Catalase domain structures

The tertiary structure of small subunit enzymes can be subdivided into four distinct regions, and the C-terminal or flavodoxin domain of the large subunit enzymes becomes a fifth region. These are indicated in Fig. 8 for clarity. The first region is the amino terminal arm (Fig. 8), which extends 50 or more residues from the amino terminus almost to the essential histidine residue (to residue 53 in PMC, 60 in PVC, 73 in BLC, and 127 in HPII). There is very little structural similarity in the N-terminal region and, in the case of HPII, the structure of the terminal 27 residues is not even defined and they do not appear in the crystal structure. Within the N-terminal arm is a 20-residue helix, helix a2 in HPII, which is the first secondary structure element common to all catalases. The presence of helix al varies among catalases, and there is no sequence or location equivalence even when it is present. 

Fig. 8. Comparison of the subunit structures of a small subunit catalase (BLC) (A) and a large subunit catalase (HPII) (B). Segments including the N-terminal domain, the beta barrel core, the wrapping domain, the alpha helical domain, and the C-terminal domain (HPII) are indicated and are described in Section V,A.

Fig. 12. A hypothetical folding and assembly pathway for catalases. In A secondary and tertiary folding first occurs in the individual subunits to form the 3-barrel (p), wrapping domain (W), a-helical segment (a), and fiavodoxin domain (F, only in HPII). In proceeding to B, heme is bound to each of the subunits, and this may serve as a catalyst for the rapid association of the i -related subunits to form the structure in C. In proceeding to D, Q-related subunits associate, resulting in the N-terminal arms being overlapped as the C-terminal portions fold back on themselves to form the fully folded structure shown in E. Only two subunits are shown in the progression from C to E, but a simultaneous folding must be occurring in the associated dimer. The fully folded tetramer is shown in two orientations.

These are difficult enzymes to work with and only recently have crystal structures become available for two catalase-peroxidases Haloarcula marismortui (HMCP) and Burkholderia pseudomallei (BpKatG). A typical subunit is approximately 80 kDa in molecular mass, with a single heme b prosthetic group. The primary structure of each subunit can be divided into two distinct domains, N terminal and C terminal. The N-terminal domain contains the heme and active site, while the C-terminal domain does not contain a heme binding motif and its function remains unclear. The clear sequence similarity between the two domains suggests gene duplication and fusion. Curiously, despite many years of study, the actual in vivo peroxidatic substrate of the catalase-peroxidases has not been identified. 

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