Question

For hemoglobin and myoglobin, discuss the mechanism, similarities, differences, and effects of O₂ binding on the enzymes. What happens to the enzyme structure, and what do these structural changes then cause in terms of functional changes, if any? What role does BPG play for the enzyme and why would this be the case? What about H⁺? How does a baby breathe in the womb?  

Answer 1

The heme is a small but important non-protein molecule, or prosthetic group, that is associated with these oxygen binding proteins. The heme is comprised of protoporphyrin IX (an organic ring structure) which has a chelated (bound) iron atom. The heme resides in a small hydrophobic cleft within each polypeptide. There is one heme associated with myoglobin. There are four hemes found in hemoglobin, one in each of the subunits. Therefore, myoglobin binds reversibly to only one oxygen molecule, while and hemoglobin binds four. These differences help to explain the different biological functions of hemoglobin and myoglobin. Hemoglobin is used to transport oxygen over large distances (from the lungs to all tissues), whereas myoglobin is present in muscle and behaves as a local "storage" reservoir of oxygen. At the beginning, myoglobin binds oxygen molecules very easily and lately become saturated. This binding process is very rapid in myoglobin than in hemoglobin. Hemoglobin initially binds oxygen with difficulty. Myoglobin can bind one oxygen molecule so called monomer, while hemoglobin can bind four oxygen molecules, so called tetramer. Myoglobin binds oxygen more tightly than does hemoglobin.

BPG affects oxygen binding affinity by binding in a small cavity at the center axis of deoxygenated hemoglobin. It is held in place by a number of electrostatic interactions with positively charged residues in hemoglobin. In oxygenated hemoglobin, this cavity is too small to effectively accommodate 2,3-BPG. It is only in the deoxygenated state that this molecule has the ability to bind into this cavity. When bound, 2,3-BPG greatly diminishes the binding of oxygen to hemoglobin and facilitates oxygen unloading to actively respiring tissues.

When deoxygenated hemoglobin returns to the lungs, H+ is low. This causes H+ to be released from hemoglobin. An unborn baby’s lungs are undeveloped, un-inflated and filled with amniotic fluid. Instead, the developing fetus receives all of the benefits of breathing, including oxygen, with help from the mother.