Question

1. How does the quaternary structure of hemoglobin influence its more efficient offloading of O2 in tissues compared to myoglobin?

2. How does the Kd of hemoglobin for O2 change in the increasingly lower pH environment of actively fermenting muscles during a sprint?

3. If muscles ferment glucose to lactate during a sprint, what biochemical process ultimately uses that lactate to produce new glucose and in what tissue is it active?

Answer 1

Ans 1) Hemoglobin is a quaternary structure and myoglobin is monomeric structured protein .Hemoglobin has 4 chains of different types named alpha, beta, delta and gamma but, myoglobin has only single polypeptide chain .In hemoglobin when oxygen bind to one sub unit of hemoglobin, it changes into the 4 chain structure and makes easier for other molecules of oxygen to be carried along with it. This feature helps for unloading of oxygen to tissue also because when one molecule is offloaded other molecules can be offloaded easily because of this tetrameric structure.

Ans 2) During sprinting the oxygen demand of actively fermenting muscles is high so it requires more offloading of oxygen from blood to the working muscles. Once the pH decreases (increase acidity) the affinity of hemoglobin for oxygen decrease in the micro circulation .This leads to increase offloading of oxygen to the working muscles. Acidosis causes right shift in oxyhemoglobin dissociation curve and facilitate oxygen delivery to tissues. Kd of hemoglobin is a disassociation constant which will be high in case of lower blood pH .

Ans 3) lactate is formed during sprint as a result of breakdown of glucose in the absence of oxygen .This lactate is then used in the same tissues where they got formed as energy source or it transported to other tissues via blood stream and used for energy . Body use lactate as a mediator for metabolizing carbohydrate .Once they are produced as result of breakdown of carbohydrate then it goes back to the general circulation and reaches the liver. In liver it is used for making liver glycogen .The heart slow-twitch fibers and respiratory muscle use lactate as fuel during exercise. The metabolic pathway of conversion of lactate to glucose is called cori cycle.