Protein Classification: Definition, Function and Different Types of These Molecules Essential for Life

We are talking about a highly complex substance that is present in all living organisms.

Proteins are of great nutritional value and are directly involved in the chemical processes essential for life.

The importance of proteins was recognized by chemists in the early 19th century, including the Swedish chemist Jöns Jacob Berzelius, who in 1838 coined the term protein, a word derived from the Greek proteios, which means “to rank first.”

Proteins are species specific, that is, the proteins of one species differ from those of another species.

They are also specific organs; for example, within a single organism, muscle proteins differ from those of the brain and liver.

A protein molecule is very large compared to sugar or salt molecules.

Amino acids come together to form long chains, like beads are arranged on a string.

There are about 20 different amino acids that occur naturally in proteins.

Proteins of similar function have a similar amino acid composition and sequence.

Although it is not yet possible to explain all the functions of a protein from its amino acid sequence, the correlations established between structure and function can be attributed to the properties of the amino acids that make up proteins.

Non-ruminant animals, including humans, obtain protein primarily from animals and their products, for example meat, milk, and eggs.

The protein content of animal organs is usually much higher than that of blood plasma.

The amount of free amino acids and peptides in animals is much less than the amount of protein.

Protein molecules are produced in cells through the gradual alignment of amino acids and are released into body fluids only after synthesis is complete.

The high protein content of some organs does not mean that the importance of proteins is related to their amount in an organism or tissue, on the contrary, some of the most important proteins, such as enzymes and hormones, are produced in extremely high quantities. little.

The importance of proteins is mainly related to their function.

All the enzymes identified so far are proteins.

Enzymes, which are the catalysts for all metabolic reactions, allow an organism to accumulate the chemicals necessary for life (proteins, nucleic acids, carbohydrates and lipids) to convert them into other substances and break them down.

Life without enzymes is not possible, and there are several protein hormones with important regulatory functions.

In all vertebrates, the respiratory protein hemoglobin acts as an oxygen carrier in the blood, carrying oxygen from the lungs to the organs and tissues of the body.

A large group of structural proteins maintains and protects the structure of the animal body.

Definition of protein

Proteins can be defined as high molar mass compounds consisting largely or entirely of chains of amino acids.

In whose structure, an amino group is present on the carbon atom adjacent to the carboxyl group.

Amino acids that have this general formula are known as alpha (α) amino acids. In its structure, four bonds of a carbon atom (C α) are occupied by NH 2, COOH, H and R molecules.

Where R can be any compound that determines the properties of an amino acid and is known as the side chain of the amino acid.

Composition of the protein molecule

Amino acid condensation

Under favorable conditions the amino acids polymerize. The a-amino group of one molecule joins the carboxyl group of another in a condensation reaction.

This results in the formation of a peptide bond. Peptides that contain two, three, and four amino acids are called dipeptides, tripeptides, and tetra-peptides, respectively.

Primary structure of the protein

The number and sequence of amino acids in a polypeptide chain and is the primary structure of the protein.

The only form of bond in the primary structure of a protein is the peptide bond.

By convention, we represent the structure of peptides beginning with the amino acid whose amino group is free and this end is called the N-terminal end.

The other end, therefore, contains a free carboxyl group and is called the C-terminal end.

Secondary structure of the protein

The primary structure describes only the amino acid sequence in the protein chain.

But from the primary structure you cannot understand the shape (conformation) of the protein molecule.

It is now established that each protein appears in nature in a unique and particular three-dimensional conformation.

The fixed configuration of the polypeptide backbone is known as the secondary structure of the protein.

Protein configuration and conformation

Two considerations are involved in the secondary structure of proteins.

One consideration is configuration, which refers to the geometric relationship between a given set of atoms and the other refers to the three-dimensional architecture of the protein.

The interconversion of the conformers involves not the breaking of the covalent bonds, but the breaking and reformation of the non-covalent forces (hydrogen bonds) that stabilize a given conformation.

The particular conformation of a protein is of immense importance in biological activities.

There may be different conformational species for a particular protein, but they cannot all participate equally in biological actions.

Quaternary structure of the protein

The quaternary structure of a protein defines the structure resulting from interactions between separate polypeptide units of a protein that contains more than one shipment.

