Analysis of heme structural heterogeneity in Mycobacterium tuberculosis catalase-peroxidase (KatG)

Stealing survival: Iron acquisition strategies of Mycobacteriumtuberculosis

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), faces iron scarcity within the host due to immune defenses. This review explores the importance of iron for Mtb and its strategies to overcome iron restriction. We discuss how the host limits iron as an innate immune response and how Mtb utilizes various iron acquisition systems, particularly the siderophore-mediated pathway. The review illustrates the structure and biosynthesis of mycobactin, a key siderophore in Mtb, and the regulation of its production. We explore the potential of targeting siderophore biosynthesis and uptake as a novel therapeutic approach for TB. Finally, we summarize current knowledge on Mtb's iron acquisition and highlight promising directions for future research to exploit this pathway for developing new TB interventions.

The Iron Response of Mycobacterium tuberculosis and Its Implications for Tuberculosis Pathogenesis and Novel Therapeutics

Most pathogenic bacteria require iron for growth. However, this metal is not freely available in the mammalian host. Due to its poor solubility and propensity to catalyze the generation of reactive oxygen species, host iron is kept in solution bound to specialized iron binding proteins. Access to iron is an important factor in the outcome of bacterial infections; iron limitation frequently induces virulence and drives pathogenic interactions with host cells. Here, we review the response of Mycobacterium tuberculosis to changes in iron availability, the relevance of this response to TB pathogenesis, and its potential for the design of new therapeutic interventions.

Genome sequencing of the vermicompost strain Stenotrophomonas maltophilia UENF-4GII and population structure analysis of the S. maltophilia Sm3 genogroup

The Stenotrophomonas maltophilia complex (Smc) is a cosmopolitan bacterial group that has been proposed an emergent multidrug-resistant pathogen. Taxonomic studies support the genomic heterogeneity of Smc, which comprises genogroups exhibiting a range of phenotypically distinct strains from different sources. Here, we report the genome sequencing and in-depth analysis of S. maltophilia UENF-4GII, isolated from vermicompost. This genome harbors a unique region encoding a penicillin-binding protein (pbpX) that was carried by a transposon, as well as horizontally-transferred genomic islands involved in anti-phage defense via DNA modification, and pili glycosylation. We also analyzed all available Smc genomes to investigate genes associated with resistance and virulence, niche occupation, and population structure. S. maltophilia UENF-4GII belongs to genogroup 3 (Sm3), which comprises three phylogenetic clusters (PC). Pan-GWAS analysis uncovered 471 environment-associated and 791 PC-associated genes, including antimicrobial resistance (e.g. blaL1 and blaR1) and virulence determinants (e.g. treS and katG) that provide insights on the resistance and virulence potential of Sm3 strains. Together, the results presented here provide the grounds for more detailed clinical and ecological investigations of S. maltophilia.

The Capacity of Mycobacterium tuberculosis To Survive Iron Starvation Might Enable It To Persist in Iron-Deprived Microenvironments of Human Granulomas

This study was conducted to investigate the role of iron deprivation in the persistence of Mycobacterium tuberculosis. We present evidence of iron restriction in human necrotic granulomas and demonstrate that under iron starvation M. tuberculosis persists, refractive to antibiotics and capable of restarting replication when iron is made available. Transcriptomics and metabolomic analyses indicated that the persistence of M. tuberculosis under iron starvation is dependent on strict control of endogenous Fe utilization and is associated with upregulation of pathogenicity and intrinsic antibiotic resistance determinants. M. tuberculosis mutants compromised in their ability to survive Fe starvation were identified. The findings of this study advance the understanding of the physiological settings that may underpin the chronicity of human tuberculosis (TB) and are relevant to the design of effective antitubercular therapies.

One-third of the world population may harbor persistent M. tuberculosis, causing an asymptomatic infection that is refractory to treatment and can reactivate to become potentially lethal tuberculosis disease. However, little is known about the factors that trigger and maintain M. tuberculosis persistence in infected individuals. Iron is an essential nutrient for M. tuberculosis growth. In this study, we show, first, that in human granulomas the immune defense creates microenvironments in which M. tuberculosis likely experiences drastic Fe deprivation and, second, that Fe-starved M. tuberculosis is capable of long-term persistence without growth. Together, these observations suggest that Fe deprivation in the lung might trigger a state of persistence in M. tuberculosis and promote chronic TB. We also identified vulnerabilities of iron-restricted persistent M. tuberculosis, which can be exploited for the design of new antitubercular therapies.

The ins and outs of metal homeostasis by the root nodule actinobacterium Frankia

BACKGROUND

Frankia are actinobacteria that form a symbiotic nitrogen-fixing association with actinorhizal plants, and play a significant role in actinorhizal plant colonization of metal contaminated areas. Many Frankia strains are known to be resistant to several toxic metals and metalloids including Pb(2+), Al(+3), SeO2, Cu(2+), AsO4, and Zn(2+). With the availability of eight Frankia genome databases, comparative genomics approaches employing phylogeny, amino acid composition analysis, and synteny were used to identify metal homeostasis mechanisms in eight Frankia strains. Characterized genes from the literature and a meta-analysis of 18 heavy metal gene microarray studies were used for comparison.

RESULTS

Unlike most bacteria, Frankia utilize all of the essential trace elements (Ni, Co, Cu, Se, Mo, B, Zn, Fe, and Mn) and have a comparatively high percentage of metalloproteins, particularly in the more metal resistant strains. Cation diffusion facilitators, being one of the few known metal resistance mechanisms found in the Frankia genomes, were strong candidates for general divalent metal resistance in all of the Frankia strains. Gene duplication and amino acid substitutions that enhanced the metal affinity of CopA and CopCD proteins may be responsible for the copper resistance found in some Frankia strains. CopA and a new potential metal transporter, DUF347, may be involved in the particularly high lead tolerance in Frankia. Selenite resistance involved an alternate sulfur importer (CysPUWA) that prevents sulfur starvation, and reductases to produce elemental selenium. The pattern of arsenate, but not arsenite, resistance was achieved by Frankia using the novel arsenite exporter (AqpS) previously identified in the nitrogen-fixing plant symbiont Sinorhizobium meliloti. Based on the presence of multiple tellurite resistance factors, a new metal resistance (tellurite) was identified and confirmed in Frankia.

