Chapter 9: Hemoglobin, an Allosteric Protein Flashcards
What is hemoglobin?
Hemoglobin is a tetramer consisting of two α subunits and two β subunits. Each subunit has a bound heme. It is an allosteric protein that displays cooperativity in oxygen binding and release. It is a red blood cell protein that carries oxygen from the lungs to the tissues.
Where does myoglobin bind oxygen? Is the binding of oxygen by myoglobin cooperative?
Myoglobin binds oxygen in muscle cells. The binding of oxygen by myoglobin is not cooperative.
How is oxygen binding measured?
Oxygen binding is measured as a function of the partial pressure of oxygen (pO2).
What is the partial pressure of oxygen in the alveoli of the lungs (in torr)?
100 torr.
What is myoglobin? What does it consist of?
Myoglobin is a single polypeptide chain consisting mainly of α helices arranged to form a globular structure. Can monomeric protein display cooperativity?
Where do myoglobin and hemoglobin bind oxygen?
Myoglobin and hemoglobin bind oxygen at a heme; a bound prosthetic group.
What does the heme group consist of?
The heme group consists of an organic component called protoporphyrin and a central iron ion in the ferrous (Fe2+) form.
How many bonding sites does the iron at the centre of the protoporphyrin have? What does the iron bond to at these sites?
- The iron is bound to four nitrogens, and can form two additional bonds called the fifth and sixth coordination sites. The fifth site is occupied by an imidazole ring called the proximal histidine. The sixth site binds oxygen.
What happens to the iron upon oxygen bonding?
Upon oxygen binding, the iron moves into the plane of the protoporphyrin ring.
What is the difference between deoxyhemoglobin and oxyhemoglobin in terms of the iron configuration? Do the properties of the iron change?
In deoxyhemoglobin the iron lies slightly out of the plane of the porphyrin, but moves into the plane on the oxyhemoglobin. The magnetic properties of the heme iron change when it moves into the plane of the protoporphyrin ring.
How do fMRI’s work? How is this used in medicine?
Functional magnetic resonance imaging (fMRI) can distinguish the relative amounts of oxy- and deoxyhemoglobin. It can be used to monitor activity in specific regions of the brain by measuring the increase in oxyhemoglobin.
How can the quaternary structure of hemoglobin best be described as?
The quaternary structure is best described as a pair of identical αβ dimers (α1β1 and α2β2).
Does deoxyhemoglobin correspond to the T state or R state of allosteric enzymes? How are the αβ dimers linked?
T state of allosteric enzymes, the αβ dimers are linked by an extensive interface.
What is the difference between T state and R state? Does the hemoglobin have to be in one of these states?
The T state (the tense state)(deoxyhemoglobin) has less of an affinity for oxygen than the R state (the relaxed state)(oxyhemoglobin). The hemoglobin must either be in its T state or R state.
When does the transition occur between deoxyhemoglobin (T) and oxyhemoglobin (R)? Where does the iron ion move? What moves with the iron? What does this structional change signal?
The transition from deoxyhemoglobin (T state) to oxyhemoglobin (R state) occurs upon oxygen binding. The iron ion moves into the plane of the heme when oxygen binds. The proximal histidine, which is a component of an α helix, moves with the iron. The structural change is communicated to the other subunits so that the two αβ dimers rotate with respect to each another, resulting in the formation of the R state.
