Testing c‐Type Heme Sources for Nontypeable Haemophilus influenzae

Abstract

The property that defines c‐type heme proteins is the covalent attachment of the heme group to the polypeptide chain of the protein, such as in cytochrome c. The covalent attachment of the c‐heme (usually by thioether linkages donated by two Cysteine residues) is energetically expensive, and the biological motivation for the attachment has been debated for decades. We propose that one biological motivation for the covalently attached c‐heme is to prevent heme‐dependent bacteria, such as Nontypable Haemophilus influenzae (NTHi), from scavenging heme from c‐heme proteins. To test this hypothesis, we grew NTHi in the presence of cytochrome c variants with zero, one, or two thioether linkages to heme, which we prepared using site‐directed mutagenesis. Our results suggest that one covalent attachment is enough to prevent NTHi from scavenging heme from cytochrome c. Microperoxidase‐11 (MP‐11) is obtained from enzymatic cleavage of cytochrome c, and contains the heme group and residues 11–21 of the protein, including the two covalent attachments to heme and the Histidine axial ligand. Our experiments show that NTHi cannot utilize MP‐11 as a heme source.