Figure 5.

Possible mechanisms for exchange of heme. Heme-binding partners (dark gray pacmans) are envisaged as transferring heme to heme proteins (red circles) by dissociative (pale gray box) or associative (pale yellow box) pathways, resembling classical ligand exchange mechanisms in coordination chemistry. In a dissociative pathway, a free molecule of heme (assumed to be coordinated by a water molecule) is formed transiently following dissociation from a heme-binding partner, and is intercepted by an apo-heme protein (faded red circles). Alternatively, an associative exchange of heme is possible and is shown here for the example of heme delivery by chaperones (C, circles). This latter mechanism may provide better selectivity toward the target heme protein. However, we do not envisage this as being exclusive to chaperones, but a mechanism which, in principle, is available to be used by heme-binding partners as well in delivering heme to apo heme proteins. The different mechanisms of heme exchange may help to fine-tune the delivery of heme to specific acceptors.