Biochem Lecture 4 Flashcards
(28 cards)
What does Myoglobin do and where is it found?
It is present in heart and skeletal muscle where it binds and stores O2, releasing when needed.
What does hemoglobin do and where is it found?
In erythrocytes where it transports O2 and CO2 to and from the tissues and lungs.
What is the structure of Myoglobin?
It is a monomer that is globular and very compact. Interior has many nonpolar amino acids.
How does myoglobin bind oxygen?
It has a globin fold domain in a crevice in which there is one heme group that binds one O2 molecule.
What is the structure of hemoglobin?
It is a tetramer with two alpha globin and two beta globin polypeptide chains. It is a tetramer of two identical dimers.
What is a prosthetic group?
It is a small molecule tightly attached to a protein and essential for its function. In Mb and Hb, the heme group is a prosthetic group that allows them to bind O2.
Why is the heme group necessary for Hb and Mb to bind O2?
When O2 and iron bind alone, it is irreversible. The heme group around the iron atom allows the O2 binding to be reversible.
What is the name for the shape of the Hb O2 dissociation curve?
Sigmoidal. It is kind of an S shape.
Which molecule has a higher affinity for O2, Hb or Mb?
Myoglobin does. This allows it to be a more efficient storage molecule for O2.
What are the two states of Hb conformations?
“T” for taught and “R” for relaxed.
In the T conformation, Hb has a low affinity for O2 and is called deoxyhemoglobin.
In the R conformation, Hb has a higher affinity for O2 and is called oxyhemoglobin.
How does Hb affinity for O2 change as it goes between conformations?
As O2 molecules bind to Hb, the heme groups holding the Iron atoms flex and become planar. This pulls on the Histidine group attached to the iron atom and changes the tertiary and quaternary structure of the Hb molecule. The more O2 that binds, the more stable the new conformation becomes.
Binding of O2 changes Hb from which conformation and into what new conformation?
Hb is in the T state when no O2 is bound and turns into the R state as O2 binds.
What is the phenomenon known as cooperativity?
The binding of a ligand to one site affects the binding of the ligand to another site. O2 does this to Hb and is apparent by the sigmoidal shape of its binding curve.
What is positive cooperativity?
Ligands that increase affinity for additional ligands is positive cooperativity.
What is allosteric protein regulation?
It is the regulation of protein activity through the binding of a ligand called an effector. Their binding causes a conformational change that affects other parts of the protein. This can only happen in proteins with quaternary structure. Allosteric proteins have sigmoidal binding curves like Hb.
What is the difference between homotropic and heterotropic effectors?
Homo means the actual ligand is the same as the effector. Hetero is different.
What are negative effectors and what is their effect on the binding curve?
They have a negative effect on the action of the protein. They are inhibitors. They shift the binding curve to the right for the original ligand.
What are positive effectors and their effect on the binding curve?
Positive effectors increase a protein’s ability to do its job. They shift a binding curve to the left.
What are the negative and positive allosteric effectors for Hb?
Negative: 2,3-BPG, CO2, Temperature, H+ (pH Bohr Effect)
Positive: CO, O2
How does 2,3 BPG affect Hb?
It binds to the two beta subunits and stabilizes Hb into the T state where it has a lower affinity for O2. This is important to allow Hb to release more O2 in the tissues. Fetal Hb doesn’t have beta units so this mechanism can’t work.
How does temperature affect Hb binding to O2?
Increased temperature has a negative effect which helps the Hb to unload O2 in the tissues during exercise.
How does CO2 affect Hb?
CO2 binds to the N terminus of the amino acid chain and has a negative effect on Hb. Some of the CO2 in the body is transported in this way to the lungs. The majority is transported as bicarbonate (HCO3-).
How does pH affect Hb binding to O2?
Increasing [H+] stabilizes the deoxy form of Hb. It therefore helps unload O2 in the tissues.
How does H+get produced in the tissues and how does Hb interact with this reaction?
Metabolically active tissues produce CO2 which reacts with water to form H2CO3 which converts easily into HCO3- and H+. Hb then acts as a buffer and accepts protons. This stabilizes Hb in the T state.
