Transcript Oxygen binding proteins RESP 312

Oxygen Binding
Proteins
Oxygen Binding Proteins are
Hemoproteins
• Oxygen binding proteins (myoglobin and hemoglobin) are hemoproteins
• Hemoproteins are a group of specialized proteins that contain heme as a
tightly bound prosthetic group.
• Heme is a complex of protoporphyrin IX & ferrous iron (Fe2+) .
• Heme Fe2+ is held in the center of the heme molecule by bonds to the four
nitrogens of the porphyrin ring.
•
It can form two additional bonds, one on each side of the planar porphyrin ring.
- one of these positions is coordinated to a histidine of globin
- the other position is available to bind oxygen
OXYGEN
PORPHYRIN IX
RING
FERROUS
ION
HISTIDINE
The heme group: Fe2+ porphyrin complex with bound O2
Myoglobin
• Location:
heart & skeletal muscles
•
Function:
a reservoir for oxygen for muscles.
an oxygen carrier that increases the rate of transport of oxygen
within the muscle cell.
• Structure:
Myoglobin is composed of a single polypeptide chain that is
structurally similar to the individual subunit polypeptide chain of
hemoglobin.
~ 80 % of its polypeptide chain is folded into 8 stretches of α-helix.
These α-helical regions, labeled A to H
Myoglobin cont.
Globin Protein (Polypeptide Chain)
The interior almost entirely of nonpolar
amino acids.
Polar amino acids are located almost
exclusively on the surface of the
molecule.
Heme group
is located in a pocket in the
molecule between helix E and
helix F which is lined by nonpolar
amino acids with exceptions
are two histidine amino acids.
Myoglobin cont.
• The first is called the proximal histidine (F8), binds directly to the iron
of heme.
• The second is called the distal histidine (E7), does not directly interact
with the heme group, but helps stabilize the binding of oxygen to the
ferrous iron.
• The protein, or globin, portion of myoglobin prevents the oxidation of iron of
heme.
Myoglobin as a diagnostic tool
• Myoglobin is used as a marker for acute myocardial
infarction
• Advantage:
It is elevated in blood of patients with myocardial infarction
than other markers as CK-MB or troponin. So, it can
diagnose myocardial infarction attack at an early stage.
• Disadvantage:
myoglobin has a reduced specificity for diagnosing
myocardial infarction.
Hemoglobin
• Location:
exclusively in red blood cells
• Function:
main function is to transport oxygen,
H+ and CO2
• Structure:
•
Hemoglobin is composed of four polypeptide chains - two α chains and two
β chains – arranged into 2 dimers held together by noncovalent
interactions.
Each subunit has 8 stretches of α-helical structure, and a heme-binding
pocket (as for myoglobin)
The oxygen-binding properties of hemoglobin are
regulated by interaction with allosteric effectors
Quaternary structure of hemoglobin
•
The hemoglobin tetramer can be envisioned as being composed of two
identical dimers, (αβ)1 & (αβ)2 (1 & 2 are numbers)
•
The two polypeptide chains within each dimer are held tightly together,
primarily by hydrophobic interactions
•
In contrast, the two dimers are able to move with respect to each other,
being held together primarily by polar bonds.
•
The weaker interactions between these mobile dimers result in the two
dimers occupying different relative positions in deoxyhemoglobin as
compared with oxyhemoglobin
Quaternary structure of hemoglobin
cont.
TAUT
Structure
RELAXED
Structure
T
R
Structural changes resulting from
oxygenation & deoxygenation of
hemoglobin
Quaternary structure of hemoglobin
cont.
T form:
The two αβ dimers interact through a network of ionic bonds
that constrain the movement of the polypeptide chains.
The T form is the low-oxygen-affinity form of hemoglobin.
R form:
The binding of oxygen to hemoglobin causes the rupture of
some of the ionic bonds between the αβ dimers.
This leads to a structure called the “R,” or relaxed form, in
which the polypeptide chains have more freedom of movement
The R form is the high- oxygen-affinity form of hemoglobin.
Binding of oxygen to myoglobin & hemoglobin
• Myoglobin can bind only one molecule of oxygen (O2),
as it contains only one heme group.