Classification of proteins

Like carbohydrates and lipids, proteins could not be classified based on structural similarities alone. Because protein molecules have great structural complexities.

Proteins can be divided into: fibrous and globular, simple and complex, and blood proteins, antibodies, enzymes, contractile and structural hormones.

Shape-based protein classification


They are elongated proteins, for example, silk fibroin, keratin, among others.

They have primarily mechanical and structural functions, which ensure flexibility and resistance and provide external protection, support and shape, they are also insoluble in water, since they contain hydrophobic amino acids.

These hydrophobic amino acids are present inside and outside the structure, the latter facilitating packaging into more complex structures.

Some examples of fibrous proteins are:


Collagen is the main protein component of connective tissue.

They are located in tissues and organs, such as tendons, organic bone matrix, in very high percentages, also in cartilage and in the cornea of ​​the eye.

In different tissues, they form different structures, each capable of satisfying a particular need.

For example, in the cornea, the molecules are arranged in an almost crystalline matrix, so that they are virtually transparent, while in the skin they form fibers that are not very intertwined and directed in all directions, which guarantees the tensile strength of the skin itself.

Las keratin α

They make up almost all of the dry weight of nails, hair, and a large part of the outer layer of the skin.


This protein provides elasticity to the skin and blood vessels.


They are spherical, compact proteins, such as egg albumin, caesin and most of the enzymes.

Fibrous proteins tend to be insoluble in water and other solvents, while globular proteins are soluble in water and in salt and water solutions.

Globular proteins are more complex than fibrous proteins.

Unlike fibrous proteins, which have structural and mechanical functions, globular proteins have functions of the following type:

  • Enzymatic.
  • Hormonal.
  • Membrane transporters and receptors.
  • Transporters in the blood of triglycerides, fatty acids and oxygen.
  • They act like antibodies.
  • Energy storage.

Myoglobin, hemoglobin, and cytochrome c are some of the globular proteins.

Classification of proteins according to their chemical composition

simple proteins

They are composed solely of amino acids and are known as homoproteins, such as plasma albumin, collagen and keratin.

Simple protein is made up of only α-amino acids. Therefore, it exclusively produces α-amino acids on hydrolysis.

These proteins are subdivided based on their solubility in various solvents.

Some examples of simple proteins are.


These are soluble in water and in dilute saline solutions.

Albumins are the most important and most common group of simple proteins.

These are present in the blood (serum albumin).


These are insoluble in water but are soluble in dilute saline solutions.

They are widely distributed groups of simple proteins and are present as antibodies in blood serum and as fibrinogen in blood.


They are soluble in water and insoluble in dilute ammonium hydroxide. Histones contain a high proportion of basic amino acids (lysine and arginine).

These are found in association with nucleic acids in the nucleoprotein of the cell.

Scleroproteins (albuminoids)

These are characterized by their insolubility in water and other solvents.

Scleroproteins have structural and protective functions in the body.

Examples of scleroproteins are keratin (present in hair, skin, and nails), collagen (present in bone, tendon, and cartilage), and elastin (elastic fibers in connective tissues).

Conjugated proteins

Sometimes also called heteroproteins, they contain a non-protein portion in their structure.

Three examples are glycoproteins, chromoproteins, and phosphoproteins.

These are made up of a-amino acids and a non-protein material.

The non-protein material of the protein conjugate is called the prosthetic group.

The different types of conjugated proteins are subdivided according to their prosthetic group.

Among the conjugated proteins are:


These are composed of α-amino acids and phosphoric acids, their prosthetic group is phosphoric acid. Caesin, present in milk, is an important member of this group.

The function of phosphoproteins is structural, such as dentin or reserve, such as caseins in milk.


Typically, they consist of branches no more than 15 to 20 carbohydrate units, where arabinose, fucose (6-deoxygalactose), galactose, glucose, mannose, N-acetylglucosamine and N-acetylneuraminic acid can be found that bind to the backbone of the polypeptide.

Some examples of glycoproteins are:

Glycophorin: the best known of the erythrocyte membrane glycoproteins.

Fibronectin: this protein anchors cells to the extracellular matrix through interactions on one side with collagen or other fibrous proteins, while on the other side with cell membranes,
all proteins in blood plasma, except albumin.

