Hemoglobin and Myoglobin: Structure, Function, and Oxygen Binding Mechanisms

What is the primary function of myoglobin?

To bind and store oxygen in muscle tissue.

What is the primary function of hemoglobin?

To transport oxygen from the lungs to the tissues.

What structural feature do myoglobin and hemoglobin share?

Both are homologous proteins that contain heme and bind oxygen.

What is the structure of myoglobin primarily composed of?

α helices.

What is the heme group composed of?

Protoporphyrin and a central iron atom.

In what form must iron be to bind oxygen?

Ferrous (Fe2+) form.

What is the sixth coordination site of iron in myoglobin used for?

Oxygen binding.

What happens to iron in deoxymyoglobin?

Iron lies slightly outside the plane of the porphyrin ring.

What is oxyhemoglobin?

Hemoglobin in its oxygen-bound form.

What does the binding curve of myoglobin indicate?

It is hyperbolic, showing the relationship between fractional saturation and partial pressure of oxygen (pO2).

What does P50 represent in relation to myoglobin?

The partial pressure of oxygen at which half of the myoglobin molecules are bound to oxygen.

How does myoglobin discriminate between oxygen and carbon monoxide?

The distal histidine creates steric constraints that favor oxygen binding over carbon monoxide.

What is the effect of carbon monoxide on myoglobin?

It binds much more strongly than oxygen, potentially interfering with oxygen transport.

What is the structure of human hemoglobin?

A tetramer consisting of two identical α chains and two identical β chains.

How many oxygen molecules can hemoglobin bind?

Four oxygen molecules.

What type of binding curve does hemoglobin exhibit?

A sigmoidal oxygen-binding curve.

What does cooperative binding in hemoglobin allow?

Increased likelihood of oxygen binding or release at one site affecting another site.

How does hemoglobin's saturation change from lungs to tissues?

It drops from ~98% in the lungs to ~32% in the tissues.

Why is myoglobin not efficient for oxygen transport?

It binds oxygen too tightly, remaining nearly saturated in both lungs and tissues.

What is the significance of cooperative binding in hemoglobin?

It allows hemoglobin to deliver significantly more oxygen to tissues than myoglobin.

What is the P50 value for human myoglobin?

Approximately 2 torr.

What happens to oxygen delivery during exercise?

Cooperative binding allows for increased oxygen delivery to tissues that need it most.

What is the pO2 in resting muscle?

~40 torr

What happens to pO2 during exercise?

It is reduced to ~20 torr.

What percentage of oxygen is released from lungs to resting muscle?

21% over a drop of 60 torr.

What percentage of oxygen is released from resting muscle to exercising muscle?

45% over a drop of 20 torr.

What is the significance of the steep part of the oxygen-binding curve?

It corresponds to the change in oxygen concentration from rest to exercise, allowing effective oxygen delivery to tissues.

What is cooperative binding in hemoglobin?

It is when the binding of oxygen at one site influences the oxygen-binding properties at other sites.

What are the two states of hemoglobin?

T (tense) state and R (relaxed) state.

What structural change occurs when oxygen binds to hemoglobin?

Two αβ dimers rotate 15 degrees relative to each other.

What does the T state of hemoglobin represent?

The quaternary structure of deoxyhemoglobin.

What does the R state of hemoglobin represent?

The quaternary structure of oxyhemoglobin.

How does oxygen binding affect hemoglobin's affinity for oxygen?

It increases the oxygen-binding affinity of the other sites.

What role does 2,3-bisphosphoglycerate (2,3-BPG) play in hemoglobin function?

It stabilizes the T state and facilitates oxygen release.

How does BPG affect the oxygen affinity of hemoglobin in red blood cells?

It lowers the oxygen affinity compared to pure hemoglobin.

What is the effect of BPG on oxygen release in tissues?

It allows ~66% oxygen release with BPG compared to ~8% without BPG.

How does fetal hemoglobin differ from adult hemoglobin?

Fetal hemoglobin contains two γ chains instead of β chains, giving it a higher affinity for oxygen.

What effect does lower pH have on hemoglobin's oxygen affinity?

It decreases the oxygen affinity, facilitating oxygen release.

How do hydrogen ions and carbon dioxide affect hemoglobin?

They act as allosteric effectors that promote oxygen release.

What happens to pH when CO2 reacts with water in the blood?

It forms bicarbonate and hydrogen ions, resulting in a drop in pH.

What is the concerted model of hemoglobin cooperativity?

The assembly exists only in the T state or R state, with O2 binding shifting the equilibrium between these states.

What is the sequential model of hemoglobin cooperativity?

O2 binding to one site increases the binding affinity of neighboring sites without full conversion to the R state.

What genetic mutation causes sickle-cell anemia?

A mutation in the β chains of hemoglobin where glutamate is replaced by valine at position 6.

What is hemoglobin S (HbS)?

The mutated form of hemoglobin associated with sickle-cell anemia.

What are the consequences of hemoglobin aggregation in sickle-cell anemia?

It distorts red blood cell shape, blocking capillaries and impairing blood flow.

How does the mutation in sickle-cell anemia affect hemoglobin solubility?

It substantially decreases the solubility of deoxyhemoglobin.

What is the role of the C-terminal end of the α helix in hemoglobin's T-to-R transition?

Its change in position favors the T-to-R transition upon oxygen binding.

What is the impact of CO2 on hemoglobin's oxygen delivery?

It decreases oxygen affinity, increasing delivery from 77% to 88% at pH 7.2.