GLOBIN: CHARACTERISTICS, STRUCTURE, FUNCTION AND ALTERATIONS - BIOLOGY - 2025

The globin is a proteinaceous structure arranged in spherical or globular shape, thus being hence its name. This structure is of the tertiary type and is characterized by being complex, since the chain of amino acids folds to form the spheroprotein. There are several types of globin chains and these were classified in Greek letters: alpha, beta, delta, gamma, epsilon, and zeta chain globin.

The amino acids that make up the primary structure of the protein vary according to the species to which they belong (humans or animals). There are also variations within the same species according to the current state of life of the organism (embryonic life, fetal life or post-birth life).

Different structures that contain globin in their composition. Sources: Wikipedia.com/BiancaDescals / The original uploader was ProteinBoxBot at English Wikipedia. /Wikipedia.com

The genetic information for the synthesis of the various globin chains are contained in various chromosomes. For example, alpha chain globins are found on chromosome 16, while the genetic information for beta, delta, gamma, and epsilon globins are on chromosome 11.

characteristics

Globin is part of important structures in the body, for example the most relevant are: hemoglobin and myoglobin.

Hemoglobin contains four globin chains (alpha 1 and alpha 2) and (beta 1 and beta 2). Each globin has a fold where it protects the heme group.

On the other hand, there is myoglobin. Which has a less complex structure than hemoglobin. This presents a globular polypeptide of a single strip arranged secondarily.

Until recently it was believed that these were the only substances that contained globin in higher beings, but today two more are known to possess globin in their constitution: cytoglobin and neuroglobin.

Cytoglobin is present in most tissues and is especially found in connective tissue, as well as it has also been found in the retina.

In turn, neuroglobin has a preference for nervous tissue, hence its name. Neuroglobin has been found in nerve cells of the brain located at the level of the cerebral cortex, as well as in other locations such as the thalamus, hypothalamus, hippocampus and cerebellum.

However, they are not the only locations, as outside the nervous system it can be found in the islets of Langerhans of the pancreas and in the retina.

Structure

There are 6 different types of globin chains that are designated by the letters of the Greek alphabet: alpha (α), beta (β), gamma (γ), delta (δ), epsilon (ε) and zeta (ζ). These chains belong to the globin family, but they differ from each other in the number of amino acids they possess.

These polypeptides have a primary, secondary and tertiary structure. The single chain of amino acids represents the primary structure. When the chain is wound into spirals or helices they make up the secondary structure.

If this structure then folds over itself many times, it forms a globular structure corresponding to the tertiary structure.

Likewise, they can only acquire the quaternary form when 4 globin molecules or chains are combined in tertiary form.

This is how it occurs in the complex structure of hemoglobin. However, in myoglobin it is different. In this case, globin appears as a monomer, that is, it has a single peptide chain that is arranged in folds, creating 8 helices (secondary structure).

Both hemoglobin and myoglobin harbor a heme group within their complex structure.

Hemoglobin

In this molecule 2 alpha globin chains and 2 beta chains bind. This is how they are perfectly coupled to house the heme group, plus iron, in their center.

Between these structures there are weak bonds and strong bonds. 19 amino acids participate in weak unions and the union occurs as follows: the alpha 1 chain joins the beta 2 chain and the alpha 2 chain joins the beta 1 chain.

Meanwhile, 35 amino acids participate in strong unions and the chains that join are: the alpha 1 chain joins the beta 1 chain and the alpha 2 chain joins the beta 2 chain.

Location of alpha 1 and alpha 2, beta 1 and beta 2 chains in the structure of hemoglobin. Source: OpenStax College edited image (translated into Spanish)

Myoglobin

A globular protein group is also present in myoglobin, but in this case there is a single peptide chain made up of 153 amino acids. Its spatial arrangement is secondary and it has 8 alpha helices.

This protein structure strategically places hydrophobic amino acids towards the inside of the structure, while hydrophilic or polar amino acids are towards the outside.

