OXYHEMOGLOBIN: CHARACTERISTICS, STRUCTURE AND BINDING CURVE - BIOLOGY - 2025

Or xyhemoglobin is the name given to hemoglobin when it binds to oxygen. Hemoglobin is a protein found within red blood cells whose main function is to transport oxygen from the lungs to the tissues.

The first living beings were unicellular and lived in a liquid environment from which they were nourished and from which they eliminated their waste, just like some of the organisms that exist today. Under these conditions, these processes are achieved by simple diffusion mechanisms, since the cell wall is in intimate contact with the medium that supplies it.

Oxyhemoglobin dissociation curve (Source: Ratznium at English WikipediaLater versions were uploaded by Aaronsharpe at en.wikipedia. Via Wikimedia Commons)

The development of increasingly complex multicellular organisms implied that the internal cells moved considerably away from the environment, so that the diffusion mechanisms as the only source of supply became insufficient.

Thus, specialized systems were developed to obtain nutrients and gases such as the digestive system and the respiratory system, as well as transport mechanisms to bring these nutrients and gases closer to the cell: the cardiovascular system.

To produce energy in the form of ATP molecules, cells need oxygen. Since ATP cannot be stored, it must be constantly synthesized, which means a permanent demand for oxygen by the cells.

Hemoglobin emerged, evolutionarily speaking, as a gas transporter that "solved" the problem of oxygen transport from the environment to the cell.

Characteristics and structure

To discuss the characteristics and structure of oxyhemoglobin it is necessary to refer to hemoglobin, since oxyhemoglobin is nothing more than hemoglobin bound to oxygen. Therefore, hereinafter, the joint characteristics of the molecule will be described in the presence or not of the gas in question.

What is hemoglobin for?

Hemoglobin is necessary to transport oxygen to the tissues in the quantity and speed that these warrant, given that oxygen has so little solubility in the blood that its transport by diffusion would be insufficient to supply tissue needs.

What is a hemoglobin molecule like?

Hemoglobin is a tetrameric protein (which has four subunits), has a spherical shape and a molecular mass of 64 kDa.

Its four subunits constitute a single functional unit, where each one reciprocally influences the other. Each subunit is made up of a polypeptide chain, globin, and a prosthetic group, the heme or "heme" group, which acts as a cofactor and is not made up of amino acids; that is, it is not protein in nature.

Globin is found in two forms: alpha globin and beta globin. The hemoglobin tetramer consists of a pair of alpha globin chains (of 141 amino acids) and a pair of beta globin chains (of 146 amino acids).

Each of the four polypeptide chains are attached to a heme group, which has in the center an iron atom in a ferrous state (Fe2 +).

What is the heme group like?

The heme group is a porphyrin ring that is made up of four pyrrolic rings (heterocyclic aromatic compounds with the formula C4H5N) linked by methyl bridges. The iron in the ferrous state in the center is bound to the structure through coordinated nitrogen bonds.

Each heme group is capable of binding to one oxygen molecule, so each hemoglobin molecule can only bind 4 molecules of the gas.

The human body contains approximately 2.5 x 1013 erythrocytes, which are the blood cells that make and transport hemoglobin. Each erythrocyte has about 280 million hemoglobin molecules and can thus carry more than 1 billion oxygen molecules.

How is oxyhemoglobin formed?

Oxyhemoglobin is formed after the union of an oxygen atom to each iron atom in the ferrous state found in each heme group of the hemoglobin molecule.

The term oxyhemoglobin refers, then, to an oxygenated and not chemically oxidized hemoglobin, since it does not lose an electron when combining with oxygen and iron remains in a ferrous state.

Oxygenation produces a change in the quaternary structure of the molecule, that is, a change in conformation that can be transmitted from the globin chains to the heme group and vice versa.

What is the maximum amount of oxygen that hemoglobin can carry?

Hemoglobin can, at most, bind four oxygen molecules in its structure. If the molar volume of ideal gases is 22.4 L / mole, one mole of hemoglobin (64,500g) will bind with 89.6 liters of oxygen (4 moles of O2 x 22.4 L / mole).

So each gram of hemoglobin must bind with 1.39 ml of O2 to be 100% saturated (89.6L / 64500g x (1000ml / L)).

In practice, blood tests give slightly lower results, since there are small amounts of methemoglobin (oxidized hemoglobin) and carboxyhemoglobin (hemoglobin + carbon monoxide (CO)) that cannot bind oxygen.

Taking this into account, the “Hüfner” rule states that, in the blood, 1g of hemoglobin has a maximum capacity to bind oxygen of 1.34ml.

Oxyhemoglobin binding curve

The number of oxygen molecules that can bind to a hemoglobin molecule depends on the partial pressure of oxygen or PO2. In the absence of oxygen, hemoglobin is deoxygenated, but as PO2 increases, the number of oxygens that bind to hemoglobin increases.

The process of binding oxygen to hemoglobin dependent on the partial pressure of oxygen. When plotted, the result is called the "oxyhemoglobin curve" and has a characteristic 'S' or sigmoid shape.

Depending on the PO2, hemoglobin will be less or more capable of “releasing” or “delivering” the oxygen it carries, as well as being loaded with it.

For example, in the region between 10 and 60 mmHg of pressure, the steepest part of the curve is obtained. In this condition, hemoglobin can easily give up large amounts of O2. This is the condition that is achieved in the tissues.

When PO2 is between 90 and 100 mmHg (12 to 13 kPa), hemoglobin is almost 100% saturated with O2; and when arterial PO2 is 60 mmHg (8 kPa), saturation with O2 is still as high as 90%.

In the lungs these are the prevailing conditions (pressure between 60 and 100 mmHg), and it is this that allows the hemoglobin molecules present in the erythrocytes to be charged with oxygen.

This sigmoid shape that draws the oxyhemoglobin curve ensures that this protein behaves as an excellent lung loader, a very efficient transporter in arterial blood and an excellent O2 donor in tissues, in proportion to the local metabolic rate. that is, on demand.

References

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