CONCLUSIONS

Each strain had a unique combination of metal import, binding, modification, and export genes that explain differences in patterns of metal resistance between strains. Frankia has achieved similar levels of metal and metalloid resistance as bacteria from highly metal-contaminated sites. From a bioremediation standpoint, it is important to understand mechanisms that allow the endosymbiont to survive and infect actinorhizal plants in metal contaminated soils.

Catalase in peroxidase clothing: Interdependent cooperation of two cofactors in the catalytic versatility of KatG

Catalase-peroxidase (KatG) is found in eubacteria, archaea, and lower eukaryotae. The enzyme from Mycobacterium tuberculosis has received the greatest attention because of its role in activation of the antitubercular pro-drug isoniazid, and the high frequency with which drug resistance stems from mutations to the katG gene. Generally, the catalase activity of KatGs is striking. It rivals that of typical catalases, enzymes with which KatGs share no structural similarity. Instead, catalatic turnover is accomplished with an active site that bears a strong resemblance to a typical peroxidase (e.g., cytochrome c peroxidase). Yet, KatG is the only member of its superfamily with such capability. It does so using two mutually dependent cofactors: a heme and an entirely unique Met-Tyr-Trp (MYW) covalent adduct. Heme is required to generate the MYW cofactor. The MYW cofactor allows KatG to leverage heme intermediates toward a unique mechanism for H2O2 oxidation. This review evaluates the range of intermediates identified and their connection to the diverse catalytic processes KatG facilitates, including mechanisms of isoniazid activation.

Tyrosyl radicals in dehaloperoxidase: how nature deals with evolving an oxygen-binding globin to a biologically relevant peroxidase

Dehaloperoxidase (DHP) from Amphitrite ornata, having been shown to catalyze the hydrogen peroxide-dependent oxidation of trihalophenols to dihaloquinones, is the first oxygen binding globin that possesses a biologically relevant peroxidase activity. The catalytically competent species in DHP appears to be Compound ES, a reactive intermediate that contains both a ferryl heme and a tyrosyl radical. By simulating the EPR spectra of DHP activated by H2O2, Thompson et al. (Thompson, M. K., Franzen, S., Ghiladi, R. A., Reeder, B. J., and Svistunenko, D. A. (2010) J. Am. Chem. Soc. 132, 17501-17510) proposed that two different radicals, depending on the pH, are formed, one located on either Tyr-34 or Tyr-28 and the other on Tyr-38. To provide additional support for these simulation-based assignments and to deduce the role(s) that tyrosyl radicals play in DHP, stopped-flow UV-visible and rapid-freeze-quench EPR spectroscopic methods were employed to study radical formation in DHP when three tyrosine residues, Tyr-28, Tyr-34, and Tyr-38, were replaced either individually or in combination with phenylalanines. The results indicate that radicals form on all three tyrosines in DHP. Evidence for the formation of DHP Compound I in several tyrosine mutants was obtained. Variants that formed Compound I showed an increase in the catalytic rate for substrate oxidation but also an increase in heme bleaching, suggesting that the tyrosines are necessary for protecting the enzyme from oxidizing itself. This protective role of tyrosines is likely an evolutionary adaptation allowing DHP to avoid self-inflicted damage in the oxidative environment.

Distinct structural and redox properties of the heme active site in bacterial dye decolorizing peroxidase-type peroxidases from two subfamilies: resonance Raman and electrochemical study

Spectroscopic data of dye decolorizing peroxidases (DyPs) from Bacillus subtilis (BsDyP), an A subfamily member, and Pseudomonas putida (PpDyP), a B subfamily enzyme, reveal distinct heme coordination patterns of the respective active sites. In solution, both enzymes show a heterogeneous spin population, with the six-coordinated low-spin state being the most populated in the former and the five-coordinated quantum mechanically mixed-spin state in the latter. We ascribe the poor catalytic activity of BsDyP to the presence of a catalytically incompetent six-coordinated low-spin population. The spin populations of the two DyPs are sensitively dependent on the pH, temperature, and physical, i.e., solution versus crystal versus immobilized, state of the enzymes. We observe a redox potential for the Fe(2+)/Fe(3+) couple in BsDyP (-40 mV) at pH 7.6 substantially more positive than those reported for the majority of other peroxidases, including PpDyP (-260 mV). Furthermore, we evaluate the potential of the studied enzymes for biotechnological applications on the basis of electrochemical and spectroelectrochemical data.

Integral role of the I'-helix in the function of the "inactive" C-terminal domain of catalase-peroxidase (KatG)

Catalase-peroxidases (KatGs) have two peroxidase-like domains. The N-terminal domain contains the heme-dependent, bifunctional active site. Though the C-terminal domain lacks the ability to bind heme or directly catalyze any reaction, it has been proposed to serve as a platform to direct the folding of the N-terminal domain. Toward such a purpose, its I'-helix is highly conserved and appears at the interface between the two domains. Single and multiple substitution variants targeting highly conserved residues of the I'-helix were generated for intact KatG as well as the stand-alone C-terminal domain (KatG(C)). Single variants of intact KatG produced only subtle variations in spectroscopic and catalytic properties of the enzyme. However, the double and quadruple variants showed substantial increases in hexa-coordinate low-spin heme and diminished enzyme activity, similar to that observed for the N-terminal domain on its own (KatG(N)). The analogous variants of KatG(C) showed a much more profound loss of function as evaluated by their ability to return KatG(N) to its active conformation. All of the single variants showed a substantial decrease in the rate and extent of KatG(N) reactivation, but with two substitutions, KatG(C) completely lost its capacity for the reactivation of KatG(N). These results suggest that the I'-helix is central to direct structural adjustments in the adjacent N-terminal domain and supports the hypothesis that the C-terminal domain serves as a platform to direct N-terminal domain conformation and bifunctionality.

