Hemoglobin and the Bohr effect

Structure and functions

Hemoglobin is a metalloprotein contained in red blood cells, responsible for the transport of oxygen in the blood stream. In fact, oxygen is only moderately soluble in water; therefore, the quantities dissolved in the blood (less than 2% of the total) are not sufficient to satisfy the metabolic demands of the tissues. The need for a specific carrier is therefore evident.

In the bloodstream, oxygen cannot bind directly and reversibly to proteins, as occurs instead for metals such as copper and iron. Not surprisingly, at the center of each protein subunit of hemoglobin, wrapped in a protein shell, we find the so-called prosthetic group EME, with a metallic heart represented by an iron atom in the Fe2 + oxidation state (reduced state), which binds oxygen in a reversible way.

Blood analysis

• Normal hemoglobin values ​​in the blood: 13-17 g / 100 ml

In women the values ​​are on average 5-10% lower than in men.

Possible Causes of High Hemoglobin

• Polycythemias
• Extended stay on the high ground
• Chronic lung diseases
• Heart disease
• Blood doping (use of erythropoietin and derivatives or substances that mimic their action)

Possible Causes of Low Hemoglobin

• Anemias
• Iron deficiency (iron deficiency)
• Copious bleeding
• Carcinomas
• Pregnancy
• Thalassemias
• Burns

The oxygen content in the blood is therefore given by the summation of the small quantity dissolved in the plasma with the fraction bound to hemoglobin iron.

More than 98% of the oxygen present in the blood is bound to hemoglobin, which in turn circulates in the bloodstream allocated within the red blood cells. Without hemoglobin, therefore, the erythrocytes could not perform their task as oxygen transporters in the blood.

Given the central role of this metal, the synthesis of hemoglobin requires an adequate intake of iron in the diet. About 70% of the iron present in the body is in fact contained in the heme groups of hemoglobin.

Hemoglobin is made up of 4 subunits that are structurally very similar to myoglobin *.

* While, hemoglobin transports oxygen from the lungs to the tissues, myoglobin carries the oxygen released by hemoglobin in the various cellular organelles that use it (eg mitochondria).

Hemoglobin is a large and complex metalloprotein, characterized by four globular protein chains respectively wrapped around a heme group that contains Fe2 +.

For each hemoglobin molecule we therefore find four heme groups wrapped in the relative globular protein chain. Since there are four iron atoms in each hemoglobin molecule, each hemoglobin molecule can bind four oxygen atoms to itself, according to the reversible reaction:

Hb + 4O2 ← → Hb (O2) 4

As is known to most, the task of hemoglobin is to take oxygen in the lungs, release it to the cells that need it, take carbon dioxide from them and release it into the lungs where the chilo begins again.

During the passage of blood in the capillaries of the pulmonary alveoli, hemoglobin binds oxygen to itself, which subsequently releases to the tissues in the peripheral circulation. This exchange occurs because the bonds of oxygen with the iron of the EME group are labile and sensitive to many factors, the most important of which is the tension or partial pressure of oxygen.

Binding of oxygen to hemoglobin and the Bohr effect

In the lungs, plasma oxygen tension increases due to the diffusion of gas from the alveoli to the blood (↑ PO2); this increase causes hemoglobin to bind avidly to oxygen; the opposite occurs in the peripheral tissues, where the concentration of dissolved oxygen in the blood decreases (↓ PO2) and the partial pressure of carbon dioxide increases (↑ CO2); this induces the hemoglobin to release oxygen and become charged with CO2. Simplifying the concept as much as possible, the more carbon dioxide is present in the blood, the less oxygen remains bound to hemoglobin.

Although the amount of oxygen physically dissolved in the blood is very low, it therefore plays a fundamental role. In fact, this quantity heavily influences the bond strength between oxygen and hemoglobin (as well as representing an "important reference value in regulating pulmonary ventilation).

Summarizing everything with a graph, the amount of oxygen linked to hemoglobin grows in relation to the pO2 following a sigmoid curve:

The fact that the plateu region is so large places an important safety margin at the maximum saturation of hemoglobin during the passage into the lungs. Although the pO2 at the alveolar level is normally equal to 100 mm Hg, observing the figure we note in fact how even a partial pressure of oxygen equal to 70 mmHg (typical occurrence of some diseases or of staying at high altitudes), the percentages of saturated hemoglobin remain close to 100%.

In the region of maximum slope, when the oxygen partial tension falls below 40 mmHg, the ability of hemoglobin to bind oxygen drops suddenly.

In resting conditions, the PO2 in intracellular fluids is approximately 40 mmHg; in this location, due to the laws of gases, the oxygen dissolved in the plasma diffuses towards the poorer tissues of O2, crossing the capillary membrane. Consequently, the plasma tension of O2 drops further and this favors the release of oxygen from the hemoglobin. . During intense physical exertion, on the other hand, the oxygen tension in the tissues drops to 15 mmHg or less, as a result of which the blood is greatly depleted of oxygen.

For what has been said, in resting conditions an important quantity of oxygenated hemoglobin leaves the tissues, remaining available in case of need (for example to face a sudden increase in metabolism in some cells).

The solid line shown in the image above is called the hemoglobin dissociation curve; it is typically determined in vitro at pH 7.4 and at a temperature of 37 ° C.

The Bohr effect has consequences both on the intake of O2 at the lung level and on its release at the tissue level.

Where there is more dissolved carbon dioxide in the form of bicarbonate, hemoglobin releases oxygen more easily and becomes charged with carbon dioxide (in the form of bicarbonate).

The same effect is obtained by acidifying the blood: the more the blood pH decreases and the less oxygen remains bound to the hemoglobin; not surprisingly, in the blood carbon dioxide is dissolved mainly in the form of carbonic acid, which dissociates.

In honor of its discoverer, the effect of pH or carbon dioxide on oxygen dissociation is known as the Bohr effect.

As anticipated, in an acid environment, hemoglobin releases oxygen more easily, while in a basic environment the bond with oxygen is stronger.

Other factors capable of modifying the affinity of hemoglobin for oxygen include temperature. In particular, the affinity of hemoglobin for oxygen decreases with increasing body temperature. This is particularly advantageous during winter and spring months, since the temperature of the pulmonary blood (in contact with the air of the external environment) is lower than that reached in the tissues, where the release of oxygen is therefore facilitated.

2.3 diphosphoglycerate is an intermediate of glycolysis which affects the affinity of hemoglobin for oxygen. If its concentrations within the red blood cell increase, the affinity of hemoglobin for oxygen decreases, thus facilitating the release of oxygen to the tissues Not surprisingly, the erythrocyte concentrations of 2,3 diphosphoglycerate increase, for example, in anemia, in cardio-pulmonary insufficiency and during the stay at high altitude.

In general, the effect of 2,3 bisphosphoglycerate is relatively slow, especially when compared to the rapid response to changes in pH, temperature and partial pressure of carbon dioxide.

The Bohr effect is very important during intense muscular work; in such conditions, in fact, in the tissues most exposed to stress there is a local increase in the temperature and pressure of carbon dioxide, therefore in blood acidity. As explained above, all this favors the release of oxygen to the tissues, shifting the hemoglobin dissociation curve to the right.