Protoporphyrin IX and heme binding properties of Staphylococcus aureus IsdC

Staphylococcus aureus is a human pathogen that results in numerous infections in hospital settings and recently also in the wider community. Its antibiotic resistant forms are causing considerable alarm. A series of surface-anchored proteins that have heme uptake and transport properties have been reported. Through the use of absorption and magnetic circular dichroism spectroscopies and mass spectrometry, the iron-free, protoporphyrin IX and the iron-containing, heme-binding characteristics of bacterial rIsdC have been obtained. Mass spectrometry showed that following isolation and purification, the rIsdC is bound predominantly to protoporphyrin IX and to a lesser extent heme, unlike the case of rIsdA, which binds predominantly heme. Magnetic circular dichroism analysis provided further information regarding porphyrin binding because the characteristic magnetic circular dichroism band envelopes for the iron-free protoporphyrin IX and the iron-containing heme can be clearly distinguished in the spectrum of the rIsdC. Analysis of these spectral data showed that the minor heme component exists as a high-intermediate spin state ferric heme when bound to rIsdC, similar to the high-spin ferric heme reported for the rIsdA protein.