Specific function of the Met-Tyr-Trp adduct radical and residues Arg-418 and Asp-137 in the atypical catalase reaction of catalase-peroxidase KatG

Catalase activity of the dual-function heme enzyme catalase-peroxidase (KatG) depends on several structural elements, including a unique adduct formed from covalently linked side chains of three conserved amino acids (Met-255, Tyr-229, and Trp-107, Mycobacterium tuberculosis KatG numbering) (MYW). Mutagenesis, electron paramagnetic resonance, and optical stopped-flow experiments, along with calculations using density functional theory (DFT) methods revealed the basis of the requirement for a radical on the MYW-adduct, for oxyferrous heme, and for conserved residues Arg-418 and Asp-137 in the rapid catalase reaction. The participation of an oxyferrous heme intermediate (dioxyheme) throughout the pH range of catalase activity is suggested from our finding that carbon monoxide inhibits the activity at both acidic and alkaline pH. In the presence of H(2)O(2), the MYW-adduct radical is formed normally in KatG[D137S] but this mutant is defective in forming dioxyheme and lacks catalase activity. KatG[R418L] is also catalase deficient but exhibits normal formation of the adduct radical and dioxyheme. Both mutants exhibit a coincidence between MYW-adduct radical persistence and H(2)O(2) consumption as a function of time, and enhanced subunit oligomerization during turnover, suggesting that the two mutations disrupting catalase turnover allow increased migration of the MYW-adduct radical to protein surface residues. DFT calculations showed that an interaction between the side chain of residue Arg-418 and Tyr-229 in the MYW-adduct radical favors reaction of the radical with the adjacent dioxyheme intermediate present throughout turnover in WT KatG. Release of molecular oxygen and regeneration of resting enzyme are thereby catalyzed in the last step of a proposed catalase reaction.

Ferric haem forms of Mycobacterium tuberculosis catalase-peroxidase probed by EPR spectroscopy: Their stability and interplay with pH

Low temperature EPR spectroscopy was used to characterise Mycobacterium tuberculosis catalase-peroxidase in its resting ferric haem state. Several high spin ferric haem forms and no low spin forms were found in the enzyme samples frozen in methanol on dry ice. The EPR spectra depended not only on the pH but also on the buffer type. As a general trend, the higher the pH, the greater the 'rhombic' fraction of the high spin ferric haem that was observed. The rhombic form was characterised by well separated two lines in the g = 6 region whereas in the 'axial' form the two lines overlap. This pH dependence of the equilibrium of axial and rhombic ferric haem forms is also seen in rapidly freeze-quenched samples. Different high spin ferric haem forms were monitored during a 3 week storage of the enzyme at 4 °C. For some forms, extremal dependences, i.e. those progressing via maxima or minima over storage time, were found. This indicates that the mechanism of the time-dependent transition from one high spin ferric haem form to another must be more complex than a simple single site oxidation.

Spectroscopic and mechanistic investigations of dehaloperoxidase B from Amphitrite ornata

Dehaloperoxidase (DHP) from the terebellid polychaete Amphitrite ornata is a bifunctional enzyme that possesses both hemoglobin and peroxidase activities. Of the two DHP isoenzymes identified to date, much of the recent focus has been on DHP A, whereas very little is known pertaining to the activity, substrate specificity, mechanism of function, or spectroscopic properties of DHP B. Herein, we report the recombinant expression and purification of DHP B, as well as the details of our investigations into its catalytic cycle using biochemical assays, stopped-flow UV-visible, resonance Raman, and rapid freeze-quench electron paramagnetic resonance spectroscopies, and spectroelectrochemistry. Our experimental design reveals mechanistic insights and kinetic descriptions of the dehaloperoxidase mechanism which have not been previously reported for isoenzyme A. Namely, we demonstrate a novel reaction pathway in which the products of the oxidative dehalogenation of trihalophenols (dihaloquinones) are themselves capable of inducing formation of oxyferrous DHP B, and an updated catalytic cycle for DHP is proposed. We further demonstrate that, unlike the traditional monofunctional peroxidases, the oxyferrous state in DHP is a peroxidase-competent starting species, which suggests that the ferric oxidation state may not be an obligatory starting point for the enzyme. The data presented herein provide a link between the peroxidase and oxygen transport activities which furthers our understanding of how this bifunctional enzyme is able to unite its two inherent functions in one system.

Nuclear receptors homo sapiens Rev-erbbeta and Drosophila melanogaster E75 are thiolate-ligated heme proteins which undergo redox-mediated ligand switching and bind CO and NO

Nuclear receptors E75, which regulates development in Drosophila melanogaster, and Rev-erbbeta, which regulates circadian rhythm in humans, bind heme within their ligand binding domains (LBD). The heme-bound ligand binding domains of E75 and Rev-erbbeta were studied using electronic absorption, MCD, resonance Raman, and EPR spectroscopies. Both proteins undergo redox-dependent ligand switching and CO- and NO-induced ligand displacement. In the Fe(III) oxidation state, the nuclear receptor hemes are low spin and 6-coordinate with cysteine(thiolate) as one of the two axial heme ligands. The sixth ligand is a neutral donor, presumably histidine. When the heme is reduced to the Fe(II) oxidation state, the cysteine(thiolate) is replaced by a different neutral donor ligand, whose identity is not known. CO binds to the Fe(II) heme in both E75(LBD) and Rev-erbbeta(LBD) opposite a sixth neutral ligand, plausibly the same histidine that served as the sixth ligand in the Fe(III) state. NO binds to the heme of both proteins; however, the NO-heme is 5-coordinate in E75 and 6-coordinate in Rev-erbbeta. These nuclear receptors exhibit coordination characteristics that are similar to other known redox and gas sensors, suggesting that E75 and Rev-erbbeta may function in heme-, redox-, or gas-regulated control of cellular function.

Antibiotic resistance in Mycobacterium tuberculosis: peroxidase intermediate bypass causes poor isoniazid activation by the S315G mutant of M. tuberculosis catalase-peroxidase (KatG)

KatG (catalase-peroxidase) in Mycobacterium tuberculosis is responsible for activation of isoniazid (INH), a pro-drug used to treat tuberculosis infections. Resistance to INH is a global health problem most often associated with mutations in the katG gene. The origin of INH resistance caused by the KatG[S315G] mutant enzyme is examined here. Overexpressed KatG[S315G] was characterized by optical, EPR, and resonance Raman spectroscopy and by studies of the INH activation mechanism in vitro. Catalase activity and peroxidase activity with artificial substrates were moderately reduced (50 and 35%, respectively), whereas the rates of formation of oxyferryl heme:porphyrin pi-cation radical and the decay of heme intermediates were approximately 2-fold faster in KatG[S315G] compared with WT enzyme. The INH binding affinity for the resting enzyme was unchanged, whereas INH activation, measured by the rate of formation of an acyl-nicotinamide adenine dinucleotide adduct considered to be a bactericidal molecule, was reduced by 30% compared with WT KatG. INH resistance is suggested to arise from a redirection of catalytic intermediates into nonproductive reactions that interfere with oxidation of INH. In the resting mutant enzyme, a rapid evolution of 5-c heme to 6-c species occurred in contrast with the behavior of WT KatG and KatG[S315T] and consistent with greater flexibility at the heme edge in the absence of the hydroxyl of residue 315. Insights into the effects of mutations at residue 315 on enzyme structure, peroxidation kinetics, and specific interactions with INH are presented.

