Calcium-dependent conformation of a heme and fingerprint peptide of the diheme cytochrome c peroxidase from Paracoccus pantotrophus

The structural changes in the heme macrocycle and substituents caused by bi nding of Ca2+ to the diheme cytochrome c peroxidase from Paracoccus pantotr ophus were clarified by resonance Raman spectroscopy of the inactive fully oxidized form of the enzyme. The changes in the macrocycle vibrational mode s are consistent with a Ca2+-dependent increase in the out-of-plane distort ion of the low-potential heme, the proposed peroxidatic heme. Most of the i ncrease in out-of-plane distortion occurs when the high-affinity site I is occupied, but a small further increase in distortion occurs when site II is also occupied by Ca2+ or Mg2+. This increase in the heme distortion explai ns the red shift in the Soret absorption band that occurs upon Ca2+ binding . Changes also occur in the low-frequency substituent modes of the heme, in dicating that a structural change in the covalently attached fingerprint pe ntapeptide of the LP heme occurs upon Ca2+ binding to site I. These structu ral changes may lead to loss of the sixth ligand at the peroxidatic heme in the semireduced form of the enzyme and activation.