Hemoglobin is a metalloprotein contained in red blood cells, responsible for transporting oxygen in the bloodstream. Oxygen, in fact, 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.
Structure and functions
In the bloodstream, oxygen cannot bind directly and reversibly to proteins, as happens instead for metals such as copper and iron. It is no coincidence that at the center of each protein subunit of hemoglobin, surrounded by a protein shell, we find the so-called EME prosthetic group, with a metallic heart represented by an iron atom in the Fe oxidation state2+ (reduced state), which binds oxygen in a reversible manner.
In women the values are on average 5-10% lower than in men.
Possible causes of High hemoglobin
Possible causes of Low hemoglobin
The oxygen content in the blood is therefore given by the sum 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 located inside the red blood cells. Without hemoglobin, therefore, erythrocytes could not carry out their role 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. Approximately 70% of the iron present in the body is in fact contained in the EME 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 into the various cellular organelles that use it (e.g. mitochondria).
Hemoglobin is a large and complex metalloprotein, characterized by four globular protein chains respectively wrapped around an EME group that contains Fe2+.
For each hemoglobin molecule we therefore find four EME groups wrapped in the relevant 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 people, the task of hemoglobin is to take oxygen into the lungs, release it to the cells that need it, take carbon dioxide from them and release it into the lungs where the cycle starts again.
During the passage of blood through the capillaries of the pulmonary alveoli, hemoglobin binds oxygen to itself, which subsequently passes 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 diffusion of gas from the alveoli into the blood (↑PO2); this increase causes hemoglobin to bind avidly to oxygen; the opposite occurs in peripheral tissues, where the concentration of dissolved oxygen in the blood decreases (↓PO2) and the partial pressure of carbon dioxide increases (↑CO2); this causes hemoglobin to release oxygen and become loaded with CO2. Simplifying the concept as much as possible, the more carbon dioxide there is 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 quantity of oxygen bound to hemoglobin grows in relation to pO2 following a sigmoid curve:
The fact that the plateu region is so large places an important safety margin on the maximum saturation of hemoglobin during passage into the lungs. Although the pO2 at the alveolar level is normally equal to 100 mm Hg, observing the figure we note that even at a partial pressure of oxygen equal to 70 mmHg (a typical occurrence of some diseases or of staying at high altitude), the percentages of saturated hemoglobin remain close to 100%.
In the region of maximum slope, when the partial oxygen tension drops below 40 mmHg, the ability of hemoglobin to bind oxygen drops sharply.
In resting conditions, the PO2 in intracellular fluids is approximately 40 mmHg; in this case, due to the laws of gases, the oxygen dissolved in the plasma diffuses towards the tissues poorer in O2, crossing the capillary membrane. As a result, plasma O2 tension drops further and this favors the release of oxygen from hemoglobin. During intense physical effort, however, the oxygen tension in the tissues drops to 15 mmHg or less, so the blood is severely depleted of oxygen.
As mentioned, in resting conditions a significant quantity of oxygenated hemoglobin leaves the tissues, remaining available in case of need (for example to deal with 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 a temperature of 37°C.
The Bohr effect has consequences both on the intake of O2 at the pulmonary level and on its transfer at the tissue level.
Where there is more carbon dioxide dissolved in the form of bicarbonate, hemoglobin releases oxygen more easily and is loaded with carbon dioxide (in the form of bicarbonate).
The same effect is obtained by acidifying the blood: the more the blood pH decreases, the less oxygen remains bound to hemoglobin; it is no coincidence that in the blood carbon dioxide is found dissolved mainly in the form of carbonic acid, which dissociates.
In honor of its discoverer, the effect of pH or carbon dioxide on the dissociation of oxygen is known as the Bohr effect.
As anticipated, in an acidic 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 the 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 at tissue level, where the transfer of oxygen is therefore facilitated .
2,3-diphosphoglycerate is an intermediate in glycolysis that affects the affinity of hemoglobin for oxygen. If its concentrations inside the red blood cell increase, the affinity of hemoglobin for oxygen decreases, thus facilitating the delivery of oxygen to the tissues. It is no coincidence that erythrocyte concentrations of 2,3 diphosphoglycerate increase, for example, in anemia, in cardiopulmonary insufficiency and during stays at altitude.
In general, the effect of 2,3 biphosphoglycerate 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 similar conditions, in fact, in the tissues most exposed to stress there is a local increase in temperature and carbon dioxide pressure, and therefore in blood acidity. As stated above, all this favors the transfer of oxygen to the tissues, shifting the hemoglobin dissociation curve to the right.