Histidine residues catalase

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. 13. Active site residues in a small-subunit catalase BLC (A) and a large-subunit catalase HPIl (B). The active site residues are labeled, and hydrogen bonds are shown between the serine (113 in BLC and 167 in HPll) and the essential histidine (74 in BLC and 128 in HPll). A single water is shown hydrogen bonded to the histidine. The equivalent water in BLC is located by analogy to the position of the water in HPll. The unusual covalent bond between the N of His392 and the C of Tyr415 in HPll is evident on the proximal side of the heme in B. The flipped orientations of the hemes are evident in a comparison of the two structures, as is the eis-hydroxyspirolactone structure of heme d in B.

The reaction of other minor or type D catalases such as methemoglo-bin and metmyoglobin is not treated in detail here, because they are minor activities, significantly lower than even that of chloroperoxidase. The orientation of residues on the distal side of the heme is not optimized for the catalatic reaction to the extent that there is even a sixth ligand of the heme, a histidine, that would preclude a close association of the heme with hydrogen peroxide without a significant side-chain movement. It is only after an extended treatment with H2O2 and oxidation of the Fe that a low level of catalatic activity becomes evident. 

In the type A catalases, there are only two active site residues in locations where they can influence the reaction, a histidine and an asparagine. A mechanism for compound I reduction in catalases was... 

In the case of beef liver catalase, distal histidine (His74) is believed to serve as a general acid-base catalyst to facilitate the heterolytic 0—0 bond of hydroperoxide bound to the heme (Scheme 2) (21). The Asnl47 residue located near the heme could assist the heterolysis by making the distal site into a polar atmosphere. The same acid-base mechanism has been attributed to peroxidases... 

As discussed in Scheme 2, the distal histidine in peroxidase and catalase is a crucial amino acid residue for the formation of compound I. In fact, the... 

The amino acid content of horse liver catalase is reported to be as follows (per gram atom of iron) total number of amino acid residues, 496 histidine, 15 or 16 arginine, 27 lysine, 32 (220). 

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