• Hemoglobin can bind four oxygen molecules (one at
each of its four heme groups)
• The degree of saturation (Y) of these oxygen-binding
sites on all myoglobin or hemoglobin molecules can vary
between zero (all sites are empty) and 100% (all sites are
full
Oxygen dissociation curve
The oxygen dissociation curve is
a plot of saturation of binding sites with O2 in hemoglobin or myoglobin (Y)
measured at
Different partial pressures of oxygen (pO2)
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Myoglobin has a higher oxygen affinity at any pO2 value than
hemoglobin as:
In myoglobin:
pO2 needed to achieve 50% saturation of
binding sites is ~ 1 mm Hg
In hemoglobin:
~ 26 mmHg
[The higher the O2 affinity (i.e. more tightly bound O2), the lower the pO2 needed]
Allosteric effects
The ability of hemoglobin to reversibly bind oxygen is affected by:
•
•
•
•
•
pO2
pH
pCO2
2,3-bisphosphoglycerate (2,3 DPG)
CO
These are collectively called allosteric (“other site”)
effectors, because their interaction at one site on the
hemoglobin molecule affects the binding of oxygen to
heme groups at other locations on the molecule.
N.B.: The binding of oxygen to myoglobin is not influenced
by allosteric effectors.
1-
pO2 (Oxygen Concentration)
Loading & unloading of oxygen depend on pO2 (oxygen
concentration
1- pO2 in alveoli of lungs (i.e. concentration of O2) is high
So, affinity of HB to O2 is increased leading to saturation of hemoglobin with O2
(loading of oxgen)
2- pO2 in peripheral tissues is low
So, affinity of HB to oxygen is decreased leading to release of oxygen to peripheral
tissues (unloading of oxygen)
AT THE SAME TIME:
Myoglobin is designed to bind oxygen released from hemoglobin at low pO2
found in muscles (as myoglobin requires very low pO2 to be fully saturated
with oxygen)
2-
Bohr effect
• Bohr effect is the change of oxygen binding in
hemoglobin due to hydrogen ions (H+ or protons) and
CO2
• In peripheral tissues, H+ & CO2 are increased, which results to
increased release of oxygen from hemoglobin.
1- H+ released from metabolism of peripheral tissues
2- CO2 resulting from cellular metabolism is converted by carbonic
anhydrase to carbonic acid which is converted to bicarbonate & H+
• In both cases, H+ increases the ionic bonds  favoring of T form of
Hb   O2 release to the tissues
• Hb O2 (OxyHb, R form) + H+
↔ HbH (deoxy Hb, T form) + O2
So, increase in H+ shift equilibrium to right
CO2
• Most of CO2 produced during metabolism
• Some CO2 is carried as carbamate bound
to the uncharged amino group
• Hg-NH2+ CO2
Hb-NH-COO- + H+
• This stabilizes the T –form
2,3-bisphosphoglycerate
Effect of 2,3-bisphosphoglycerate on oxygen affinity:
• 2,3- Bisphosphoglycerate (2,3-BPG) is an important regulator of the binding
of oxygen to hemoglobin.
• 2,3-BPG is synthesized from an intermediate of the glycolysis.
Binding of 2,3-BPG to deoxyhemoglobin (T form):
• 2,3-BPG decreases the oxygen affinity of hemoglobin by binding to
deoxyhemoglobin but not to oxyhemoglobin.
• This preferential binding stabilizes the T conformation of deoxyhemoglobin.
Response of 2,3-BPG levels to chronic hypoxia or anemia:
 The concentration of 2,3-BPG in the red blood cell increases in response to
chronic hypoxia (in certain lung diseases or high altitude) or chronic anemia
(oxygen available to Hb is low in both cases)
• Elevated 2,3-BPG levels lower the oxygen affinity of hemoglobin, permitting
greater unloading of oxygen in the capillaries of tissues.
Binding of Carbon monoxide (CO)
• Carbon monoxide (CO) binds tightly (but reversibly) to the
hemoglobin iron, forming carboxyhemoglobin (carboxyHb)
Dangers of CO binding to hemoglobin (in CO poisoning)
1- The affinity of hemoglobin for CO is 220 times greater than for oxygen
(in availability of both, hemoglobin binds CO more)
2- When carbon monoxide binds to one or more of the four heme sites,
hemoglobin shifts to the relaxed conformation (R-form), causing the
remaining heme sites to bind oxygen with high affinity (tightly).
As a result, the affected hemoglobin is unable to release oxygen to the
tissues leading to tissue hypoxia.
CO poisoning is treated with 100% oxygen therapy to facilitate the
dissociation of CO from hemoglobin.
Factors favoring the T-form of Hb. are:
• Deoxygenation
• Low pH (  H+)
• CO2
• Lactic acid
• 2,3 bisphosphoglycerate
Factors favoring the R-form of Hb. are:
• O2
• CO