Structure
Mycobacterium Tuberculosis Catalase Peroxidase (mtCP) is a homodimer with each monomer consisting of two domains. The overall structure is stabilized by 703 water molecules.
blue - n terminus hook
magenta - n terminus
light pink - c terminus
Monomer Structure
The two domains of each monomer are primarily alpha helical and have similar foldings. The similar foldings suggests that the monomer results from a gene duplication event; however, the C-terminal domain does not contain the heme b prosthetic group, while the N-terminal does. The active site is therefore located within the N-terminal domain. The two monomers interact through an interlocking hook formed by the N-terminal domains that stabilizes the formation of the dimer.
The N-terminal is formed through hydrophobic interactions between residues Tyr-28 and Tyr-197 and residues Trp-38 and Trp-204. This interlocking loop region is also found in similar conformations of other catalase peroxidase structures such as: hmCP and bpCP.
Active Site
In hmCP, which share 55% and 69% identity with mtCP, the heme is buried inside HmCP-N, and substrate access to the active site is thorugh a narrow channel that prevents access of a large substrate (3).
Catalase Peroxidases
Catalase-peroxidases are enzymes that degrade hydrogen peroxide. Catalase converts two equivalents of hydrogen peroxide into water and oxygen via a two-step reaction cycle in which H202 alternately oxidizes and reduces the heme iron at the active site. Within peroxidases, oxidation of heme iron involves a H202 molecules, similar to that in the catalase-catalyzed reaction. Reduction of the heme iron, however, involves hydrogen donors such as NADH, not a second H202 molcule (3). Catalase-Peroxidases that have been characterized are either homodimers or homotetramers and contain a single heme b cofactor at the active site. Usually, the primary struture of the subunit can be divided into two halves that have a high level of sequence similarity, most likely due to a gene duplication event.
Clinical Applications
Isoniazid Structure
Isoniazid Role
INH is a prodrug susceptible to oxidative reactions catalyzed by KatG (4). INH action against mycobacteria requires catalase-peroxidase (KatG) function.
Mechanism
Formation of IN-NAD Adduct
Activation or oxidation of INH by KatG has recently been measured in the terms of production of an IN-NAD adduct molecule that serves as a tight binding inhibitor of InhA. InhA is an enzyme involved in the biosynthesis of mycolic acids, which are components of the mycobacterial cell wall. Therefore, inhibition of InhA alone is sufficient enough to inhibit mycolic acid biosynthesis and induce cell lysis after exposure of bacteria to INH (4).
Mutations
Mutation of Ser315 to a Thr in mtCP results in a loss of the activation to the anti-tuberculosis drug (INH) with no loss of either peroxidase or catalase activity (3).
References
1) Bertrand, T.; Eady, N. A. J.; Jones, J. N.; Jesmin, Nagy, J. M.; Jamart-Gregorie, B.; Raven. E. L.; Brown, K. A. Crystal Structure of Mycobacterium Tubercuosis Catalase-Peroxidase. The J. of Biol. Chem. 2004, 279, 38991-38999.
2) Carpena, X.; Loprasert, Suvit,; Mongkolsuk, S.; Switala, J.; Loewen, P. C.; Fita, I. Catalase-peroxidase KatG of Burkholderia pseudomallei at 1.7 Å resolution. J. Mol. Biol. 2003, 327, 475-489.
3) Yamada, Y.; Fujiwara, T.; Sato, T.; Igarashi, N.; Tanaka, N. The 2.0 Å crystal structure of catalase-peroxidase from Haloarcula marismortui Natur Structural Biol. 2002, 9, 691-695.
4) Zhao, X.; Yu, H.; Yu, S.; Wang, F.; Sacchettini, J. C.; Magliozzo, R. S.; Hydrogen Peroxide-Mediated Isoniazid Activation Catalyzed by Mycobacterium tuberculosis Catalase-Peroxidase (KatG) and Its S315T Mutant. Biochemistry. 2006, 45, 4131-4140.
