Transcript CHMI 2227E Biochemistry I

CHMI 2227E
Biochemistry I
Proteins:
-
Quaternary structure
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
hydrophobic

Quaternary structure involves several
polypeptides:


hydrophilic
Porin: a trimeric membrane-bound protein
Oligomers
Heteromers

These subunits interact with each
other through the usual weak
interaction forces (H bonds, Van der
Waals, ionic interactions, hydrophobic
interactions) and/or though disulfide
bonds;

For aquous proteins, frequently, but
not always, the interface between two
subunits is made of hydrophobic
amino acids.

For membrane-bound proteins, the
amino acids at the interface between
the subunits are usually hydrophilic;
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
Hemoglobin

Made up of 4 polypeptide
chains:
2 copies of a-subunit (or
HbA): yellow and blue;
 2 copies of b-subunit (or
HbB): red and pink


Each subunit binds its own
heme group: so each subunit
can bind O2

Each subunit is highly similar
in structure to myoglobin;

Both hemoglobin and
myoglobin bind O2 in a very
similar fashion
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
Hemoglobin HbA vs myoglobin
HbA
Myoglobin
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
Oxygen binding by Hb and myoglobin

4 major residues
surround the heme
group:




CHMI 2227 - E.R. Gauthier, Ph.D.
Phe 43
His 64
Val 68
His 93

These amino acids
create a hydrophobic
environment while help
hold the heme group in
place;

Also: His 93 binds the
Fe2+ atom;
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Quaternary structure
Oxygen binding by Hb and myoHb

O2 binds the Fe2+ atom of the heme
group, and is held in place with His 64;

Oxygen-bound myoglobin/Hb is called
oxymyoglobin/oxyHb

Oxygen-free myoglobin/Hb is called
deoxymyoglobin/deoxyHb

Now, if both Myo and Hb can bind O2,
why is it that Hb is a multimeric protein,
while myoglobin is monomeric???
WHY????
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
Oxygen binding by Hb and myoHb

O2 binding to myoglobin shows a
simple equilibrium where the
amount of O2 bound-myoglobin
(y) directly depends on the
concentration of O2 present;

However, O2 binding to Hb is
more complex:

At low O2 concentration, very
little Hb binds O2 even as the
concentration of O2 increases
(part A of the Hb curve);

However, at a certain threshold
of O2 concentration, Hb becomes
rapidly saturated with O2 (part B
of the Hb curve);
CHMI 2227 - E.R. Gauthier, Ph.D.
B
A
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Quaternary structure
Oxygen binding by Hb and myoHb

At high O2 concentrations, both
myoglobin and Hb are
saturated, meaning there are
no more O2-binding spots
available.

Interestingly: the affinity of
myoglobin and Hb for oxygen
varies by a factor of 10:
B

A
Only 2.8 Torr are required to
get 50% of myoglobin
saturated;
 However, 26 Torr are required
to half-saturate Hb.
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
O2 binding changes the 3-D shape of Hb

In the deoxyHb form, Fe2+ is
bonded to 5 ligands: His 93 and 4
amines from the heme group;

When one subunit of Hb binds O2,
the Fe2+ atom moves foward the
plane of the heme group, pulling
with it the His 93 and the a-helix;

This causes a slight but significant
change in the tertiary structure of
all the other Hb subunits, even if
they are in the deoxyHb form;
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
O2 binding changes the 3-D shape of Hb

The consequence of this slight
change in conformation is an
increase in the affinity of these
other Hb subunits for O2;

This phenomenon, where a change
in the shape in one subunit trigger
similar changes in other subunits of
the same molecule, is called
cooperativity;

Molecules exhibiting cooperativity
are also called allosteric
molecules;
http://upload.wikimedia.org/wikipedia/commons/0/07/Hb-animation2.gif
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
Hb is an allosteric protein

This phenomenon explains very well
the behaviour of Hb in the presence of
O2:

At low pO2, all of the Hb subunits in the
molecule are in the deoxy form with low
affinity for O2: they bind O2 very poorly;

At higher pO2, one of the 4 subunits
binds O2, changes its conformation to
the one with high affinity, and transmits
this change in 3D structure to the other
3 subunits;

The other 3 subunits, now having high
affinity for O2, readily bind the molecule
and rapidly become saturated.
O2 O2
O2 O2
O2
O2
O2
O2
O2
O2
O2 O2
O2 O2
Low affinity
High affinity
Saturated
CHMI 2227 - E.R. Gauthier, Ph.D.
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Quaternary structure
Why Hb is allosteric, while Myoglobin is not?

If Hb behaved like Myoglobin,
then most of the Hb molecules
would remained tightly bound
to O2 and would not unload O2
in tissues;

Conversely, if myoglobin
behaved like Hb, it would
readily let go of its O2,
drastically limiting our muscles
ability to perform aerobic work;
CHMI 2227 - E.R. Gauthier, Ph.D.
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The Bohr effect
The Bohr effect concerns the
observed decrease in O2
binding by hemoglobin when
the pH is lowered;

This effect explains why
hemoglobin binds O2 in the
lungs, and releases it in the
tissues;
CHMI 2227 - E.R. Gauthier, Ph.D.
http://www.aw-bc.com/mathews/ch07/fi7p16.htm

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Gas exchange
In the tissues
H2O
Glucose + O2
CO2
Carbonic
anhydrase
CO2
H2CO3
H2O + Cl-
Hb-4O2
H+
HCO3-
HCO3(to lungs)
ATP
Hb-H+
4O2
4O2
Erythrocytes
Plasma
CHMI 2227 - E.R. Gauthier, Ph.D.
Tissues
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Gas exchange
In the lungs
H2O
CO2
CO2
CO2
Carbonic
anhydrase
H2CO3
Air
H2O + Cl-
Hb-4O2
H+
HCO3-
HCO3-
Hb-H+
4O2
O2
Erythrocytes
Plasma
CHMI 2227 - E.R. Gauthier, Ph.D.
Lungs
O2
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