Calcium and its role in the activation mechanism

Another characteristic of P. denitrificans CCP is the presence of a monomer-dimer equilibrium where only the last form has peroxidatic activity. This equilibrium is shifted to the dimer state at high concentrations of the enzyme and in the presence of Ca. If we take into consideration the Ca dissociation constants of sites I and II and the dimer dissociation constant (pM), and at the same time, the physiological calciiun and peroxidase concentrations and ionic strength in the P. denitrificans periplasm (close to 200 pM), we can conclude that the physiological state of the enzyme is the dimeric form with both calciiun sites filled. 

The visible spectrum of the oxidized P. denitrificans CCP is typical of a heme protein [18]. The reduced form shows a splitting of the a-band. The active form of 

UV/visible spectroscopy has contributed, because of its simplicity, to the evaluation of the spin transitions associated with conformational changes induced by the reduction of the HP heme and its dependence on the presence of calcium. The data now described demonstrate clearly that, in the presence of calcium, the reduction of the HP heme induces a conformational change in both hemes the heme that is reduced converts to a low-spin configuration state, and the LP heme sequentially 

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