chmi 2227e biochemistry i

CHMI 2227 - E.R. Gauthier, Ph.D. 1

CHMI 2227EBiochemistry I

Proteins:- Quaternary structure


Quaternary structure


Quaternary structure Quaternary structure involves several

polypeptides: 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;

Porin: a trimeric membrane-bound protein

hydrophobic

hydrophilic


Quaternary structureHemoglobin

Made up of 4 polypeptide chains: 2 copies of -subunit (or

HbA): yellow and blue; 2 copies of -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


Quaternary structureHemoglobin HbA vs myoglobin

HbA Myoglobin


Quaternary structureOxygen binding by Hb and myoglobin

4 major residues surround the heme group: 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;


Quaternary structureOxygen 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????



A

B

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);



A

B

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: Only 2.8 Torr are required to

get 50% of myoglobin saturated;

However, 26 Torr are required to half-saturate Hb.


Quaternary structureO2 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 -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;


Quaternary structureO2 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


Quaternary structureHb 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

O2O2

Low affinity High affinity Saturated

O2 O2 O2


Quaternary structureWhy 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;


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;

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Gas exchangeIn the tissues

TissuesErythrocytes

Glucose + O2

ATP

CO2

CO2H2O

Carbonic anhydrase

H2CO3

HCO3-

H+

Plasma

HCO3-

(to lungs)

Hb-4O2

Hb-H+

4O2 4O2

H2O + Cl-


Gas exchangeIn the lungs

LungsErythrocytes

CO2H2O

Carbonic anhydrase

H2CO3

HCO3-

H+

Plasma

HCO3-

Hb-4O2

Hb-H+

4O2 O2

H2O + Cl-

CO2

Air

CO2

O2

chmi 2227e biochemistry i

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