General Index


Hemoglobin: Structure and Function


In this demo we'll use two models: In the left panel a model of Deoxyhemoglobin, that is, the T-state of the system. In the right panel, a model of Carbomonoxyhemoglobin, structurally identical to Oxyhemoglobin, the R-state of the system.


1. Introduction

Hemoglobin is an allosteric system that follows the Monod-Wyman-Changeux model. Hemoglobin is a tetramer formed by four subunits. In the most abundant adult hemoglobin, Hemoglobin A1, these subunits are called a and b. Thus, quaternary structure of hemoglobin is described as a2b2.

Subunit a has 141 aminoacids, and subunit b 146. Both have a strong sequence homology with one another and with other oxygen transporter, Myoglobin, present in muscle cells. Myoglobin and hemoglobin subunits have a prosthetic group consisting in a Protoporphyrin IX coordinated to a ferrous ion in a complex of octahedral symmetry. Four of this coordinate bonds are established with the porphyrin. The other two with (a) Nitrogen of His 87 (a subunit) or His 92 b subunit; (b) an oxygen molecule, the physiological ligand, in the case of oxyhemoglobin, or remains void in the case of deoxyhemoglobin. The protein is loaded with oxygen in the lung alveoli and discharges it in the peripheral tissues.

Hemoglobin is an Allosteric System:

1. It presents positive cooperativity in oxygen binding, that gives sigmoid saturation curves (homotropic effect). This positive cooperativity does not appear either in isolated subunits or in myoglobin; then it can be concluded that depend on quaternary structure and on intersubunit interactions.

2. Cooperativity may be affected by other ligands (heterotropic effects). Thus, the proton H+, carbon dioxide CO2 and 2,3-bisphosphoglycerate enhance the sigmoid character of the dissociation curve, behaving as Allosteric Inhibitors.

3. The system is composed of four protomers (the four chains). Being slightly different, the a and b subunits behave as if they were identical.

4. The T-state corresponds to deoxyhemoglobin; the R-state to oxyhemoglobin. In the R-state, the sixth coordination position of the ferrous ion is occupied by an oxygen molecule. Other ligands, such as carbon monoxide CO, have the same effect as oxygen. Thus, carbomonoxyhemoglobin is structurally equivalent to oxyhemoglobin.

Thanks to the classical studies of Perutz we have a clear idea of the conformational transitions that occur in hemoglobin upon oxygenation. In the T-state the subunits are more tightly bound that in the R-state. Many of these bonds are salt bridges between the different subunits.

The different subunits are:

T-state

a1 subunit
b1 subunit
a2 subunit
b2 subunit

R-state

a1 subunit
b1 subunit
a2 subunit
b2 subunit

Back to the initial representation, R-state: And T-state:

We'll study now the structural differences between the two states, looking at the intersubunit contacts.

Index

2. The a - a contact

In the T-state, Interactions between subunits are as follows: (a) the side chain of Arg 141 (C-terminal) with That of Asp 126 of the other subunit: (b) Carboxyl group of Arg 141 with the N-terminal amino group of the other chain: and (c) The same interactions at the other end of both chains: In spacefill rendering,

On the contrary, in the R-state these interactions are much weaker or don't occur at all: Note the distance between Arg 141 and Asp 126 in the R-state. In spacefill rendering,

Index

3. The a - b contact

In the T-state, occur the following interactions: (a) Arg 92 of the a chain with Glu 43 of the b chain: and (b) Lys 40 of the a chain with the el C-terminal carboxyl of His 146 in the b chain:

In spacefill rendering,

While in the R-state these interactions do not exist:

In spacefill rendering,

Index

4. The b - b contact

In the b - b contact the gap between subunits is wider in the T-state:

The compound 2,3-bisphosphoglycerate is an allosteric inhibitor and can only bind to the T-state: through salt bridges with His 2, Lys 82 and His 143:

In the R-state, the gap between subunits has narrowed and cannot bind 2,3-bisphosphaglycerate: What can be better seen in spacefill display:

Index

5. Heme group environment

The heme group is located in a hydrophobic pouch of the molecule, and only the propionate side chains interact with the solvent:

In the T-state:

We change the display to cartoons:

In the R-state:

We change to cartoons display:

In both states, the ferrous ion is coordinated to the four nitrogens of the porphyrin and to the nitrogen of His 92 (helix F) in the b chain (His 87 in the a chain), the Proximal histidine. There is another histidine in the heme neighborhood, the Distal histidine.

Proximal histidine in the T-state:

Distal histidine in the T-state:

Proximal histidine in the R-state:

Distal histidine in the T-state:

In the R-state we can see the ligand (carbon monoxide) bound to the sixth coordination position. Also note the different diameter of the ferrous ion in R- and T-states, the ion being bigger in the T-state:

Index


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