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ID: 532091 • Letter: P
Question
Please try to actually answer the question, and do not post spam. I really need help with this problem. Thank you.
(5) A myoglobin similar to the example we did in class had the protonation of a histidine residue coupled to the oxidation of a heme. The histidine had a pKA of 6.0 when the heme is oxidized and 7.1 when the heme is reduced. At pH 9.5, the reduction potential of the heme is +275 mV vs NHE. (a) Draw the thermodynamic box that describes this system (b) Predict the reduction potential at pH 3. (c) The net charge at the iron center really cycles between 0 and +1, as the nitrogens at the center of the porphyrin ring have a total net charge of -2. Assuming a dielectric constant of 6, predict the distance between the heme iron and the histidine side chain.Explanation / Answer
In myoglobin and hemoglobin, heme is covalently linked with histidine F8(eighth residue of F helix). because of covalent bond this histidine is closer to heme iron and named as proximal histidine (closer histidine), while other key histidine which responsible for stabilization of oxygen in E7 (seventh residue of E helix) is far from heme iron so named as distal.
Imagine the Fe(2+) in the plane of the protoporphyrin (prosthetic group). Above and below this plane Fe (2+) has two more sites to bind, called the fifth and sixth coordination sites. On one of these sites, the iron binds a histidine and we call that the proximal histidine, and on the other side, it will bind the incoming oxygen. Since these bonds are rather weak and you don't want a highly reactive oxygen that is coming on or off the globin, the oxygen itself is then bonded by another histidine for protection which we call the distal histidine.