Protids or proteins are biopolymers, are made up of large numbers of simple repetitive structural units (monomers). These units are amino acids. Only twenty amino acids are part of living matter. The combination and link by peptide bond give rise to the different proteins of living things. Hundreds of thousands of these amino acids may participate in the formation of large polymer molecule of a protein.

All proteins contain carbon, hydrogen, oxygen and nitrogen, and most also possess sulfur. Although there are slight variations in different proteins, the nitrogen content represents, on average, 16% of the total mass of the molecule, ie, each 6,25 g of protein contains 1 g of  N.

The amino acid sequence of a protein is encoded in the gene (DNA portion) by the genetic code.


Most proteins fold into unique 3-dimensional structures. The shape into which a protein naturally folds is known as its native conformation.Although many proteins can fold unassisted, simply through the chemical properties of their amino acids, others require the aid of molecular chaperones to fold into their native states. Biochemists often refer to four distinct aspects of a protein’s structure:

  • Primary structure: the amino acid sequence.
  • Secondary structure: regularly repeating local structures stabilized by hydrogen bonds. The most common examples are the alpha helix, beta sheet and turns. Because secondary structures are local, many regions of different secondary structure can be present in the same protein molecule.
  • Tertiary structure: the overall shape of a single protein molecule; the spatial relationship of the secondary structures to one another. Tertiary structure is generally stabilized by nonlocal interactions, most commonly the formation of a hydrophobic core, but also through salt bridges, hydrogen bonds, disulfide bonds, and even posttranslational modifications. The term “tertiary structure” is often used as synonymous with the term fold. The tertiary structure is what controls the basic function of the protein.
  • Quaternary structure: the structure formed by several protein molecules (polypeptide chains), usually called protein subunits in this context, which function as a single protein complex.


All proteins perform elementary functions for cellular life, but each of them also has a more specific function in the face of our organization. Because of its functions can be classified into:

1. Catalysis: It consists of protein enzymes that are responsible for performing chemical reactions in a more quick and efficient. Processes that are critical to the body. For example pepsin, this enzyme is found in the digestive system degrade and are responsible for food.

2. Regulatory: Hormones are a class of proteins which help to have a balance between the functions performed by the body. Such is the case of the insulin regulates the glucose found in the blood.

3. Structural: This type of proteins whose function is to give strength and elasticity that allows the formation of tissue and other structures supporting. This is the case of the tubulin is in the cytoskeleton.

4. Defensive: They are responsible for defending the body. Glycoproteins that are responsible for producing immunoglobulins that defend the body against foreign bodies, or keratin that protects the skin as well as fibrinogen and prothrombin forming clots.

5. Transport: The function of these proteins is carried substances throughout the body where they are needed. Proteins such as hemoglobin which carries oxygen through the blood.

6. Receiving: This type of protein found in the cell membrane and perform the function of receiving signals for the cell to perform its function. Receiving the acetylcholine contraction signals to produce.

7. Nutritional functions.  As in egg ovalbumin, or casein in milk.

Dietary sources

Dietary sources of protein include meat, eggs, soy, grains, legumes and dairy products such as cheese or yogurt. Animal sources of proteins have 20 amino acids as well as vegetable sources, with cereals and legumes, possess the necessary amino acids total.

Different proteins have different levels of biological family for the human body. Many foods have been introduced to measure the rate of utilization and retention of proteins in humans. These include biological value, NPU (Net Protein Utilization), NPR (net protein ratio) and PDCAAS (Protein Digestibility Corrected Amino Acids Score), which was developed by the FDA to improve the PER (Protein Efficiency Ratio). These methods examine what proteins are more efficiently used by the body. Overall, they concluded that animal protein containing all essential amino acids (milk, eggs, meat) and soy protein are most valuable to the organization.

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