(S,R,S)-AHPC

Molecular testing is rapidly becoming an integral component of global tuberculosis (TB) control. Uncommon mechanisms of resistance escape detection by these platforms and undermine our ability to contain outbreaks. This article is a systematic review of published articles that reported isoniazid (INH) resistance-conferring mutations between September 2013 and December 2019. The geneskatG,inhA, andfabG1, and the intergenic regionoxyR'-ahpCwere considered in this review. Fifty-two articles were included that described 9,306 clinical isolates (5,804 INH resistant [INHr] and 3,502 INH susceptible [INHs]) from 31 countries. The three most frequently mutated loci continue to be locus 315 ofkatG(katG315;n= 4,271), locus -15 ofinhA(inhA-15;n= 787), and locus -8 ofinhA(inhA-8; 106). However, the diagnostic value ofinhA-8 is far lower than previously thought, as it only appears in 25 (0.4%) of the INHrisolates lacking the first two mutations. I catalogued 45 new loci (29katG, nineinhA, and sevenahpC) associated with INH resistance and identified 59 loci (common to this and previous reviews) as a reliable basis for molecular diagnostics. Including all observed mutations provides a cumulative sensitivity of 85.6%. In 14.4% of resistant isolates, no mechanism of resistance was detected, making them likely to escape molecular detection, and in the case of INH monoresistance, likely to convert to multidrug-resistant TB (MDR-TB). Integrating the information cataloged in this study into current diagnostic tools is essential for combating the emergence of MDR-TB, and its exclusion can lead to an unintended selection against common mechanisms and to diversifying evolution. Observation of many low-frequency resistance-conferring mutations points to an advantage of whole-genome sequencing (WGS) for diagnostics. Finally, I provide five recommendations for future diagnostic platforms.

Peroxiredoxins make up a ubiquitous family of cysteine-dependent peroxidases that reduce hydroperoxide or peroxynitrite substrates through formation of a cysteine sulfenic acid (R-SOH) at the active site. In the 2-Cys peroxiredoxins, a second (resolving) cysteine reacts with the sulfenic acid to form a disulfide bond. For all peroxiredoxins, structural rearrangements in the vicinity of the active site cysteine(s) are necessary to allow disulfide bond formation and subsequent reductive recycling. In this study, we evaluated the rate constants for individual steps in the catalytic cycle of Salmonella typhimurium AhpC. Conserved Trp residues situated close to both peroxidatic and resolving cysteines in AhpC give rise to large changes in fluorescence during the catalytic cycle. For recycling, AhpF very efficiently reduces the AhpC disulfide, with a single discernible step and a rate constant of 2.3 × 10(7) M(-1) s(-1). Peroxide reduction was more complex and could be modeled as three steps, beginning with a reversible binding of H2O2 to the enzyme (k1 = 1.36 × 10(8) M(-1) s(-1), and k-1 = 53 s(-1)), followed by rapid sulfenic acid generation (620 s(-1)) and then rate-limiting disulfide bond formation (75 s(-1)). Using bulkier hydroperoxide substrates with higher Km values, we found that different efficiencies (kcat/Km) for turnover of AhpC with these substrates are primarily caused by their slower rates of binding. Our findings indicate that this bacterial peroxiredoxin exhibits rates for both reducing and oxidizing parts of the catalytic cycle that are among the fastest observed so far for this diverse family of enzymes.

An enzyme system protecting bacteria from oxidative stress includes the flavoprotein AhpF and the peroxiredoxin AhpC. The N-terminal domain of AhpF (NTD), with two fused thioredoxin (Trx) folds, belongs to the hyperthermophilic protein disulfide oxidoreductase family. The NTD is distinct in that it contains a redox active a fold with a CxxC sequence and a redox inactive b fold that has lost the CxxC motif. Here we characterize the stability, the (15)N backbone relaxation, and the hydrogen-deuterium exchange properties of reduced [NTD-(SH)(2)] and oxidized (NTD-S(2)) NTD from Salmonella typhimurium. While both NTD-(SH)(2) and NTD-S(2) exhibit similar equilibrium unfolding transitions and order parameters, R(ex) relaxation terms are quite distinct with considerably more intermediate time scale motions in NTD-S(2). Hydrogen exchange protection factors show that the slowly exchanging core corresponds to residues in the b fold in both NTD-(SH)(2) and NTD-S(2). Interestingly, folded-state dynamic fluctuations in the catalytic a fold are significantly increased for residues in NTD-S(2) compared to NTD-(SH)(2). Taken together, these data demonstrate that oxidation of the active site disulfide does not significantly increase stability but results in a dramatic increase in conformational heterogeneity in residues primarily in the redox active a fold. Differences in dynamics between the two folds of the NTD suggest that each evolved a specialized function which, in the a fold, couples redox state to internal motions which may enhance catalysis and specificity and, in the b fold, provides a redox insensitive stable core.