Lecture 7 Flashcards
Myoglobin and Hemoglobin
Big Picture Items
• Myoglobin contains a heme group where an Fe(II) binds O
• Hemoglobin contains four myoglobin-like subunits
• Myoglobin’s oxygen binding curve is hyperbolic, while hemoglobin’s is S-shaped
• Conformational changes occur in hemoglobin upon
oxygen binding
• BPG increases oxygen delivery at high altitude by decreasing hemoglobin’s affinity for oxygen
• Hemocyanins perform the same function as hemoglobin through a very different mechanism
• Sickle cell anemia arises from a mutation in hemoglobin that causes formation of fibers.
Myoglobin
Waiting in muscle cells for oxygen to arrive
Heme group in red with spherical Fe(II) ion in center.
- The eight helices are labeled A to H.
- Helix-connecting loops are AB, BC, etc
Oxy-myoglobin’s heme group with Fe(II) binding oxygen
Histidine F8 interacts with the Fe(II), as do four nitrogens of the heme group.
• In Oxy-Mb one oxygen atom of O2 also binds to the Fe(II).
• The Fe(II) in oxy-Mb is octahedrally coordinated by six ligands.
The key features shown of oxygen binding and heme group are the same in myoglobin and all four hemoglobin chains.
In hemoglobin and myoglobin, residue “F8” means: the 8th amino acid in helix F.
Myoglobin – the “Heme complex”
Heme group is packed between hydrophobic side chains Val E11 and Phe CD1
• When O2 is bound it also interacts with Nε2 of the imidazole of His E7.
• The oxygen binding pocket is quite tight
(several additional residues are not shown for clarity).
Oxygen Binding Curve of Myoglobin
General: The fractional saturation Y of a protein by ligand L, is the fraction of binding sites occupied by L relative to the total available sites.
Specific: The fractional oxygen saturation YO2 , is the fraction of O2-binding sites occupied by O2
pO2 is the partial pressure of O2.
(pO2 in venous blood is ~30 torr; 760 torr = 760 mm Hg = 1 atmosphere)
Hemoglobin
The oxygen transporter in the blood of most animals
It is usually embedded in red blood cells.
Hemoglobin (Hb) is much more sophisticated than myoglobin:
• Hb has four myoglobin-like subunits
• Two Hb chains are called α chains, the other two β chains
• The two α chains have the same amino acid sequence
• The two β chains have also the same sequence
• The α chain and β chain are ~57% different (in humans)
• As a result of cross-talk between these four subunits, the O2 binding curve of Hb is NOT hyperbolic but “S” (sigmoidal) shaped
• This is called cooperativity
• Increased efficiency of oxygen transport from lung to other organs.
• Its oxygen-affinity is pH-dependent.
• Its oxygen-affinity can be regulated “allosterically” by BPG.
Deoxy versus Oxy Hemoglobin
Shift in structure in the T R transition involves both tertiary and quaternary structure changes.
Deoxy-Hb “T-state”
Oxy-Hb “R-state”
- Note the change in size of the central cavity.
- Salt bridges are an important component of the interactions in Deoxy-Hb
O2 binding to heme in Hemoglobin
• Fe moves into the plane of the heme when O2 binds.
(The reason for that is fascinating - but you do not have to know)
• The heme becomes planar when O2 binds.
• No oxidation or reduction of Fe(II) occurs.
• Movement of His F8 acts like a lever and moves helix F
• Salt bridges are changed between the subunits.
• The subunits move relative to one another.
• The shift in pKa of the groups in the salt bridges causes proton release.
Changes in the heme upon O2 binding: the “trigger”
Upon oxygenation: the movement of the Fe2+ into the plane of the heme group, and the greater planarity of the heme after this move, is the “trigger” of the Tstate (Deoxy-Hb) to R-state (Oxy-Hb) transition
Oxygen-binding curves in Mb and Hb
The shapes of the curves are drastically different
The Mb curve is hyperbolic; the Hb curve is S-shaped (sigmoidal)
(The dotted curve is a hyperbolic O2-binding curve with the same P50 as Hb)
Hb picks up O2 in lungs and delivers O2 to muscle
with MUCH greater efficiency than Mb would be able to do:
Hb saturation varies between ~ 96% in the lung and 35 – 65% in the tissue.
(The dotted curve is a hyperbolic O2-binding curve with the same P50 as Hb)
The S-shaped curve of Hb-O2 binding can be described by the Hill equation.
The larger the Hill coefficient, the steeper the S-shaped oxygen binding Hill plot.
(Except for the name, you do not need to know specifics of the Hill plot for this course)
Allosteric Proteins – Symmetric model
Allosteric Effect:
• The binding of one ligand at one site affects the binding of another ligand at another site.
• This often, but not necessarily always, requires interactions between subunits of oligomeric proteins.
Four models of allosteric transitions in hemoglobin
• Sequential model
• Symmetry (“concerted”) model
• Multistate model: complex, combining features of sequential and symmetry models
• Dynamic model: changes due to dynamic properties rather than conformational changes per se
We will only consider the symmetry model of
hemoglobin allostery.
Allosteric Proteins – Symmetric model
The symmetric model assumes:
• An oligomer of symmetrically related subunits
• Each subunit can exist in two states, designated R and T;
• The ligand can bind to either conformation
• Molecular symmetry is maintained, i.e. the tetramer is either all squares or all spheres.
The key point is that hemoglobin cannot adopt an intermediate conformation.
The tetramer is either in the deoxy (T) state or in the oxy (R) state.
Allosteric Proteins – Sequential model
The sequential model assumes:
• Ligand binding induces conformational change in subunit to which it binds
• These conformational changes alter the neighboring subunits and their affinity for the ligand: cooperative interactions
• Subunits can adopt multiple conformations
• Symmetry needs not be maintained during ligand binding.
BPG : our friend at high altitudes
At high altitudes the [BPG] in human red blood cells (RBCs) increases quickly from ~ 4 mM to ~ 8 mM.
(Animals living at high altitudes, such as llama’s, have an other solution: they have hemoglobin variants with higher O2-binding affinities than their sea-level cousins.)
BPG binds at one mole BPG per mole deoxy-Hb tetramer
(Thereby decreasing the O2 affinity of Hb)
- BPG binds in the central cavity between the β-chains in deoxy Hb
- BPG does not bind to oxy-Hb.
- [BPG] is increased in short term altitude adaptation.
- The net effect of increased [BPG] is to decrease Hb’s affinity for O2.
- The net effect of increased [BPG] at high altitude is that the delivery of O2 from lung to tissue is almost as efficient as before at sea level!
BPG only binds to deoxyHb
