bio 98 - lecture 7 oxygen binding proteins myoglobin and hemoglobin
TRANSCRIPT
Bio 98 - Lecture 7
Oxygen Binding Proteins
Myoglobin and Hemoglobin
(Mb) (Hb)
Tetrapyrrole ring system
Protophorphyrin IX
Octahedral coordination
Heme biosynthesis
Octahedral coordination
Erythropoietic porphyrias are associated with accumulation of porphyrins in erythrocytes and are rare. The rarest is congenital erythropoetic porphyria (CEP) otherwise known as Gunther’s disease. The signs may present from birth and include severe photosensitivity, brown teeth that fluoresce in ultraviolet light due to deposition of type I porphyrins and later hypertrichosis. Hemolytic anemia usually develops.
Recent use: the comic book Ultimate Comics Avengers Vol. 3 #1 (October 2010), in which writer Mark Millar employed porphyria as the explanation for vampires in that fictional universe.
Werewolves?
His64
His93
N
C
I. Similarities and differences
Similarities• polypeptide length (~140 residues) & sequence (27 residues / 20% identical) are similar• presence of heme; mechanism of O2 binding
Differences Mb HbLocation: muscle bloodSubunits: 1 4Function: O2 reserve O2 transportO2 source: blood lungO2 destination: mitochondria peripheryO2 binding: simple complex
hyperbolic sigmoidal(cooperativity)
II. O2 binding curves
Y
(fraction of Mb with O2 bound, aka Mb:O2)
[O2]0
0.5
1.0
[O2]0.5
A. O2 binding curve for myoglobin (Mb)
How does the availability of oxygen or [O2] influence the amount of Mb that has oxygen bound to it?
Y = fraction of ligand binding site occupied (Mb:O2)
[Mb:O2] bound Mb(1) Y = ——————— = ————— [Mb:O2] + [Mb] total Mb
[Mb] [O2]Kd = ————— = dissociation constant
[Mb:O2] (just like the Ka of a weak acid)
(2) binding rxn: Mb:O2 Mb + O2
An equation of the form f(x) = x/(x + z) describes a hyperbola
[O2](3) Y = ————— Kd + [O2]
Kd = [O2] when Mb is half-saturated (Y = ½), i.e. when [Mb] = [Mb:O2].
At this condition, Kd = [O2], often written as [O2]0.5
Another name for Kd is [O2]0.5
Solve eq. 2 for [Mb:O2], then substitute into eq. 1 ...
[Mb] [O2]/Kd [Mb] [O2]
(1) Y = ————— = ————— [Mb] [O2]/Kd + [Mb] [Mb] (Kd + [O2])
Mb:O2 Mb + O2
Use pO2 (partial pressure) in place of [O2]
pO2
Y = ————— P50 + pO2
Buffering of O2: Mb:O2 Mb + O2
blood
mitochondria
[O2](3) Y = ————— [O2]0.5 + [O2] Y
B. O2 binding curve for hemoglobin
Hb has a different function - transport, not buffering
1. Hb needs to pick up O2 efficiently in the lung where pO2 is high (100 mm Hg or Torr = 13.3 kPa).
(1 Torr = 0.133 kPa)
2. Hb needs to drop off O2 efficiently in the tissues where pO2 is lower (30 mm Hg = 4 kPa).
3. Normal or “hyperbolic” binding won’t suffice; not enough difference in saturation when pO2 drops to only ~1/3 the maximal value (30 vs. 100 mm Hg).
p
O2[mm Hg]
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100 120
P50 = 2.8 mm Hg
Simple (hyperbolic) myoglobin-like binding does not suit a transport protein well.
pO2 in lung >
< pO2 in tissues
P50 = 26 mm Hg
Y[mm Hg]
The state with a higher affinity for O2 has a lower P50 value (aka [O2]0.5) compared to the P50 value of the lower affinity state.
8%
25%
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100 120
Sigmoidal (cooperative) binding does suit a transport protein well (Hb, magenta curve)
pO2 in lung >
< pO2 in tissues
YT-form
R-form
[mm Hg]O2p
55 %
Change in conformation of heme as Hb goes from T -> R state
His93
His93His93
Val68Val68Val68
T (Tense): lower affinity for O2 (deoxy state)R (Relaxed): higher affinity for O2 (oxy state)
Change in conformation of subunits in tetramer as Hb goes from T -> R state
Hb is an allosteric protein whose properties are affected by changes in quaternary structure, which are mediated by interactions with small molecules: i.e. O2 (and other effectors such as pH, see last two slides)
III. Models of Cooperativity
A. A simple 2-subunit model
B. 4-subunit models (Hb)
1. Concerted model
2. Sequential model
A simple 2-subunit model of cooperative ligand binding
B.1: Concerted model of the cooperative transition of Hb subunits: symmetry
Monod, Wyman, Changeux
T (Tense): lower affinity for O2 (deoxy state)R (Relaxed): higher affinity for O2 (oxy state)
B.2: Sequential modelof the cooperative transition of Hb subunits:induced fit
More oxygen bound. More and more subunits next to oxygen-occupied sites are switching to the strong-binding state.
Some oxygen bound. Each binding of an oxygen molecule favors the transition of adjacent subunits to the strong-binding state and promotes their binding of oxygen
Koshland, Nemethy, Filmer
T (Tense): lower affinity for O2 (deoxy state)R (Relaxed): higher affinity for O2 (oxy state)
Bohr Effect blood pH alters affinity of Hb for O2 in a useful way
lung
CO2 + H2O H+ + HCO3-
H+ and CO2 both bind to Hb and stabilize the T (deoxy) state:
H+ and CO2 are allosteric effectors
In the lungs, [CO2] is reduced (pH 7.6) compared to tissues (pH 7.2)
tissue
Y
70 % 40 %
BPG (2,3 bisphosphoglycerate) helps us adapt when climbing Mt. Whitney!
Y
BPG binds to positively charged groups stabilizing the T (deoxy) state.
Increasing [BPG] lowers [O2] affinity, i.e. increases the P50.
As you climb to higher altitude your body adapts to lower pO2 by increasing [BPG] from 5 to 8 mM.
