PH-LINKED BINDING OF MN(II) TO MANGANESE PEROXIDASE

The stability of Mn(II) binding to manganese peroxidase (MnP) has been studied as a function of pH by spectrophotometric and potentiometric titrations. The sensitivity of the potentiometric titrations allows co llection of data that are consistent with a high-affinity and a low-af finity Mn(II) binding site on the peroxidase. The two sites differ in affinity by 4 to 900-fold between pH 4 and 6.5. The stability of Mn(II ) binding to the high-affinity site increases with increasing pH, whil e the stability of Mn(II) binding to the low-affinity site decreases w ith increasing pH. Interestingly, at pH values above 5.0, the high-aff inity site appears to be partially unavailable for binding Mn(II), A p i-I-dependent structural change in the Mn(II) binding site is proposed to account for this partial inactivation at elevated pH.