Calcium-binding sites hydrogen bonds

Fig. 9. The calcium binding site in P. aeruginosa CCP. One of the propionates of the high-potential (HP) heme is hydrogen bonded to two bound water (OW) molecules.

Figure 18. Key catalytic residues for (A) CCP (B) LIP and (C) HRP determined from X-ray coordinates [137, 143, 211], The heme group is coordinated by a histidine and the sixth position is vacant. Calcium-binding sites for LIP and HRP are shown as spheres. Dashed lines represent hydrogen-bonding interactions.

Asp-49/35 is essential to optimal calcium binding and catalysis. Replacement of Asp-49 with other amino acids or chemical modification of the side-chain carboxylate reduces Acat to less than 5% of native rates (Fleer etai, 1981 Van den Berghe/a/., 1989). The close spatial coupling of Asp-49/35 with its respective catalytic histidine (His-48/34) ensures a fixed active site geometry. Stability of this bihelical substructure is critical because His-48/34 is supported by a side chain from an adjacent segment of the same helix (Tyr-52) and must hydrogen bond with a residue from the opposed helix (Asp-99/64) for function. 

HRP C contains two different types of metal center (i.e., iron(III) protoporphyrin IX-heme group and two calcium atoms) that are fundamental for the integrity of the enzyme. The heme group is attached to the enzyme at His 170 by a coordinate bond between the histidine side-chain NE2 atom and the heme iron atom. The second axial coordination site is unoccupied in the resting state of the enzyme but available to hydrogen peroxide during enzyme turnover. Small molecules such as carbon monoxide, cyanide, fluoride, and azide bind to the heme iron atom at this distal site, giving six-coordinated PX complexes. 

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