA in complex with NAD(+) and a C16 fatty acyl substrate, trans-2-hexadecenoyl-(N-acetylcysteamine)-thioester, reveals th at the substrate binds in a general "U-shaped" conformation, with the trans double bond positioned directly adjacent to the nicotinamide ring of NAD(). The side chain of Tyr(158) directly interacts with the thioester carbony l oxygen of the C16 fatty acyl substrate and therefore could help stabilize the enolate intermediate, proposed to form during substrate catalysis, Hyd rophobic residues, primarily from the substrate binding loop (residues 196- 219), engulf the fatty acyl chain portion of the substrate. The substrate b inding loop of InhA is longer than that of other enoyl-ACP reductases and c reates a deeper substrate binding crevice, consistent with the ability of I nhA to recognize longer chain fatty acyl substrates.