10 julio, 2024

Globular proteins: characteristics, structure, examples

Globular proteins are those proteins that have a tertiary structure in which the peptide chains are arranged in a globular-looking conformation. A large part of the cellular proteins corresponds to this group, being the most important the proteins with enzymatic activity.

Proteins represent a very special type of molecules in the cells that make up all living things. Its structure consists of a combination of 20 amino acids that are repeated in different proportions and that are joined together through chemical bonds, in a genetically determined order or sequence.

They are extremely abundant and fulfill essential functions from many points of view of cellular life, to such an extent that without them the existence of life as we know it would not be possible.

Each species of living being on Earth has a specific set of proteins and, furthermore, multicellular organisms have cells that differ from each other mainly by the proteins they produce.

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globular and fibrous proteins

Protein scientists have traditionally classified proteins according to many parameters, but one of the most important is structure. Thus, depending on the three-dimensional structure that they adopt, proteins can be fibrous or globular.

Fibrous proteins are those that have an elongated appearance, since their peptide chains are generally parallel to each other. These proteins have many functions, but the most important have to do with cellular structure, support, and biomechanics.

Two classic examples of fibrous proteins in the human and other animal bodies are keratin and collagen, which are involved in the formation of hair and nails (the former) and skin, bones, and tendons (the latter).

Globular proteins, on the other hand, are proteins that have a more rounded or spherical three-dimensional conformation, so they can appear a little more compact and irregular. These proteins do not participate directly in the cell structure, but they do have a fundamental functional role.

Examples of globular proteins are enzymatically active proteins (enzymes) such as hemoglobin, which is involved in transporting oxygen through the blood, and immunoglobulins, which function in the mammalian immune system.

Characteristics of globular proteins

Solubility

Globular proteins are partially soluble in water, an aspect of great importance, since they are truly abundant in the aqueous environment of the cytosol and in the lumen of the different cell organelles where they perform their functions.

Structure

While fibrous proteins are almost always formed by a repetitive type of secondary structure, globular proteins are more heterogeneous, since they are characterized by presenting different types of secondary structures along their peptide chains that fold together.

functions

In the group of globular proteins are all the enzymes, a large number of transport proteins, regulatory proteins, motor proteins and many more, so it is a very diverse group, both from the point of view of structure and size as well as of function.

Conformation

As is true for fibrous proteins, all the information necessary to achieve the folding and structural conformation of globular proteins is determined by the amino acid sequence, which, in turn, depends on the information contained in the genes that encode them.

Classification

Generally these proteins are classified according to their function, and each category is further divided into many subcategories. A good example of this is the classification of enzymes, which is usually based on the type of reaction in which they participate.

Structure of globular proteins

Globular proteins are defined as such thanks to the native conformation of their tertiary structures, in which the amino acid chains are arranged to form a relatively spherical structure, generally lined with hydrophilic amino acids (which interact with water) that protect a more hydrophobic core. (which does not interact with water).

Primary and secondary structure

Like fibrous proteins, globular proteins have a primary structure formed by the linear chain of amino acids that make them up, which are arranged in alpha helices or beta sheets, giving rise to the secondary structure.

Tertiary and Quaternary structure

The tertiary structure of globular proteins is formed spontaneously and is maintained by the interactions between the amino acid chains that make them up.

It is a compact and hemispherical conformation, so compact that it closely resembles that of a crystal. It is determined by the interactions between the different secondary structures that may exist in the same polypeptide chain.

It has been determined that the forces that maintain the interaction between these chains are usually of a weak nature, such as van der Waals interactions between the most hydrophobic amino acids (apolar bonds), or hydrogen bonds between the most hydrophilic amino acids (polar bonds). ).

In addition, many globular proteins, especially large ones, have different «lobes» or «domains,» which can have different functions within the same molecule.

Likewise, some globular proteins are found in nature as large protein complexes, which are composed of discrete (separate) polypeptide chains, also known as subunits, so they are said to be proteins with quaternary structures.

Examples of globular proteins

There are many examples of globular proteins, some essential for cell functions and others not so much, but whatever the case, their structure is always related to their function.

At the cellular level we can talk, then, about some of the proteins that participate in the most important metabolic pathways, such as:

hexokinase

It is a relatively small globular protein found in almost all living cells, where it is responsible for catalyzing the phosphorylation reaction of glucose residues in the first part of the glycolytic pathway.

Succinate dehydrogenase

It is a mitochondrial protein complex consisting of four subunits (AD) and participating in both the tricarboxylic acid cycle (Krebs cycle) and the electron transport chain, two fundamental processes for the production of cellular energy in the form of ATP.

In the human body and that of other animals there are also other very important proteins such as hemoglobin and immunoglobulins.

hemoglobin

It is, like succinate dehydrogenase, a globular protein with a quaternary structure, as it is made up of two different pairs of subunits, known as alpha chains and beta chains. This is found inside the red blood cells, where it participates in the transport of oxygen to the tissues.

mioglobin

It is also a globular oxygen-binding protein, but this has only a tertiary structure and is found exclusively in the skeletal muscle cells of vertebrate animals.

Yoimmunoglobulins

They are globular glycoproteins present in many animals, particularly in the blood, in the lymph and in some vascularized tissues, where they perform functions as members of the immune system.

Like hemoglobin and succinate dehydrogenase, these proteins have a quaternary structure, as they are made up of two pairs of subunits: two heavy chains and two light chains.

aquaporin

Another globular protein, common in animal and plant cells, is the protein that forms membrane channels for water transport, better known as aquaporin.

Aquaporins are classified as globular proteins, but they are integral membrane proteins that are arranged in quaternary structures made up of several identical subunits.

References

Chan, HS, & Dill, KA (1990). Origins of structure in globular proteins. Proceedings of the National Academy of Sciences, 87(16), 6388-6392.
Grattendick, K., & Pross, S. (2007). Immunoglobulins.
Gromiha, MM (2010). Protein bioinformatics: from sequence to function. academic press.
Gromiha, MM, Nagarajan, R., & Selvaraj, S. (2019). Protein Structural Bioinformatics: An Overview.
Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. macmillan.
Verkman AS (2013). Aquaporins. Current biology: CB, 23(2), R52–R55. https://doi.org/10.1016/j.cub.2012.11.025

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