Role of the oxyferrous heme intermediate and distal side adduct radical in the catalase activity of Mycobacterium tuberculosis KatG revealed by the W107F mutant

Catalase-peroxidase (KatG) is essential in Mycobacterium tuberculosis for oxidative stress management and activation of the antitubercular pro-drug isoniazid. The role of a unique distal side adduct found in KatG enzymes, involving linked side chains of residues Met255, Tyr229, and Trp107 (MYW), in the unusual catalase activity of KatG is addressed here and in our companion paper (Suarez, J., Ranguelova, K., Jarzecki, A. A., Manzerova, J., Krymov, V., Zhao, X., Yu, S., Metlitsky, L., Gerfen, G. J., and Magliozzo, R. S. (2009) J. Biol. Chem. 284, in press). The KatG[W107F] mutant exhibited severely reduced catalase activity yet normal peroxidase activity, and as isolated contains more abundant 6-coordinate heme in high spin and low spin forms compared with the wild-type enzyme. Most interestingly, oxyferrous heme is also found in the purified enzyme. Oxyferrous KatG[W107F] was prepared by photolysis in air of the carbonyl enzyme or was generated using hydrogen peroxide decayed with a t1/2 of 2 days compared with 6 min for wild-type protein. The stability of oxyenyzme was modestly enhanced in KatG[Y229F] but was not affected in KatG[M255A]. Optical stopped-flow experiments showed rapid formation of Compound I in KatG[W107F] and facile formation of oxyferrous heme in the presence of micromolar hydrogen peroxide. An analysis of the relationships between catalase activity, stability of oxyferrous enzyme, and a proposed MYW adduct radical is presented. The loss of catalase function is assigned to the loss of the MYW adduct radical and structural changes that lead to greatly enhanced stability of oxyenzyme, an intermediate of the catalase cycle of native enzyme.

Two [Fe(IV)=O Trp*] intermediates in M. tuberculosis catalase-peroxidase discriminated by multifrequency (9-285 GHz) EPR spectroscopy: reactivity toward isoniazid

We have characterized the intermediates formed in the peroxidase cycle of the multifunctional heme-containing enzyme KatG of M. tuberculosis. Selected Trp variants from the heme proximal (W321F) and distal (W107F and W91F) sides were analyzed together with the wild-type enzyme with regard to the reaction with peroxyacetic acid and hydrogen peroxide (in the catalase-inactive W107F). The 9 GHz EPR spectrum of the enzyme upon reaction with peroxyacetic acid showed the contribution of three protein-based radical species, two Trp* and a Tyr*, which could be discerned using a combined approach of multifrequency Electron Paramagnetic Resonance (EPR) spectroscopy with selective deuterium labeling of tryptophan and tyrosine residues and site-directed mutagenesis. Trp321, a residue in H-bonding interactions with the iron through Asp381 and the heme axial ligand His270, was identified as one of the radical sites. The 9 GHz EPR signal of the Trp321 radical species was consistent with an exchange-coupled species similar to the oxoferryl-Trp radical intermediate in cytochrome c peroxidase. On the basis of the possibility of distinguishing among the different radical intermediates of the peroxidase cycle in M. tuberculosis KatG (MtKatG), we used EPR spectroscopy to monitor the reactivity of the enzyme and its W321F variant with isoniazid, the front-line drug used in the treatment of tuberculosis. The EPR experiments on the W321F variant preincubated with isoniazid allowed us to detect the short-lived [Fe(IV)=O Por*+] intermediate. Our results showed that neither the [Fe(IV)=O Por*+] nor the [Fe(IV)=O Trp321*+] intermediates were the reactive species with isoniazid. Accordingly, the subsequent intermediate (most probably the other Trp*) is proposed to be the oxidizing species. Our findings demonstrate that the protein-based radicals formed as alternative intermediates to the [Fe(IV)=O Por*+] can play the role of cofactors for substrate oxidation in the peroxidase cyle of KatGs.

Redox intermediates in the catalase cycle of catalase-peroxidases from Synechocystis PCC 6803, Burkholderia pseudomallei, and Mycobacterium tuberculosis

Monofunctional catalases (EC 1.11.1.6) and catalase-peroxidases (KatGs, EC 1.11.1.7) have neither sequence nor structural homology, but both catalyze the dismutation of hydrogen peroxide (2H2O2 --> 2H2O + O2). In monofunctional catalases, the catalatic mechanism is well-characterized with conventional compound I [oxoiron(IV) porphyrin pi-cation radical intermediate] being responsible for hydrogen peroxide oxidation. The reaction pathway in KatGs is not as clearly defined, and a comprehensive rapid kinetic and spectral analysis of the reactions of KatGs from three different sources (Synechocystis PCC 6803, Burkholderia pseudomallei, and Mycobacterium tuberculosis) with peroxoacetic acid and hydrogen peroxide has focused on the pathway. Independent of KatG, but dependent on pH, two low-spin forms dominated in the catalase cycle with absorbance maxima at 415, 545, and 580 nm at low pH and 418 and 520 nm at high pH. By contrast, oxidation of KatGs with peroxoacetic acid resulted in intermediates with different spectral features that also differed among the three KatGs. Following the rate of H2O2 degradation by stopped-flow allowed the linking of reaction intermediate species with substrate availability to confirm which species were actually present during the catalase cycle. Possible reaction intermediates involved in H2O2 dismutation by KatG are discussed.

