Oxygen Transport

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Oxygen Transport
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blood myoglobin haem group dissociation curves
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Oxygen transport and the role of blood
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• To use oxygen, aerobic organisms must overcome
• the low aqueous solubility of oxygen (about 1 mM)
• its slow diffusion rate (oxygen will travel
  about 0.06 mm in one second by diffusion).
• Large, aerobic organisms solve these problems by
  using hemoglobin in the blood to increase oxygen
  solubility and by pumping the blood to all parts
  of the body using the circulatory system (heart,
  lungs, blood vessels).

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Composition of Blood red cells
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The average human body contains 5 litres (11
pints) of blood. About half of blood volume is
made up of cells the remaining volume is known
as blood plasma. The blood cells originate in
the bone marrow.

• Red blood cells (erythrocytes)
• flat, biconcave disks (7 µm in diameter)
• carry O2 in and CO2 out
• contain high concentrations of theoxygen-binding
  molecule haemoglobin
• population 5 x 106 cells/mm3
• about 3 x 106 cells replaced per second

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Blood as an oxygen transporter
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Oxygen is 50 times more soluble in blood than in
water (thanks to the red blood cells). To
transport the blood around the body, the heart
pumps blood to the lungs where it is oxygenated
(carbon dioxide is released and oxygen is taken
up). Oxygenated blood returns to the heart and is
pumped via the arteries to all parts of the body
where it gives up its oxygen and picks up carbon
dioxide. The blood returns via the veins to the
heart to complete the cycle.
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Myoglobin (Mb) - oxygen storer
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Tissues which require a readily accessible store
of oxygen (e.g. muscles) contain an
oxygen-storage molecule, myoglobin. We will first
examine the properties and function of myoglobin
before examining haemoglobin which is, in
essence, a specialised oligomeric form of
myoglobin.

• Properties of Mb
• single polypeptide chain (monomer)
• 153 amino acids (17.8 kDa)
• globular (45Å x 35Å x 25Å)
• 75 ?-helix 8 helices in all, labelled A-H (no
  ?-sheet)

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Myoglobin (Mb) - oxygen storer
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• Properties of Mb
• contains a non-covalently bound prosthetic group
  haem
• oxygen binds reversibly to an iron atom (Fe) at
  the centre of haem

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Special features of myoglobin
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1. Isolated haem can bind oxygen but in doing so
risks having its Fe oxidised from Fe(II) to
Fe(III), a form of iron that no longer binds
oxygen. This reaction requires the formation of
a haem-oxygen-haem sandwich. Mb encloses haem in
a deep cleft and thus sterically hinders the
formation of this sandwich intermediate and
prevents oxidation to Fe(III).
Picket-fence Fe porphyrin
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Special features of myoglobin
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2. The binding environment of the haem reduces
its affinity for carbon monoxide (CO) from 25,000
to 200 times greater than its affinity for
oxygen. Under normal conditions about 1 of Mb
is occupied by CO rather than O2.
Distal Histidine
ProximalHistidine
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Special features of myoglobin
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2. (cont.) In particular, the presence of the
distal His on one side of the haem disfavours the
preferred binding orientation of CO - weakening
its interaction but has no effect on oxygen
affinity
DistalHistidine
ProximalHistidine
CO preferred orientation
O2 preferred orientation
CO forced orientation
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Oxygen dissociation curve for Mb
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We observe a hyperbolic binding curve. Y is
defined as the fractional occupancy of O2 binding
sites on Mb.
Let us consider the binding equilibrium Mb
O2 ltgt Mb-O2 The equilibrium dissociation
constant is
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Oxygen dissociation curve for Mb
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By definition, Substituting from the
expression for Kd At O2 Kd, Y 0.5.
Thus Kd represents the concentration at which Mb
is half-occupied. Thus, since O2 ??pO2, we can
write where p50 is the partial pressure of
oxygen at which Mb is half-occupied.
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Oxygen dissociation curve for Mb
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For Mb, p50 0.0013 atm (1 torr). In humans,
pO2(lungs) 0.13 atm and pO2(tissue) 0.026
atm Mb would be an inefficient O2 transporter
(less than 4 of the oxygen picked up in the
lungs is released in the tissue) because it binds
O2 too tightly. It stores a reservoir of oxygen
in cells that are likely to need rapid access to
it under certain conditions thus Mb will release
O2 under conditions of strenuous exercise when
the tissue concentration drops to very low levels.
Ylungs 0.990Ytissue 0.952 DY 0.038
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• Oxygen transport
• Haemoglobin - oxygen transporter
• Structure
• Oxygen binding curve
• Monod-Wyman-Changeux model of oxygen binding
• Molecular mechanism of oxygen-binding

