Hemopexin heme-protein interactions

A basic tenet in biology is that structure and function are linked, and the model above is consistent with functional studies on heme uptake from and gene regulation by heme-hemopexin (see below), Thus, with the recent details of the structures of hemopexin (M. Paoli, H. M. Baker, B. F. Anderson, W. T. Morgan, A. Smith, N. Baker, unpublished results) and ceruloplasmin [74], the metal-protein interactions of two key players in redox metal transport and metabolism are now established. 

The ferro-complex CD spectrum shows that reduction of the heme iron alters the heme environment. Redox-induced protein conformation changes could alter the S5unmetry in the heme pocket or produce two binding modes for the reduced complex whose asymmetries nearly cancel each other. Redox-linked conformational changes are especially interesting in view of recent findings of oxido-reductase activity associated with the heme-hemopexin-receptor interaction (89). 

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

The thermodynamic stability of hemopexin has been examined using DSC (Fig. 13), which showed apo-hemopexin to be a stable protein with a single Tm of 54°C (AH 185 kcal/mol), which increases to 66.5°C (AH 290 kcal/mol) upon binding heme (130). The N-domain of hemopexin is less stable (Tm 52°C, AH 95 kcal/mol) but is even more strikingly stabilized by ferri-heme (T 78°C, AH 370 kcal/mol). The presence of C-domain (Tii 49.5°C, AH 140 kcal/mol) slightly destabilizes heme-N-domain (Tm 75°C, AH 320 kcal/mol) (130), showing another effect of interdomain interactions that may act in heme release. 

Interactions between the iron and heme transport systems are also manifest at the level of secreted protein synthesis. When Hepa cells are incubated with increasing concentrations of heme-hemopexin complexes transferrin is no longer detectable in the medium presumably due to a lack of synthesis and secretion (Figure 5-4). This may contibute to the pathology in vivo of conditions of heme and iron overload. 

---