Characterization of the Binding of Isoniazid and Analogues to Mycobacterium tuberculosis Catalase-Peroxidase

The first-line antituberculosis drug isonicotinic hydrazide (INH) is a prodrug whose bactericidal function requires activation by Mycobacterium tuberculosis catalase-peroxidase (KatG) to produce an acyl-NAD adduct. Peroxidation of INH is considered a required catalytic process for drug action. The binding of INH and a series of hydrazide analogues to resting KatG was examined using optical and calorimetric techniques to provide thermodynamic parameters, binding stoichiometries, and kinetic constants (on and off rates). This work revealed high-affinity binding of these substrates to a small fraction of ferric enzyme in a six-coordinate heme iron form, a species most likely containing a weakly bound water molecule, which accumulates during storage of the enzyme. The binding of hydrazides is associated with a large enthalpy loss (>100 kcal/mol); dissociation constants are in the range of 0.05-1.6 microM, and optical stopped-flow measurements demonstrated kon values in the range of 0.5-27 x 10(3) M-1 s-1 with very small koff rates. Binding parameters did not depend on pH in the range 5-8. High-affinity binding of INH is disrupted in two mutant enzymes bearing replacements of key distal side residues, KatG[W107F] and KatG[Y229F]. The rates of reduction of KatG Compound I by hydrazides parallel the on rates for association with the resting enzyme. In a KatG-mediated biomimetic activation assay, only isoniazid generated in good yield the acyl-NAD adduct which is considered a key molecule in INH action, providing a better understanding of the action mechanism of INH.

Axial coordination of heme in ferric CcmE chaperone characterized by EPR spectroscopy

In Escherichia coli cytochrome c maturation requires a set of eight proteins including the heme chaperone CcmE, which binds heme transiently, yet covalently. Several variants of CcmE were purified and analyzed by continuous-wave electron paramagnetic resonance, electron nuclear double resonance, and hyperfine sublevel correlation spectroscopy to investigate the heme axial coordination. Results reveal the presence of a number of coordination environments, two high-spin heme centers with different rhombicities, and at least one low-spin heme center. The low-spin species was shown to be an artifact induced by the presence of available histidines in the vicinity of the iron. Both of the high-spin forms are five-coordinated, and comparison of the spectra of the wild-type CcmE with those of the mutant CcmE(Y134H) proves that the higher-rhombicity form is coordinated by Tyr134. The low-rhombicity (axial) form does not have a histidine residue or a water molecule as an axial ligand. However, we identified exchangeable protons coupled to the iron ion. We propose that the axial form can be coordinated by a carboxyl group of an acidic residue in the flexible domain of the protein. The two species would represent two different conformations of the flexible alpha-helix domain surrounding the heme. This conformational flexibility confers CcmE special dynamic properties that are certainly important for its function.

Modification of the active site of Mycobacterium tuberculosis KatG after disruption of the Met-Tyr-Trp cross-linked adduct

Mycobacterium tuberculosis catalase-peroxidase (Mtb KatG) is a bifunctional enzyme that possesses both catalase and peroxidase activities and is responsible for the activation of the antituberculosis drug isoniazid. Mtb KatG contains an unusual adduct in its distal heme-pocket that consists of the covalently linked Trp107, Tyr229, and Met255. The KatG(Y229F) mutant lacks this adduct and has decreased steady-state catalase activity and enhanced peroxidase activity. In order to test a potential structural role of the adduct that supports catalase activity, we have used resonance Raman spectroscopy to probe the local heme environment of KatG(Y229F). In comparison to wild-type KatG, resting KatG(Y229F) contains a significant amount of 6-coordinate, low-spin heme and a more planar heme. Resonance Raman spectroscopy of the ferrous-CO complex of KatG(Y229F) suggest a non-linear Fe-CO binding geometry that is less tilted than in wild-type KatG. These data provide evidence that the Met-Tyr-Trp adduct imparts structural stability to the active site of KatG that seems to be important for sustaining catalase activity.

Hydrogen peroxide-mediated isoniazid activation catalyzed by Mycobacterium tuberculosis catalase-peroxidase (KatG) and its S315T mutant

Inhibition of the enzyme Mycobacterium tuberculosis InhA (enoyl-acyl carrier protein reductase) due to formation of an isonicotinoyl-NAD adduct (IN-NAD) from isoniazid (INH) and nicotinamide adenine dinucleotide cofactor is considered central to the mode of action of INH, a first-line treatment for tuberculosis infection. INH action against mycobacteria requires catalase-peroxidase (KatG) function, and IN-NAD adduct formation is catalyzed in vitro by M. tuberculosis KatG under a variety of conditions, yet a physiologically relevant approach to the process has not emerged that allows scrutiny of the mechanism and the origins of INH resistance in the most prevalent drug-resistant strain bearing KatG[S315T]. In this report, we describe how hydrogen peroxide, delivered at very low concentrations to ferric KatG, leads to efficient inhibition of InhA due to formation of the IN-NAD adduct. The rate of adduct formation mediated by wild-type KatG was about 20-fold greater than by the isoniazid-resistant KatG[S315T] mutant under optimal conditions (H2O2 supplied along with NAD+ and INH). Slow adduct formation also occurs starting with NADH and INH, in the presence of KatG even in the absence of added peroxide, due to endogenous peroxide. The poor efficiency of the KatG[S315T] mutant can be enhanced merely by increasing the concentration of INH, consistent with this enzyme's reduced affinity for INH binding to the resting enzyme and the catalytically competent enzyme intermediate (Compound I). Origins of drug resistance in the KatG[S315T] mutant enzyme are analyzed at the structural level through examination of the three-dimensional X-ray crystal structure of the mutant enzyme.

Probing the structure and bifunctionality of catalase-peroxidase (KatG)

Catalase-peroxidases (KatGs) exhibit peroxidase and substantial catalase activities similar to monofunctional catalases. Crystal structures of four different KatGs reveal the presence of a peroxidase-conserved proximal and distal heme pocket together with features unique to KatG. To gain insight into their structure-function properties, many variants were produced and very similar results were obtained irrespective of the origin of the KatG mutated. This review focuses mainly on the electronic absorption and resonance Raman results together with the combined analysis of pre-steady and steady-state kinetics of various mutants involving both the peroxidase-conserved and the KatG-specific residues of recombinant KatG from the cyanobacterium Synechocystis. Marked differences in the structural role of conserved amino acids and hydrogen-bond networks in KatG with respect to the other plant peroxidases were found. Typically, the catalatic but not the peroxidatic activity was very sensitive to mutations that disrupted the KatG-typical extensive hydrogen-bonding network. Moreover, the integrity of this network is crucial for the formation of distinct protein radicals formed upon incubation of KatG with peroxides in the absence of one-electron donors. The correlation between the structural architecture and the bifunctional activity is discussed and compared with data obtained for KatGs from other organisms.