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Haemoglobin (Hb) - oxygen transporter
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Haemoglobin is similar in some ways to myoglobin
but has a number of important differences. Hb is
an ?2?2 tetramer (4 polypeptide chains) 2 ?
chains (141 amino acids) 2 ? chains (146 amino
acids)
Max Perutz (1914-2002)
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Haemoglobin (Hb) - oxygen transporter
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• and ? have about 20 sequence identity with
  myoglobin and are structurally homologous to it.
• Both chains possess a haem group and bind oxygen
  in the same way. Haemoglobin can bind up to 4
  molecules of oxygen (one per monomer).

Haemoglobin
Myoglobin
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Haemoglobin (Hb) - oxygen transporter
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Hb binds a co-factor, 2,3-diphosphoglycerate
(DPG) at the ??? interface.
DPG
DPG
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Haemoglobin (Hb) - oxygen transporter
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The oxygen binding curves for Hb and Mb are
significantly different.
Y
Lungs
Tissue

• In contrast to Mb, Hb
• binds O2 less tightly
• displays co-operativity (i.e. binding of one
  molecule of O2 increases the affinity for
  subsequent O2 binding).
• How do we account for this behaviour?

1.0
Mb
Hb
0.5
0.0013
0.026
0.13
Oxygen partial pressure (atm)
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Oxygen dissociation curve for Hb
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The co-operative binding of oxygen to Hb is an
example of allostery. Allosteric effects can
arise when the binding of one molecule to a
protein alters the binding of subsequent
molecules. Allosteric effects regulate protein
function and arise because of communication
between binding sites on the molecule. We can
understand the allosteric behaviour of
haemoglobin using the Monod-Wyman-Changeux model.
Jacques Monod
Jean-Pierre Changeux
Jeffries Wyman
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Oxygen dissociation curve for Hb
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• Monod-Wyman-Changeux model
• The model makes a number of simple assumptions
• The protein is an oligomer (gt1 polypeptide chain)
• The protein can exist in 2 states Tense (T) and
  Relaxed (R)
• T-state has low affinity for oxygen (KT large)
• R-state has high affinity for oxygen (KR small)
• All the subunits of any one molecule are either
  in the T-state or the R-state (concerted model)

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Oxygen dissociation curve for Hb
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We wish to develop an expression for Y (the
fractional number of sites occupied) as a
function of the oxygen concentration.
Definition of terms in the kinetic
scheme S substrate (in this case, S
O2) KT oxygen dissociation constant for T-state
Hb KR oxygen dissociation constant for R-state
Hb L equilibrium constant for the conversion of
T to R L T0/R0 L1 T1/R1, L2
T2/R2, etc.
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Oxygen dissociation curve for Hb
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We can analyse each binding step. Consider
first, the binding of S to R0
The 4 is included in the numerator because there
are 4 free binding sites on R0. In other words,
the concentration of free binding sites is 4R0.
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Oxygen dissociation curve for Hb
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Now consider the binding of S to R1
This time, we have 3 in the numerator (because
there are 3 free binding sites on R1) and 2 on
the denominator (because there are 2 occupied
sites on R2, either of which might dissociate to
yield R1).
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Oxygen dissociation curve for Hb
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We can treat the binding equilibria involving R3
and R4 in the same way
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Oxygen dissociation curve for Hb
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We can then use exactly the same approach to
analyse the binding equilibria involving S and
T-states
Y is defined as the fraction of occupied sites,
so
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Oxygen dissociation curve for Hb
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To simplify this expression, we can substitute
using our expressions for KT and KR and make the
following definitions
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Oxygen dissociation curve for Hb
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This gives us
Plotting Y against ? ,we get