Proteins: definition, functions, types and characteristics

We explain what proteins are, their characteristics and functions. Also, what are its types, sources and recommended intake.

What are proteins?

The proteins are macromolecules present in all living organisms and constitute a fundamental nutrient for human beings.

They are macromolecules formed by the union of several amino acids which are acquired through food. Amino acids are organic molecules that have an amino functional group (-NH2) at one end of their structure, while at the other end they have a carboxyl functional group (-COOH).

Proteins are essential to perform various body functions, such as the formation or repair of muscles, bones and other tissues. They also intervene in cell division and in the body’s defense.

Low levels of protein in the body can cause diseases such as anemia, especially during pregnancy or early childhood.

Protein characteristics

Proteins are composed of different types of amino acids linked together, forming a long chain with a complex structure. The sequence of the amino acids determines the shape of the protein and, consequently, its function.
There are around twenty different types of amino acids, of which eight are essential or indispensable because the body cannot manufacture them and must acquire them through food. The remaining twelve types of amino acids are those that the body can synthesize or manufacture from essential amino acids.
Some organs or tissues have higher protein content than others. However, the importance of protein is not related to its concentration but to its function. Some of the most important proteins, such as enzymes and hormones, are found in reduced proportions.
Proteins are very specific in the function they perform and must also be stable in the environment in which they carry out their activity.

Protein structure

Proteins have different levels of organization in their structures that have been used to study them. There are basically four types of structures:

Primary structure. It is the amino acid sequence of the peptide chains, which do not reveal information about the conformation but about the composition of these chains. Peptide chains can be straight or bent. The simplest way to describe a protein is by using its primary structure.
Secondary structure. It is the spatial arrangement of the different types of peptide chains of the protein. The presence of hydrogen bonds determines the formation of local folds, such as alpha helices or beta sheets.
Tertiary structure. It is the folding of the peptide chain on itself, which generates a globular-type structure. This folding occurs on the already existing secondary structure.
quaternary structure. It is the structure of proteins that have more than one chain of amino acids with an already existing tertiary structure. That is, it is the union of different peptide chains. Proteins that have a quaternary structure are called oligomeric proteins. An example of this type of protein is hemoglobin.

Functions of proteins

The functions that proteins can perform are very varied and essential for the proper functioning of the organism, such as composing the cellular structures of living beings and making chemical reactions possible.

Among the main functions are:

They make certain biological processes happen faster. This process is called catalysis. For example: enzymes (they are proteins that catalyze some chemical reactions).
They control body processes, such as growth and metabolism. For example: insulin (hormone that regulates glucose levels in the human body).
They protect the body from diseases. For example: antibodies, such as IgG generated to combat Covid-19.
They transport various substances in the body, such as oxygen, iron and lipids. For example, hemoglobin, transferrin and lipoproteins.
They form the structure of various tissues, such as skin and hair. For example, collagen and keratin.
They carry out muscle contraction movements. For example, myosin and actin.

Types of proteins

There are two main groups of proteins:

Simple proteins. They are made up only of amino acids or their derivatives. For example, globular proteins form almost spherical and highly coiled structures, or fibrous, which have an elongated structure.
Proteins conjugated with other compoundsThey combine with another component that is not an amino acid and is called a “prosthetic group”. For example, lipoproteins (which combine with lipids), glycoproteins (which combine with sugars) or chromoproteins (which combine with pigments).

Protein denaturation

Protein denaturation is a process in which, under certain conditions, physicochemical changes occur that cause the loss of their structures secondary, tertiary and quaternary. Only the polypeptide chain is maintained, without any three-dimensional structure.

The most common factors that cause denaturation are sudden changes in temperature, pH and hydrostatic pressure (pressure exerted by the liquid). On the other hand, when certain organic solvents such as acetone or toluene are added, protein denaturation can also occur.

An example of this process occurs with the cooking of an egg.in which the egg white (which is very rich in albumin) is denatured when exposed to high temperatures. The egg white stops being transparent, fluid and soluble and becomes white, solid and insoluble, as occurs with a fried egg or a hard-boiled egg.

Protein sources

Protein sources They are diverse and each organism obtains and synthesizes them differently.. For example, ruminant animals (such as cows) need to consume a large amount of vegetables to meet the adequate protein level. Non-ruminant animals (like humans) need to consume animal derivatives, legumes, seeds and nuts.

Excess protein

Excess protein of animal origin may pose health risks due to the concentration of saturated fats that they contain, which contribute to increasing cholesterol and clogging the arteries, causing cardiovascular disorders, obesity and affecting the kidneys or liver.

The body cannot store excess protein. To get rid of this excess, it changes the body’s pH, making it more acidic than necessary. The process of counteracting and balancing the pH can overload and damage some vital organs.

Daily protein intake

Recommended daily protein intake varies according to various factors, such as activity intensity, age, health status, among others. The recommended protein dose, which is not the same as the amount of total food needed, is calculated based on the concentration of protein contained in a food, and its suggested proportions are:

For people who carry out moderate activity. They should consume between 0.8 and 1 gram of protein per kilo of their body weight, which is equivalent to about 56 and 90 grams of protein per day for an average weight individual.
For sportsmen or women with a lot of activityThey should consume between 2 and 2.5 grams of protein per kilogram of their body weight, which is equivalent to about 100 and 160 grams of protein per day for an individual of average weight.

It may be useful to you: Organic compounds

References:

“What are some characteristics of protein?” in Sciencing.
“Protein” in Britannica.
“What are proteins and what are they for?” in Alimente.
“Structure and properties of proteins” at the University of Valencia.
“Protein, how much should you consume per day? At AARP.
“How much protein does your body need every day?” in La Vanguardia.
Cuban surgery magazine. Alterations of the acid-base balance. Dr. Benito Saínz Menéndez. ISSN 1561-2945