Unit III- Hemoglobin Flashcards
1
Q
Life Needs Oxygen
A
- most living organisms need oxygen to carry out basic metabolic functions
- the rate of oxygen transport in tissues is inversely proportional to the square of the distance it must diffusion
- this makes the diffusion rate of Oxygen through tissue thicker than ~1mm too slow to support life
- the evolution of larger organisms required the aquisition of oxygen carriers and circulatory systems
- hemoglobin is the predominant oxygen carrier in the circulatory system
- myoglobin in an intracellular oxygen transport and storage protein
2
Q
Oxygen transport the big picture
A
- Atomosphere: 760 Torr
- Blood in alveolar capillaries: 100 torr
- Blood in muscle capillaries: 20 torr
- cytoplasm of myocyte: 5 torr
- Km of Cyt oxidase for O2- <1 torr
- oxygen transport is downhill, we just need to speed up diffusion
3
Q
Myoglobin folding
A
- myoglobin is a small intracellular protein of 153 amino acids that is highly abundant in vertebrate muscle
- Mb has a globular shape with 44 X 44 X 25 A.
- the heme group is tightly wedged in a hydrophobic pocket formed between helices E and F
- myoglobin functions in intracellular transport and temporary storage of oxygen needed for aerobic metabolism of muscle
4
Q
Quaternary Structure
A
- hemoglobin is a tetrameric protein with the quaternary structure alpha2beta2 (dimer of alphabeta protomers)
- Hb has a globular shape with approximate dimensions 64 x 55 x 50 A
- each alpha-globin and beta-globin subunit is bound non-covalently to a prosthetic group (heme). So the tetrameric Hb molecule contains four hemes
ApoHb- empty Hb tetramer consisting of alpha1beta1, alpha2beta2
HoloHb- is ApoHb + 4 Hemes
-4 hemes = 4 O2 binding sites
5
Q
Assembly of hemoglobin
A
- hemoglobin monomers first assemble into rather stable, rigid alphabeta heterodimers. Two of these dimers come together to form a tetramer. The association between two alphabeta dimers in the tetramer is loose and flexible
- hemoglobin an switch between T (low affinity for O2) and R (high affinity)
- this is the basis for the cooperativity resulting in the sigmoid binding curve
6
Q
Heme: the prosthetic group
A
- heme is a big heteroxycle with Fe coordinated in the center
- Heme is Fe-protoprophyrin IX, which in turn is a porphyrin with the specific substituent side chains depicted
- prophyrin is a heteroxyclic ring system- with four pyrrole rings linked by briding carbons
- the nitrogens of each pyrrole faces inwards to the center, creating a metal chelating site where the Fe binds
- four methyl, two vinyl, two proprionate substituent groups
7
Q
deoxyHb
A
- hemoglobin with no oxygen bound
- the iron is 5-coordinate, coordinated by 5 nitogen atoms, four from pyrroles, one from the Proximal Histidine side chain
- this iron has electronic state Fe (II) which is known as ferrous
- the distal hisidine is too far away and there is an empty space on the distal side of Fe
8
Q
O2 binds the heme iron from the side opposite the proximal histidine
A
- O2 binds with one of its atoms providing the 6th ligand of Fe
- the other atom of O2 forms a H-bond with the distal histidine, which thus increases the affinity for O2
- in turn, O2 is bound between the Fe (II) and Histidine E7 also known as distal histidine
9
Q
Binding of Oxygen to Heme
A
-oxygen binding to the heme group has dramatic consequences for the physciochemical properties of Hb. First of all O2 changes the absorption properties of Hb and consquently its color
DeoxyHb- dark red
OxyHb- brilliant scarlet
-small molecules like CO, NO, and H2S bind the heme with higher affinity than oxygen, acting as poisons
10
Q
Hemoglobin nomenclature
A
- apo/holo refers to whether or not the cofactor is present
- ferro/ferri indicates reduced/oxidized iron
- oxidation state refers to whether the Fe is Fe2+ (ferrous) or Fe3+ (ferric)
- oxygenation state- whether oxygen is bound or not and is close to the coordination state
- deoxyhemoglobin is ferrous. Ferric hemoglobin is called met-hemoglobin and does not function as oxygen carrier
11
Q
Oxygen dissociation curves of Hb (and Mb)
A
- O2 concentration in our circulatory system is:
- ~20-30 torr in venous blood (depends on level of exercise)
- ~100 torr in arterial blood
- P50 is the pressure at which binding is half-maximal, or the pressure value at which 50% of the maximal O2 load has been release- 26 torr in hemoglobin, 2.6 torr in myoglobin
- Mb hyperbolic curve, Hb sigmoidal curve (cooperative interaction)
- Mb binds O2 under conditions in which Hb releases it
12
Q
Cooperativity
A
- Hb displays a characteristic sigmoidal binding curve. What does it mean?
- The sigmoidal shape of this curve can be thought as the convulution of two molecular states: the high affinity of the R state, the low affinity of the T state
- the two Hb states are in equilibrium and O2 binding drives the T R conversion
- at high O2 concentration the dissociation curve is close to the R curve; as O2 concentration decreases, it approaches the T curve
13
Q
Regulation of Hemoglobin in vivo
A
- the cooperativity of O2 binding to Hb is a classical model for allosteric interaction
- in the case of Hb, binding of O2 to one site increases the affinity of other binding sites on the same protein. Other small molecules that bind to Hb decrease the affinity for O2
- the allosteric nature of O2 binding results from the peculiar quaternary structure of the tetramer which is eqilibrium between two forms (T and R as oxy)
- positive effectors- O2
- negative effectors- BPG, CO2, H+, and Cl-
- myoglobin, which is a monomer, has no T/R transition and shows no allosteric effects
14
Q
Allostery and Cooperativity
A
Allostery: refers to an effect of something happening at another site on the active site. The other site can be another active site in a multimeric protein
Cooperativity: a type of allostery in which what is happening at one site promotes the same thing happening at another identical site, as oxygen binding at one site increases the affinity of the other sites
15
Q
Allosteric Regulation of Hemoglobin
A
- positive effectors shift the O2 dissociation curve to the left
- negative effectors shift the O2 dissociation curve to the right
