Porphyrin-containing proteins binding

The hydrophobic core of the porphyrin is the most important in driving the binding to the proteins, while the peripheral functionalization can orient the selectivity towards different proteins (Fig. 23). The detection of metal ion containing proteins was based on the fluorescence quenching of porphyrin upon protein binding porphyrins are highly fluorescent compounds, ensuring the necessary sensitivity, and furthermore this feature avoids the functionalization of the protein... 

Figure 4. A schematic depiction of the fluorescence quenching-based binding assay. The synthetic receptor is fluorescent when unbound but when die porphyrin macrocycle is forced into proximity with the Fe(III) containing heme, fluorescence is quenched. The ratio between free and bound receptor can be equated to the change in fluorescence due to protein binding.

Hemerythrin is a respiratory protein isolated from sipun-culids (marine worms). All sipunculids examined have, in the coelomic fluid, erythrocytes loaded with the protein which in most species so far examined is octameric, but sometimes tri-meric (18, 19) and in one instance dimeric and tetrameric (20, 21). From the retractor muscle of Themiste zostericola, the protein has been characterized as a monomer (22). The monomer (23) and the subunits of the trimer (24) and octamer (25) are remarkably similar in tertiary structure, having a M.W. of about 13,500 daltons. Each subunit contains one binuclear iron site. There is no porphyrin ring and the irons are coordinated only to amino acids, some of which, as well as probably an oxy group, form the binding atoms (26). 

Heme proteins in their various forms contain mainly the ferrous or ferric porphyrin moieties [6—77] (R some organic side chain of the protein A a small molecule act-ingas a-donor-TT-acceptor ligand, e.g., CO, 02, NO, CH3CN, CH3NC) (7, 20-34). In fact the binding of dioxygen to the pentacoordinate species [6] and [7] — an essential... 

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