Question

In the Bohr effect, protonation of the imidazolium rings of the C-terminal rings of the C-terminal residues (His 126B) of the hemoglobin's two B chains is responsible for around 40% of the .60 mole of H+ that combine with Hb on the release of 1.00 mole of O2 at pH 7.40. Assuming that this imidazolium group has pKa = 7.10 in R-state oxyHb, what is its pKa value in T-state deoxyHb? (Hint: the pH of the blood - because it's buffered - doesn't change when going from R-state oxyHb to T-state deoxyHb)... Also please note that this is really a biochemistry problem, but i'm desperate if anyone has ANY idea how to even start this problem. Thanks in advance!

Answer 1

I am giving an idea to solve this problem, not solution as per your request.

At a given pH, the contribution of each hydrogen ion binding site to the total number of Bohr protons that can be additionally bound in deoxyHb is determined by the magnitude of the deoxygenation induced increase in pKa, and the closeness of the pKa values in oxy and deoxy Hb to the given pH. The individual contribution of a Bohr siteH+
i is given by two times the difference in fractional occupation of the hydrogen ion binding site between T-state deoxyHb and R-state oxyHb:

[H71 or 10-PH AH 2 10TP 10-PH 10-P I0-PH10-P

where K^T_ai and K^R_ai are the acid dissociation constants of an individual (i) site in T-state and R-state Hb, respectively.

R state ("oxy" conformation, high O2 binding affinity) stabilized by O2 binding (O2 is a homotropic effector) T state ("deoxy" conformation, low O2 binding affinity) stabilized by binding of protons (H+) , CO2, and/or 2,3- bisphosphoglycerate (2,3-BPG) (all heterotropic effectors)

I would like to suggest you to please go through the following journal for more information

Evolution of vertebrate haemoglobins: Histidine side chains,specific buffer value and Bohr effect - Title of journal

Respiratory Physiology & Neurobiology 154 (2006) 165–184

or download pdf by copy paste below one exactly in google search engine and download 1st link

ΕΞΕΛΙΞΗ ΤΗς ΑΙΜΟΣΦΑΙΡΙΝΗΣ.pdf

Thank you