This design is perfect to house the heme group inside (hydrophobic part). This is attached to the protein by non-covalent bonds.

Cytoglobin

It was discovered in 2001 and is said to be a type of hemoglobin, but it differs in that it is hexacoordinated, while hemoglobin and myoglobin are pentacoordinated. This has to do with the position that the amino acid histidine takes close to the heme group.

Neuroglobin

Its discovery was made in 2000. Neuroglobin is a monomer that has 150 amino acids, therefore it is very similar to myoglobin. The structure of neuroglobin is 21% to 25% similar to myoglobin and hemoglobin.

Features

Because globin is not found alone in the body, but as part of certain structures, the functions that each of them fulfill are mentioned:

Hemoglobin

It is found inside erythrocytes. It is responsible for fixing and transporting oxygen from the lungs to the tissues. As well as purifies the body of carbon dioxide, doing the opposite route.

Myoglobin

The heme group located in globin has the function of storing oxygen molecules to oxygenate the heart muscle and skeletal muscle.

Cytoglobin

This protein is believed to be able to influence the protection of hypoxic and oxidative stress states in tissues. It is also thought that it can carry arterial oxygen to the brain.

Neuroglobin

Neuroglobin is thought to have the ability to bind oxygen, carbon monoxide, and nitric oxide.

However, the role of neuroglobin is not yet known with certainty, but it is believed to be related to the regulation of hypoxia and cerebral ischemia. Especially it would act as a neuroprotective.

Because neuroglobin has a similar structure to hemoglobin and myoglobin, it is speculated that it could participate in the supply of oxygen at the neuronal level. It is also believed that it can eliminate free radicals and nitrogen that are produced in the respiratory chain.

In relation to nitric oxide, it is thought that it eliminates it when oxygen is normal and produces it in hypoxic processes from NO ₂.

Alterations

The alpha and beta globin chains are encoded by different genes located on chromosomes 16 and 11 respectively.

Individuals with hemoglobin S (sickle cell or sickle cell anemia) have a defect in the beta globin chain. The defect consists of a substitution of nitrogenous bases at the level of nucleotide number 20 of the involved gene, where there is a change of adenine for a thymine.

Mutations in the gene β ^s of chromosome 11 causes different haplotypes called globin: Senegal, Cameroon, Benin, Bantu or CAR and Asian or Arab-Indian.

Knowing the type of haplotype that patients with sickle cell anemia present is epidemiologically important, since it allows us to know the distribution of the different haplotypes, but this information also provides important data to know the prognosis of the disease.

For example: the Bantu haplotype is known to be more severe, while the Senegal and Asian type are milder.

The differences between one haplotype and another lie in the amount of hemoglobin F they have. The higher the percentage of hemoglobin F and the lower hemoglobin S, the better the prognosis. The lower the amount of hemoglobin F and the higher the hemoglobin S, the worse the prognosis.

These mutations are inherited autosomal along with the hemoglobin S mutation.

References

1. Globin. Wikipedia, The Free Encyclopedia. 19 Oct 2018, 13:44 UTC. 11 Jul 2019, 17:26, wikipedia.org
2. "Myoglobin." Wikipedia, The Free Encyclopedia. 7 Jul 2019, 21:16 UTC. 11 Jul 2019, 20:42, wikipedia.org
3. Durán C, Morales O, Echeverri S, Isaza M. Haplotypes of the beta globin gene in carriers of hemoglobin S in Colombia. Biomédica 2012; 32: 103-111. Available at: scielo.org
4. Forrellat M, Hernández P. Neuroglobin: a new member of the globin family. Rev Cubana Hematol Inmunol Hemoter 2011; 27 (3): 291-296. Available at: scielo.sld
5. "Cytoglobin." Wikipedia, The Free Encyclopedia. 1 Sep 2017, 17:26 UTC. 12 Jul 2019, 00:28 wikipedia.org