Role of the Met−Tyr−Trp Cross-Link in Mycobacterium tuberculosis Catalase-Peroxidase (KatG) As Revealed by KatG(M255I)

Catalase-peroxidases (KatGs) are bifunctional enzymes possessing both catalase and peroxidase activities. Four crystal structures of different KatGs revealed the presence of a novel Met-Tyr-Trp cross-link which has been suggested to impart catalatic activity to the KatGs. To decipher the individual roles of the two cross-links in the Met-Tyr-Trp adduct, we have focused on recombinant Mycobacterium tuberculosis KatG(M255I). UV-visible spectroscopic and mass spectrometric studies of the peptide fragments resulting from tryptic digestion of KatG(M255I) confirmed the presence of the single Tyr-Trp cross-link, as well as a 2e- oxidized form which is postulated to be an intermediate generated during Met-Tyr-Trp cross-link formation. KatG(M255I) lacking the Tyr-Trp cross-link was also prepared, and incubation with peroxyacetic acid, but not 2-methyl-1-phenyl-2-propyl hydroperoxide, resulted in complete formation of the Tyr-Trp cross-link. A mechanism for Tyr-Trp autocatalytic formation by KatG compound I is proposed from these studies. Optical stopped-flow studies with KatG(M255I) were performed, allowing characterization of compounds I, II, and III. Interestingly, two compound II intermediates were identified: (KatG*)(Por)Fe(III)-OH, where KatG* represents a protein-based radical, and oxoferryl (KatG)(Por)Fe(IV)=O. Insight into the contributions of the individual Tyr-Trp and Met-Tyr cross-links to catalase activity is presented, as is the overall contribution of the Met-Tyr-Trp cross-link to the structure-function-spectroscopy relationship and catalase-peroxidase mechanism in KatG.

Single-site mutations on the catalase–peroxidase from Sinorhizobium meliloti: role of the distal Gly and the three amino acids of the putative intrinsic cofactor

KatB is the only catalase-peroxidase identified so far in Sinorhizobium meliloti. It plays a housekeeping role, as it is expressed throughout all the growth phases of the free-living bacterium and also during symbiosis. This paper describes the functional and structural characterization of the KatB mutants Gly303Ser, Trp95Ala, Trp95Phe, Tyr217Leu, Tyr217Phe and Met243Val carried out by optical and electron spin resonance spectroscopy. The aim of this work was to investigate the involvement of these residues in the catalatic and/or peroxidatic reaction and falls in the frame of the open dispute around the factors that influence the balance between catalatic and peroxidatic activity in heme enzymes. The Gly303 residue is not conserved in any other protein of this family, whereas the Trp95, Tyr217 and Met243 residues are thought to form an intrinsic cofactor that is likely to play a role in intramolecular electron transfer. Spectroscopic investigations show that the Gly303Ser mutant is almost similar to the wild-type KatB and should not be involved in substrate binding. Mutations on Trp95, Tyr217 and Met243 clear out the catalatic activity completely, whereas the peroxidatic activity is maintained or even increased with respect to that of the wild-type enzyme. The k (cat) values obtained for these mutants suggest that Trp95 and Tyr217 form a huge delocalized system that provides a pathway for electron transfer to the heme. Conversely, Met243 is likely to be placed close to the binding site of the organic molecules and plays a crucial role in substrate docking.

Correlation between Isoniazid Resistance and Superoxide Reactivity inMycobacterium tuberculosisKatG

Isoniazid is an antituberculosis prodrug that requires activation by the catalase-peroxidase (KatG) of Mycobacterium tuberculosis. The activated species, presumed to be an isonicotinoyl radical, couples to NADH forming an isoniazid-NADH adduct that ultimately confers antitubercular activity. We have compared the catalytic properties of three KatGs associated with isoniazid resistance (resistance mutation KatGs, (RM)KatGs: R104L, H108Q, S315T) to wild-type enzyme and two additional lab mutations (wild-type phenotype KatGs, (WTP)KatGs: WT KatG, Y229F, R418L). Neither catalase nor peroxidase activities, nor the presence/absence of the Met-Tyr-Trp cross-link (as probed by LC/MS on tryptic digests of the protein), exhibited any correlation with isoniazid resistance. The yields of isoniazid-NADH adduct formed were determined to be 1-5, 4-12, and 20-70-fold greater for the (WTP)KatGs than the (RM)KatGs for the compound I, II, and III pathways, respectively, strongly suggesting a role for oxyferrous KatG (supported by superoxide consumption measurements) that correlates with drug resistance. Stopped-flow UV-visible spectroscopic studies revealed that all KatGs were capable of forming both compound II and III intermediates. Rates of compound II decay were accelerated 4-12-fold in the presence of isoniazid (vs absence) for the (WTP)KatGs but were unaffected by the drug for the (RM)KatGs. A mechanism for isoniazid resistance which accounts for the observed reactivity for each of the compound I, II, and III intermediates is proposed and suggests that the compound III pathway may be the primary factor in determining overall isoniazid resistance by specific KatG mutants, with secondary contributions arising from the compound I and II pathways.

The Met-Tyr-Trp Cross-link in Mycobacterium tuberculosis Catalase-peroxidase (KatG)

Catalase-peroxidases (KatG) are bifunctional enzymes possessing both catalase and peroxidase activities. Three crystal structures of different KatGs revealed the presence of a novel Met-Tyr-Trp cross-link that has been suggested to impart catalatic activity to the KatGs. High-performance liquid chromatographic separation of the peptide fragments resulting from tryptic digestion of recombinant Mycobacterium tuberculosis WT KatG identified a peptide with unusual UV-visible spectroscopic features attributable to the Met(255)-Tyr(229)-Trp(107) cross-link, whose structure was confirmed by mass spectrometry. WT KatG lacking the Met-Tyr-Trp cross-link was prepared, making possible studies of its formation under oxidizing conditions that generate either compound I (peroxyacetic acid, PAA) or compound II (2-methyl-1-phenyl-2-propyl hydroperoxide, MPPH). Incubation of this "cross-link-free" WT KatG with PAA revealed complete formation of the Met-Tyr-Trp structure after six equivalents of peracid were added, whereas MPPH was unable to promote cross-link formation. A mechanism for Met-Tyr-Trp autocatalytic formation by KatG compound I is proposed from these studies. Optical stopped-flow studies of WT KatG and KatG(Y229F), a mutant in which the cross-link cannot be formed, were performed with MPPH and revealed an unusual compound II spectrum for WT KatG, best described as (P.)Fe(III), where P. represents a protein-based radical. This contrasts with the oxoferryl compound II spectrum observed for KatG(Y229F) under identical conditions. The structure-function-spectroscopy relationship in KatG is discussed with relevance to the role that the Met-Tyr-Trp cross-link plays in the catalase-peroxidase mechanism.

