Limited proteolysis of myoglobin opens channel in ferrochelatase-globin complex for iron to zinc transmetallation

Recombinant ferrochelatase (BsFECH) from Bacillus subtilis expressed in Escherichia coli BL21(DE3) was found by UV-visible spectroscopy to bind the model substrate tetraphenylporphyrin-sulfonate, TPPS, with Ka = 3.8 105 mol/L in aqueous phosphate buffer pH 5.7 at 30 °C, and to interact with metmyoglobin with Ka = 1.07 ± 0.13 105 mol/L at 30 °C. The iron/zinc exchange in myoglobin occurring during maturation of Parma hams seems to depend on such substrate binding to BsFECH and was facilitated by limited pepsin proteolysis of myoglobin to open a reaction channel for metal exchange still with BsFECH associated to globin. BsFECH increased rate of zinc insertion in TPPS significantly and showed saturation kinetics with an apparent binding constant of Zn(II) to the [enzyme-TPPS] complex of 1.3 104 mol/L and a first-order rate constant of 6.6 10−1 s−1 for dissociation of the tertiary complex, a similar pattern was found for zinc/iron transmetallation in myoglobin.