Lecture 17 + 18 Flashcards
General structure of myoglobin
- monomer
- stores oxygen
- high affinity for oxygen in lungs
- present in tissues
- unaffected by pH, [CO2], or (BPG)
- binds 1 O2 molecule
- doesn’t bind BPG
- 153 amino acids
- 77% alpha-helical, 8 alpha-helices, interior residues are non-polar except residue 7 of helix E (HisE7 and HisF8)
- exterior residues include both polar and non-polar aa
- binds to oxygen via a permanently bound cofactor/prosthetic group -> heme
- iron in ferras state -> reduced state otherwise would not bind to oxygen
General structure of hemoglobin
- heterotetramer: 2 alpha (141aa) 2 beta (146aa) subunits
- oxygen transport in all vertebrates and some invertebrates
- removes CO2 from tissues
- variable affinity for oxygen: high in lungs, low in tissue
- in tissues want low affinity for O2, need to release O2 at destination
- present in blood
- sensitive to pH, [CO2], and [BPG]
- binds 4 O2 molecules
- hemoglobin’s cooperative binding to O2
- hemoglobin’s allosteric regulation by CO2, H+, and BPG
Myoglobin and Hemoglobin
- critical proteins for our survival: oxygen transport and storage
- O2 not very soluble in aqueous solutions like blood and cant be transported freely to tissues- does not diffuse well across tissues -> must be transported to the tissues and stored there until needed
- partial pressure of oxygen low in tissues and high in lungs = more oxygen available
Structure of heme
- cofactor in hemoglobin and myoglobin
- heme= protoporphyrin IX + Fe2+
- iron storing transport molecules must be able to bind to O2, not allow it to oxidize to any other substance, and release it on demand
- several enzymes are required to synthesize heme
Octahedral coordination of Fe 2+ by heme, myoglobin, and O2
- capacity of globins to bind to oxygen depends on the presence of bound heme, which is responsible for the distinct red color of blood and muscles
- heme prosthetic group is wedged between the hydrophobic E and F alpha-helices
- Fe2+ is coordinated by His F8
- hydrophobic environment of the protein in the heme binding site keeps the iron in a reduced (Fe2+) form
Structural changes of hemoglobin upon O2 binding
T state - deoxy, low O2 affinity
R state - oxy, high O2 affinity
- R state has increased affinity for O2 - so binding of O2 at 2 subunits increases the affinity of other subunits for O2 - cooperativity
- R state differs from the T state by a rotation of about 15 deg of the a1b1 dimer with respect to a2b2 together with a shift that brings the beta subunits closer together and narrows central cavity
Oxygen binding initiates structural changes
- prior to binding o2 the Fe is outside the plane of the heme and heme is puckered
- upon binding of O2 the iron ion moves into plane of the heme -> flat
O2 binding curves for hemoglobin and myoglobin
- myoglobin - hyperbolic
- hemoglobin - sigmoidal due to cooperative binding
hemoglobin binds cooperatively
- very efficient since it permits full saturation of the protein in the lungs where pO2 is high and efficient O2 release in tissues, where pO2 is low
- hemoglobin has 2 states: deoxyhemoglobin (T state), oxyhemoglobin (R state)
If O2 binding was not cooperative
O2 would either bind well at high pO2 but not release well at low pO2 or release well as low pO2 but not bind well at high pO2
Comparison between hemoglobin and myoglobin affinities
- ensures that O2 bound to hemoglobin in the lungs is released to myoglobin in the muscles
- oxygen delivery system is efficient because the tissue pO2 corresponds to the part of the hemoglobin binding curve where the O2 affinity falls off the most sharply
Compounds that influence oxygen binding to hemoglobin
- CO, H2S, CN- are very toxic, they bind to hemoglobin and block O2 binding
- BPG reduces O2 affinity of hemoglobin
- reduction in pH (found in respiring cells) reduces O2 affinity of hemoglobin
- release (accumulation) of CO2 in respiring tissues reduces O2 affinity of hemoglobin
- higher concentrations of H+ and CO2 in respiring tissue help to stabilize the deoxygenated T state of HB, promotes the release of O2 and works against rebinding of O2
