Most people know the definition of protein within a nutritional context, but they have functions in our body that go far beyond that concept. These nutrients are essential in the makeup of living beings.
To learn more about these biomolecules, we will explain what proteins are and what type of synthesis and structure they present. We will also see their functions, classification, and high protein foods.
What are proteins?
Proteins consist of long chains of amino acids that are connected to each other by chemical bonds between the carboxyl group of each amino acid and the amine group of the next. These bonds are known as peptide bonds. The chains that contain only a few amino acids are called polypeptides.
Some of the amino acids are essential, i.e., they must be obtained through diet. Others, on the other hand, are known as non-essential because they can be produced by the body itself.
We are surrounded by proteins. They are present in our body, in clothes, in food and even in medicines. In particular, within the organisms of living beings, they perform essential functions that include, for example, digesting food and converting it into energy or allowing some substances to pass through cell membranes.
They also allow cells to detect and react to certain hormones and toxins in the environment. When they act as antibodies, they protect us from pathogens.
Structure of proteins
We can divide them into 4 different levels: primary, secondary, tertiary and quaternary.
The primary structure corresponds to a single sequence of amino acids, i.e., the two-dimensional linear sequence in the peptide chain of these components.
Secondary and tertiary structures correspond to the three-dimensional shape in which a chain folds. They differ from each other only because secondary structures were discovered before tertiary structures.
Finally, the quaternary structure refers to the way in which many chains associate with each other, i.e., a protein composed of a chain of amino acids.
Protein synthesis
These amino acids are generally determined by DNA. In other words, a person's genetic information will decide what proteins will have each cell in the body. These basic sources of life are generally synthesized according to the genes that encode them.
Their synthesis or biosynthesis is an anabolic —or metabolic— construction process through which the chains of these nutrients are formed. This transformation consists of 2 phases: the translation of the messenger RNA, through which the amino acids read and execute the information that is in the DNA and the post-translation process where the necessary modifications are produced.
Types of proteins
Proteins represent the highest percentage of our body weight after water. Its main functions refer to its ability to regulate fluids and the immune system, among others.
They are also necessary for the body because they are essential to perform specific bodily functions. These are some examples:
- Structure and produce creatine in the musculoskeletal system
- Build and maintain muscles and tissues
- Help regulate proper cell function
- Control chemical reactions through enzymes
- Act as antibodies that protect us from harmful agents
Although they can be classified in many different types —composition, complexity or even conformation— proteins can also be differentiated according to the function they perform in the body. In this case, they are divided into 6 main types:
1. Structural proteins
They are found in the structures that make up bones, muscles, cartilage, and tendons. They are also located in the hair or nails but in lesser number. Their main function is to give resistance, rigidity, and elasticity to the tissues.
2. Enzymes
These are the result of metabolic and catalytic processes that make chemical reactions faster and more efficient. Pepsin, for example, is an enzyme that degrades food.
3. Signaling proteins
This type is produced by our endocrine system, specifically by its main organs: the pituitary gland and the thyroid. Insulin is a protein hormone that is responsible for regulating the blood glucose level.
4. Receptor proteins
We can find them in the cell membrane. They receive and capture signals from the body that the cell in which they are found needs to carry out its basic tasks. An example of a receptor protein is acetylcholine, which receives the signals necessary for the muscle to contract.
5. Defense proteins
They are also known as antibodies and are activated in the presence of a harmful agent. Their main function is to defend the body. An example of these proteins are glycoproteins, which are produced by immunoglobulins, or keratin, responsible for protecting the skin.
6. Transport proteins
Transport proteins carry specific substances throughout the body. An example is hemoglobin, which transports oxygen throughout the bloodstream.
High protein foods
Meat, milk, eggs, poultry, and seafood are considered high quality and high protein foods because they contain all the necessary essential amino acids.
Animal sources are considered more complete than vegetable proteins, such as beans or peas, which are also considered rich in proteins but not that complete.
Except for soy, vegetable sources (nuts, beans, seeds, and grains) are deficient in one or more of the essential amino acids. Dietitians recommend that a healthy diet consist of different types of foods and include at least 10 to 20% of daily intake from both animal and vegetable sources.
The needs for this amino acid are highest in childhood and gradually decrease as we grow old, except in pregnant and breastfeeding women.
The recommended daily intake is determined by the person's age. However, this also depends on body weight but is not a linear relationship.
The average Western diet contains large amounts of protein. In fact, most people in industrialized countries eat more of this nutrient than they need.
In general, the consequences of an inadequate intake can include difficulties and problems in the body such as:
- Loss of muscle mass
- Increased risk of infection
- Muscle atrophy
- Edemas
- Malnutrition and weight loss
References
Proteins. (2002). In N. Schlager (Ed.), Science of Everyday Things (Vol. 3, pp. 18-23). Detroit: Gale.
Small, P. K. (2003). Proteins. In R. Robinson (Ed.), Genetics (Vol. 3, pp. 198-204). New York: Macmillan Reference USA.