Effect of metal on the porphyrin dynamics

Free poiphyrin motions could be considerably altered in the presence of a metal ion, such as the iron, which covalently links the porphyrin to the protein. Linkages between heme and protein involve iron and the porphyrin part of the heme. Iron is linked to an imidazole group of the proximal histidine, His-93. The other heme protein bonds involve, on the heme side, the propionic acid groups, the vinyl groups and the porphyrin as a whole. The two heme propionates help to stabilize the heme by making hydrogen bonds to the side chains of the distal histidine (His-64) and an arginine. 

If linkages of the porphyrin part of the heme have a great importance in the iron-protein bond for stabilizing the heme protein complex, the presence of iron which links covalently the porphyrin to the protein could alter considerably the free porphyrin motion. 

Addition of oxygen to the solutions of proteins decreases the emission intensities of the porphyrins. Figs 9.17 and 9.18 show the traditional mode of presentation for such data  

For the Stem-Volmer plots are linear. At 20 °C the value of the kinetic 

The value obtained for the quenching constant k in Mb is equal to 8.4 x 10° M s at 20°C. This value corresponds to an inteimediate one between simultaneous contributions of different rates oxygen diffusion in water (k = 1.3 x lO M s ), oxygen penetration into the protein (k = 3.8 x 10 M s ), migration inside the protein (k = 2 x 10 M s ) and the effective quenching rate which is too large to be measured 

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