The picture of hemoglobin (the x-ray crystal structure shown in the picture) was

Question

The picture of hemoglobin (the x-ray crystal structure shown in the picture) was taken at pH 7.2. The pKa of free histidine (i.e., unattached to a protein) is 6.0. Assuming all of the beta2 H146 residues are positively charged at pH 7.2, what is your estimate for the pKa of this residue? Also, explain why you do or do not think that the pKa of the beta2-His146 in hemoglobin changes relative to free His?

A. 4.0 Zoom showing a highly schematic drawing of the key interactions that are broken going the T- and R-state crystal structures. Here the B-globins (blue) are transparent so that interactions at the -globin (pink) N-termini can be visualized a, K40 R141 2 D94 R141 , K40 4D94 a2 K127 2 K127 B2 H146 interactions a2 V1 Key interactions are formed at the B-termini in the T state. Key interactions are formed at the -termini in the T state. Key interactions at the B-termini are disrupted in the R state. Key interactions at the -termini are disrupted in the R state Copyright 2019, 2016 Pearson Education, Inc.

Explanation / Answer

I believe the correct answer to be

D) 8.0 due to the fact that it is positively charged even at ph 7.2 so it hda to be higher than that.

The pka of beta2-His146 in hemoglobin changes relative to free His due to the fact that it is present in a long chain of aminoacids with different side chains which can have affect the electronegativity of each other due to which it can be harder or easier for a single aminoacid to release its Hydrogen (in this case) which increses its pKa.

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