A myoglobin similar to the example we did in class had the protonation of a hist
ID: 532217 • Letter: A
Question
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.
(please give full and correct solutions for each part)
Explanation / Answer
The nitrite adducts of globins can potentially bind via O- or N- linkage to the heme iron. We have used EPR (electron paramagnetic resonance) and DFT (density functional theory) to explore these binding modes to myoglobin and hemoglobin. We demonstrate that the nitrite adducts of both globins have detectable EPR signals; we provide an explanation for the difficulty in detecting these EPR features, based on uniaxial state considerations. The EPR and DFT data show that both nitrite linkage isomers can be present at the same time and that the two isomers are readily interconvertible in solution. The millisecond-scale process of nitrite reduction by Hb is investigated in search of the elusive Fe(II)-nitrite adduct.
c. The Nernst equation is the second link between thermodynamics and ... The SI unit of electricalcharge is the coulomb (C): 1 A = 1 C s -1. ..... E = 0 V. To be a standard cell, the hydrogen ions and the hydrogen gas must ... potential for the reduction of Fe3+ to Fe (metallic iron) by Zn metal. ..... and nine iron-sulfur centers.
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