Immunoglobulins or antibodies: contain a carbohydrate or a carbohydrate derivative as a prosthetic group. Mucin, a component of saliva, is a glycoprotein.

chromium proteins

They are proteins that contain colored prosthetic groups, some typical examples are: hemoglobin and myoglobin, which bind, respectively, to one and four heme groups, the rhodopsins, which bind to the retina.

Its prosthetic group is a compound pigment, such as: hemoglobin, which has the pigment that contains iron.


Here the prosthetic groups are complex polymers with high molar masses and are called nucleic acids (DNA and RNA). Nucleo-proteins are present in all plant and animal cells.


They consist of phospholipid and cholesterol esters attached to protein molecules.

They are often classified as compound lipids. Most of the lipids in mammalian blood are transported in the form of lipoprotein complexes.

The electron transport system in mitochondria contains a large amount of lipoproteins, these are also found in the myelin sheath of nerves and different cell organelles.

Classification by function

Structural proteins

These proteins participate in the formation of different parts of the body. More than half of the total protein in the mammalian body is collagen found in skin, cartilage, and bone.

contractile proteins

These special types of proteins are responsible for the contraction and relaxation of muscle cells, for example actin and myosin.

These proteins are also present in single-celled animals.


They represent the largest class of proteins, almost 2,000 different types of enzymes are known.

Enzymes are called biological catalysts and are vital for any activity in the living organism.


Many of the hormones are proteins in nature, for example, insulin. Some other hormones are steroids.


Higher organisms produce antibodies to destroy any foreign material (antigen) released into the body by an infectious agent. Gamma globulins are examples of antibodies in mammals.

Blood proteins

Albumins, globulins, and fibrinogen are the three main protein constituents of the blood.

Classification of proteins based on their biological functions

From a functional point of view, they can be divided into several groups.

Enzymes (biochemical catalysts)

Almost all known enzymes, and in the human body, are proteins (except for some catalytic RNA molecules called ribozymes, that is, ribonucleic acid enzymes).

transport proteins

Many small organic and inorganic molecules are transported in the bloodstream and in extracellular fluids, through cell membranes, and within cells from one compartment to another, by specific proteins.

Some examples of transport proteins are:

  • It carries oxygen from the alveolar blood vessels to the capillaries of the tissues (hemoglobin).
  • Iron transport in the blood (Transferrin) and
    membrane carriers.
  • Fatty acid binding proteins, that is, the proteins involved in the intracellular transport of fatty acids.
  • Plasma lipoprotein proteins, macromolecular complexes of proteins and lipids responsible for the transport of triglycerides, which are otherwise insoluble in water.
  • Albumin, which contains free fatty acids, bilirubin, thyroid hormones.
Storage proteins
  • Ferritin, responsible for storing iron intracellularly in a non-toxic way.
  • Caseins, which act as a reserve of amino acids for milk.
  • Phosvitine which contains high amounts of phosphorus.
  • Prolamins and glutelins, which are the storage proteins in cereals.
Proteins that contribute to mechanical support

Proteins play a fundamental role in the stabilization of many structures, some examples are α-keratins, collagen and elastin.

Movement-generating proteins

They are responsible, among others, for the contraction of muscle fibers (of which myosin is the main component), for
the propulsion of sperm and microorganisms with flagella.

Also the separation of chromosomes during mitosis, in nerve transmission such as acetylcholine receptors in synapses.


Many hormones such as insulin, glucagon, and thyroid-stimulating hormone are proteins.

Protein proteins against harmful agents

Antibodies or immunoglobulins are glycoproteins that recognize antigens expressed on the surface of viruses, bacteria, and other infectious agents.

In this group are interferon, fibrinogen, and blood clotting factors.

Proteins and energy storage

Proteins, and in particular the amino acids that constitute them, act as energy storage, secondly only adipose tissue, which under particular conditions, such as prolonged fasting, can become essential for survival.

According to the nature of the molecules

  • Acidic proteins: exist as anions and contain acidic amino acids, such as blood groups.
  • Basic proteins: they exist as cations and are rich in basic amino acids, such as lysine, arginine, among others.