In the experiments of Barrick et al. (see the figure below), it was observed tha
ID: 193113 • Letter: I
Question
In the experiments of Barrick et al. (see the figure below), it was observed that replacement of histidine by a noncovalently bonded imidazole not only reduced cooperativity but also increased the oxygen affinity of the hemoglobin.
The effect of replacing the proximal histidine in hemoglobin with a glycine residue and adding a noncovalently bonded imidazole.
Part A
Suggest an explanation.
Suggest an explanation.
The effect of replacing the proximal histidine in hemoglobin with a glycine residue and adding a noncovalently bonded imidazole.
Part A
Suggest an explanation.
Suggest an explanation.
In native hemoglobin, the initial binding of one O2 to a heme group promotes the binding of O2 to the other heme site on the molecule. In the imidazole replacement, the initial binding of one O2 to a heme group promotes the binding of two O2 to the other heme sites on the molecule. This effectively increases the oxygen affinity of the hemoglobin. Imidazole is significantly more acidic (lower pKa) than histidine. Thus, at the pH of blood, native hemoglobin is more acidic than the imidazole replacement. This lowering in pH causes the oxygen affinity of the hemoglobin to increase. Unlike histidine imidazole binds to ATP forming a fairly stable complex. ATP binds to hemoglobin more strongly than does the ATP-imidazole complex. Imidazole, therefore, tends to increase oxygen affinity of hemoglobin by blocking the ATP-hemoglobin interaction. In native Hb, the binding of oxygen is actually hindered by the fact that pulling on helix F must move it against constraints within the molecule. In the imidazole replacement, there is no need to do the extra work of moving helix F. This difference shows up as a more favorable free energy for binding. F helix F helix HN HN Fe O. (a) The effect of O2 binding according to the Perutz model: The F helix is drawn toward the heme. F helix F helix HN Fe (b) Now lacking a covalent connection to the heme, the F helix is not disturbed by O2 binding and there is significantly reduced cooperativity.Explanation / Answer
Answer
In native Hb, the binding of oxygen is actually hindered by the fact that pulling on helix F must move it against constraints within the molecule. In the imidazole replacement, there is no need to do the extra work of moving helix F. This difference shows up as a more favorable free energy for binding.