Oxygen Transport II

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Oxygen Transport (II)
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2
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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Oxygen dissociation curve for Mb
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Mb O2 ltgt Mb-O2
By definition, Substituting from the
expression for Kd
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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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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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• 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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Oxygen dissociation curve for Hb
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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
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Oxygen dissociation curve for Hb
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Limiting Cases Note that for Hb, L 3 x 105 and
c 0.01. 1. Low oxygen concentration (? 1,
i.e. S KR)
This is a hyperbolic equation describing low
affinity binding.
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Oxygen dissociation curve for Hb
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Limiting Cases 2. High oxygen concentration (?
gtgt 1, i.e. S gtgt KR)
This is a hyperbolic equation describing high
affinity binding.
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Oxygen dissociation curve for Hb
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The observed oxygen-binding curve for Hb shows
that over the range of oxygen partial pressures
at which the molecule has to operate (between
0.13 atm in the lungs and 0.026 atm in the
tissues), there is a large ?Y.
Thus, Hb is well tuned to its transport
function.
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Molecular mechanism of Hb co-operativity
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The tense state is stabilised by the binding of
the negatively charged co-factor, DPG.
DPG
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Molecular mechanism of Hb co-operativity
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DPG plays a major role in T-state stabilisation
in the absence of DPG, Hb switches to the R-state
(high affinity) and is not capable of displaying
co-operativity.
Hb without DPG
Y
Hb
a