Mycobacterium tuberculosis KatG(S315T) catalase-peroxidase retains all active site properties for proper catalytic function

Mycobacterium tuberculosis (Mtb) KatG is a catalase-peroxidase that is thought to activate the antituberculosis drug isoniazid (INH). The local environment of Mtb KatG and its most prevalent INH-resistant mutant, KatG(S315T), is investigated with the exogenous ligands CO and NO in the absence and presence of INH by using resonance Raman, FTIR, and transient absorption spectroscopy. The Fe-His stretching vibration is detected at 244 cm(-)(1) in the ferrous forms of both the wild-type enzyme and KatG(S315T). The ferrous-CO complex of both enzymes exhibits nu(CO), nu(Fe-CO), and delta(Fe-C-O) vibrations at 1925, 525, and 586 cm(-)(1), respectively, indicating a positive electrostatic environment for the CO complex, which is probably weakly hydrogen-bonded to a distal residue. The CO geometry is nonlinear as indicated by the unusually high intensity of the Fe-C-O bending vibration. The nu(Fe(III)-NO) and delta(Fe(III)-N-O) vibrations are detected at 596 and 571 cm(-)(1), respectively, in the ferric forms of wild-type and mutant enzyme and are indicative of a nonlinear binding geometry in support of the CO data. Although the presence of INH does not affect the vibrational frequencies of the CO- and NO-bound forms of either enzyme, it seems to perturb slightly their Raman intensities. Our results suggest a minimal, if any, perturbation of the distal heme pocket in the S315T mutant. Instead, the S315T mutation seems to induce small changes in the KatG conformation/dynamics of the ligand access channel as indicated by CO rebinding kinetics in flash photolysis experiments. The implications of these findings for the catalytic mechanism and mechanism of INH resistance in KatG(S315T) are discussed.

Crystal Structure of Mycobacterium tuberculosis Catalase-Peroxidase

The Mycobacterium tuberculosis catalase-peroxidase is a multifunctional heme-dependent enzyme that activates the core anti-tuberculosis drug isoniazid. Numerous studies have been undertaken to elucidate the enzyme-dependent mechanism of isoniazid activation, and it is well documented that mutations that reduce activity or inactivate the catalase-peroxidase lead to increased levels of isoniazid resistance in M. tuberculosis. Interpretation of the catalytic activities and the effects of mutations upon the action of the enzyme to date have been limited due to the lack of a three-dimensional structure for this enzyme. In order to provide a more accurate model of the three-dimensional structure of the M. tuberculosis catalase-peroxidase, we have crystallized the enzyme and now report its crystal structure refined to 2.4-A resolution. The structure reveals new information about dimer assembly and provides information about the location of residues that may play a role in catalysis including candidates for protein-based radical formation. Modeling and computational studies suggest that the binding site for isoniazid is located near the delta-meso heme edge rather than in a surface loop structure as currently proposed. The availability of a crystal structure for the M. tuberculosis catalase-peroxidase also permits structural and functional effects of mutations implicated in causing elevated levels of isoniazid resistance in clinical isolates to be interpreted with improved confidence.

Enzyme-catalyzed Mechanism of Isoniazid Activation in Class I and Class III Peroxidases

There is an urgent need to understand the mechanism of activation of the frontline anti-tuberculosis drug isoniazid by the Mycobacterium tuberculosis catalase-peroxidase. To address this, a combination of NMR spectroscopic, biochemical, and computational methods have been used to obtain a model of the frontline anti-tuberculosis drug isoniazid bound to the active site of the class III peroxidase, horseradish peroxidase C. This information has been used in combination with the new crystal structure of the M. tuberculosis catalase-peroxidase to predict the mode of INH binding across the class I heme peroxidase family. An enzyme-catalyzed mechanism for INH activation is proposed that brings together structural, functional, and spectroscopic data from a variety of sources. Collectively, the information not only provides a molecular basis for understanding INH activation by the M. tuberculosis catalase-peroxidase but also establishes a new conceptual framework for testing hypotheses regarding the enzyme-catalyzed turnover of this compound in a number of heme peroxidases.

Properties of catalase-peroxidase lacking its C-terminal domain

Catalase-peroxidases have a two-domain structure. The N-terminal domain contains the bifunctional active site, but the function of the C-terminal domain is unknown. We produced catalase-peroxidase containing only its N-terminal domain (KatG(Nterm)). Removal of the C-terminal domain did not result in unexpected changes in secondary structure as evaluated by CD, but KatG(Nterm) had neither catalase nor peroxidase activity. Partial recovery of both activities was achieved by incubating KatG(Nterm) with the separately expressed and isolated KatG C-terminal domain. Spectroscopic measurements revealed a shift in heme environment from a mixture of high-spin species (wtKatG) to exclusively hexacoordinate, low-spin (KatG(Nterm)). Moreover, a > 1000-fold lower kon for CN- binding was observed for KatG(Nterm). EPR spectra for KatG(Nterm) and the results of site-specific substitution of active site histidines suggested that the distal histidine was the sixth ligand. Thus, one important role for the C-terminal domain may be to support the architecture of the active site, preventing heme ligation by this catalytically essential residue.

System for the expression of recombinant hemoproteins in Escherichia coli

Expression of recombinant hemoproteins in Escherichia coli is often limited because a vast majority of the protein produced lacks the heme necessary for function. This is compounded by the fact that standard laboratory strains of E. coli have a limited capacity to withdraw heme from the extracellular environment. We are developing a new tool designed to increase the heme content of our proteins of interest by simply supplementing the expression medium with low concentrations of hemin. This hemoprotein expression (HPEX) system is based on plasmids (pHPEX1-pHPEX3) that encode an outermembrane-bound heme receptor (ChuA) from E. coli O157:H7. This heme receptor, and others like it, confers on the host the ability to more effectively internalize exogenous heme. Transformation of a standard laboratory E. coli protein expression strain (BL-21 [DE3]) with the pHPEX plasmid led to the expression of a new protein with the appropriate molecular weight for ChuA. The receptor was functional as demonstrated by the ability of the transformant to grow on iron-deficient media supplemented with hemin, an ability that the unmodified expression strain lacked. Expression of our proteins of interest, catalase-peroxidases, using this system led to a dramatic and parallel increase in heme content and activity. On a per-heme basis, the spectral and kinetic properties of HPEX-derived catalase-peroxidase were the same as those observed for catalase-peroxidases expressed in standard E. coli-based systems. We suggest that the pHPEX plasmids may be a useful addition to other E. coli expression systems and may help address a broad range of problems in hemoprotein structure and function.

