A mechanism for isoniazid resistance

Resistance to isoniazid can occur by mutations that reduce the affinity of InhA enzyme for the NADH cofactor (Quemard et al 1995). Sequence analysis of inhA in resistant clinical isolates found that most of the amino acid substitutions are located within the enzyme's NADH-binding site (Fig. 4; L. Basso, personal communication 1998). These mutations cause lower affinity to NADH, indicated

FIG. 3. The proposed pathway for formation of the isonicotinic acyl-NADH inhibitor of InhA (Rozwarski et al 1998). According to this model, isonicotinic acyl anion or isonicotinic acyl radical covalently attaches to a form of NADH (NAD+ or NAD-) in the active site of InhA. The free radical pathway is the favoured mechanism because isoniazid-dependent inhibition of InhA occurs at a faster rate in the presence of NADH than with NAD+ (Johnsson et al 1995, Rozwarski et al 1998). Rozwarski proposed that Mn3+ ions induce formation of isonicotinic acyl radicals or isonicotinic anions and that KatG facilitates this activation by catalysing the oxidation of Mn2+ to Mn3+ (Rozwarski et al 1998, Zabinski & Blanchard 1997, Magliozzo & Marcinkeviciene 1997). Reprinted from Rozwarski et al (1998) with permission (Copyright 1998 American Association for the Advancement of Science).

FIG. 3. The proposed pathway for formation of the isonicotinic acyl-NADH inhibitor of InhA (Rozwarski et al 1998). According to this model, isonicotinic acyl anion or isonicotinic acyl radical covalently attaches to a form of NADH (NAD+ or NAD-) in the active site of InhA. The free radical pathway is the favoured mechanism because isoniazid-dependent inhibition of InhA occurs at a faster rate in the presence of NADH than with NAD+ (Johnsson et al 1995, Rozwarski et al 1998). Rozwarski proposed that Mn3+ ions induce formation of isonicotinic acyl radicals or isonicotinic anions and that KatG facilitates this activation by catalysing the oxidation of Mn2+ to Mn3+ (Rozwarski et al 1998, Zabinski & Blanchard 1997, Magliozzo & Marcinkeviciene 1997). Reprinted from Rozwarski et al (1998) with permission (Copyright 1998 American Association for the Advancement of Science).

FIG. 4. Positions of substitutions in InhA that are correlated with resistance in clinical tuberculosis isolates. Shown in lighter grey is the alpha-carbon backbone of a single subunit of InhA. The subunit is a single domain, in which the central core contains a Rossmann fold. Shown in darker grey is a CPK model of the bound isonicotinic-acyl-NADH inhibitor, sitting on top of the shelf created by the Rossmann fold. Shown in black are portions of the alpha-carbon backbone which correspond to the locations of clinical isolate mutations. Interestingly, all of these locations are near the isonicotinic-acyl-NADH inhibitor.

by an increased dissociation constant (at least a 10-fold increase) while other enzymatic parameters (e.g. Vmax) are not significantly altered. These substitutions may confer resistance by lowering the enzyme's affinity to isonicotinic acyl-NADH or by altering the kinetic mechanism of the enzyme (L. Basso, personal communication 1998; Rozwarski et al 1998).

FIG. 4. Positions of substitutions in InhA that are correlated with resistance in clinical tuberculosis isolates. Shown in lighter grey is the alpha-carbon backbone of a single subunit of InhA. The subunit is a single domain, in which the central core contains a Rossmann fold. Shown in darker grey is a CPK model of the bound isonicotinic-acyl-NADH inhibitor, sitting on top of the shelf created by the Rossmann fold. Shown in black are portions of the alpha-carbon backbone which correspond to the locations of clinical isolate mutations. Interestingly, all of these locations are near the isonicotinic-acyl-NADH inhibitor.

0 0

Post a comment