Protein functions

The proteins, their structural diversity are macromolecules with more functions.

• Reservation function.
• Structural and mechanical support function.
• Homeostatic function.
• Transport function.
• Defensive and protective function.
• Hormonal function.
• Contractile function.
• Catalytic or enzymatic function.
• Chemical signal recognition function.

Some proteins contain amino acids that are used as nutritional elements and building blocks by the developing embryo. For example, ovalbumin from egg, the casein of the milk or the gliadin of the seed of wheat.

Proteins are the plastic elements from which most cellular and organic structures are built. For instance:

The glycoproteins form the cell membranes where they perform functions such as transport of substances between exterior and interior, neurotransmitter receptors or hormones, etc.

Another group of proteins is the cytoskeleton of the cell, the fibers of the spindle, the cilia and the flagella, the ribosomes, etc.

The histones form part of the structure of chromosomes in eukaryotes, performing functions of regulating the activity of genes.

The collagen, which is found in connective tissue, tendons form, bone, etc.

The elastin, the elastic connective tissue (ligaments, walls of blood vessels, etc.).

The keratin in the epidermal cells, forming flakes of reptiles, the stratum corneum of the epidermis, the grip, nails, hairs and barbs of mammals, the feathers of birds, etc.

The fibroin protein secreted by spiders and silkworms to build the web or silk cocoon.

There are proteins that are specialized in transporting molecules and ions where necessary, both through the plasma membrane and to other regions of the body. The following are transporter proteins:

• Permeases and pumps, which transport ions and molecules from one side of the membrane to the other.
• Lipoproteins, which carry insoluble lipids in the blood plasma.
• Cytochromes, electron transporting proteins in the cell membrane of bacteria and in the inner membrane of mitochondria and chloroplasts in cellular respiration and photosynthesis.
• Hemoglobin. It carries oxygen from the respiratory system to the cells in the blood of vertebrates.
• Hemocyanin. It performs the same function as hemoglobin, but in invertebrates.
• Myoglobin. Very similar to hemoglobin, it is responsible for storing and transporting oxygen in the muscles.
• Seroalbumin. Transports fatty acids between adipose tissue and other organs.

The proteins that have a more important defensive function are the immunoglobulins of the blood or antibodies, which are responsible for reacting against substances foreign to the body (antigens).

Other proteins with a protective function are, for example, thrombin and fibrinogen, which intervene in coagulation, preventing blood loss in wounds, or mucins, with a bactericidal function, protective of the mucous membranes and secreted in the digestive tracts, respiratory and urogenital.

Some hormones are made up of one or more polypeptide fragments, such as insulin and glucagon synthesized by the pancreas, which regulate the metabolism of carbohydrates, the hormones secreted by the pituitary (growth hormone or somatotropin, gonadotropic hormones and other), the thyroxine of the thyroid, etc.

They allow the movement of unicellular and multicellular organisms. For instance:

• The dynein, responsible for the movement of cilia and flagella.
• The actin and myosin, which are protein filaments involved in muscle contraction.

Some proteins are enzymes, and they have a very important function, since they act as biocatalysts for the chemical reactions that take place in living beings, reducing the energy necessary for these reactions to take place.

Due to the importance of this function, we will discuss enzymes in a separate section.

The glycoproteins on the outer surface of the plasma membrane recognize chemical signals (hormones, neurotransmitters, antibodies, viruses, bacteria, ...).