Protein-based radicals in the catalase-peroxidase of synechocystis PCC6803: a multifrequency EPR investigation of wild-type and variants on the environment of the heme active site

Catalase-peroxidases are bifunctional heme enzymes with a high structural homology to peroxidases from prokaryotic origin and a catalatic activity comparable to monofunctional catalases. These unique features of catalase-peroxidases make them good systems to study and understand the role of alternative electron pathways both in catalases and peroxidases. In particular, it is of interest to study the poorly understood role of tyrosyl and tryptophanyl radicals as alternative cofactors in the catalytic cycle of catalases and peroxidases. In this work, we have used a powerful combination of multifrequency EPR spectroscopy, isotopic labeling of tryptophan and tyrosine residues, and site-directed mutagenesis to unequivocally identify the reactive intermediates formed by the wild-type Synechocystis PCC6803 catalase-peroxidase. Selected variants of the heme distal and proximal sides of the Synechocystis enzyme were investigated. Variants on the aromatic residues of the short stretch located relatively close to the heme and spanning the distal and proximal sides were also investigated. In the wild-type enzyme, the EPR signal of the catalases and peroxidases (typical) Compound I intermediate [Fe(IV)=O por.+] was observed. Two protein-based radical intermediates were also detected and identified as a Tyr. and a Trp. . The site of Trp. is proposed to be Trp 106, a residue belonging to the conserved short stretch in catalase-peroxidases and located at a 7-8 A distance to the heme propionate groups. An extensive hydrogen-bonding network on the heme distal side, involving Trp122, His123, Arg119, seven structural waters, the heme 6-propionate group, and Trp106, is proposed to have a key role on the formation of the tryptophanyl radical. We used high-field EPR spectroscopy (95-285 GHz) to resolve the g-anisotropy of the protein-based radicals in Synechocystis catalase-peroxidase. The broad gx component of the HF EPR spectrum of the Tyr. in Synechocystis catalase-peroxidase was consistent with a distributed electropositive protein environment to the tyrosyl radical.

Properties of a novel periplasmic catalase–peroxidase from Escherichia coli O157:H7

A subset of catalase-peroxidases are distinguished by their periplasmic location and their expression by pathogens. Kinetic and spectral properties have not been reported for any of these enzymes. We report the cloning, expression, isolation, and characterization of KatP, a periplasmic catalase-peroxidase from Escherichia coli O157:H7. Absorption spectra indicated a mixture of heme states dominated by the pentacoordinate and hexacoordinate high-spin forms. Apparent k(cat) values for catalase (1.8x10(4) s(-1)) and peroxidase (77 s(-1)) activities were greater than those of other catalase-peroxidases. However, apparent K(M) values for H2O2 were also higher (27 mM for catalase and 3 mM for peroxidase). Ferric KatP reacted with peracetic acid to form compound I (8.8x10(3) M(-1) s(-1)) and with CN(-) to form a ferri-cyano complex (3.9x10(5) M(-1) s(-1)) consistent with other catalase-peroxidases. The isolation and characterization of KatP opens new avenues to explore mechanisms by which the periplasmic catalase-peroxidases may contribute to bacterial virulence.

Reduced affinity for Isoniazid in the S315T mutant of Mycobacterium tuberculosis KatG is a key factor in antibiotic resistance

Catalase-peroxidase (KatG) from Mycobacterium tuberculosis is responsible for the activation of the antitubercular drug isonicotinic acid hydrazide (INH) and is important for survival of M. tuberculosis in macrophages. Characterization of the structure and catalytic mechanism of KatG is being pursued to provide insights into drug (INH) resistance in M. tuberculosis. Site-directed mutagenesis was used to prepare the INH-resistant mutant KatG[S315T], and the overexpressed enzyme was characterized and compared with wild-type KatG. KatG[S315T] exhibits a reduced tendency to form six-coordinate heme, because of coordination of water to iron during purification and storage, and also forms a highly unstable Compound III (oxyferrous enzyme). Catalase activity and peroxidase activity measured using t-butylhydroperoxide and o-dianisidine were moderately reduced in the mutant compared with wild-type KatG. Stopped-flow spectrophotometric experiments revealed a rate of Compound I formation similar to wild-type KatG using peroxyacetic acid to initiate the catalytic cycle, but no Compound I was detected when bulkier peroxides (chloroperoxybenzoic acid, t-butylhydroperoxide) were used. The affinity of resting (ferric) KatG[S315T] for INH, measured using isothermal titration calorimetry, was greatly reduced compared with wild-type KatG, as were rates of reaction of Compound I with the drug. These observations reveal that although KatG[S315T] maintains reasonably good steady state catalytic rates, poor binding of the drug to the enzyme limits drug activation and brings about INH resistance.

Conformational differences in Mycobacterium tuberculosis catalase-peroxidase KatG and its S315T mutant revealed by resonance Raman spectroscopy

KatG from Mycobacterium tuberculosis is a heme-containing catalase-peroxidase, which belongs to the class I peroxidases and is important for activation of the prodrug isoniazid (INH), a front-line antituberculosis drug. In many clinical isolates, resistance to INH has been linked to mutations on the katG gene, and the most prevalent mutation, S315T, suggests that modification of the heme pocket has occurred. Electronic absorption and resonance Raman spectra of ferric wild-type (WT) KatG and its INH-resistant mutant KatG(S315T) at different pH values and their complexes with INH and benzohydroxamic acid (BHA) are reported. At neutral pH, a quantum mechanically mixed spin state (QS) is revealed, which coexists with five-coordinate and six-coordinate high-spin hemes in WT KatG. The QS heme is the major species in KatG(S315T). Addition of either INH or BHA to KatG induces only minor changes in the resonance Raman spectra, indicating that both compounds do not directly interact with the heme iron. New vibrational modes are observed at 430, 473, and 521 cm(-1), and these modes are indicative of a change in conformation in the KatG heme pocket. The intensity of these modes and the relative population of the QS heme are stable in KatG(S315T) but not in the WT enzyme. This indicates that there are differences in heme pocket stability between WT KatG and KatG(S315T). We will discuss the stabilization of the QS heme and propose a model for the inhibition of INH oxidation by KatG(S315T).