Proteins and enzymes



Proteins are (after water) the most abundant biological molecules in the human body and in all living organisms; they are found in all cells and make up at least 50% of their dry weight. Generally, these are very large molecules (macromolecules), consisting of carbon, nitrogen, oxygen and hydrogen as the basic structure, but they can also contain sulfur, phosphorus and sometimes metals, for example: iron, zinc, copper.

Proteins are formed by long sequences of amino acids (the constitutive unit of the protein), which are joined to each other through particular bonds, called peptides, to form long chains (polypeptide chains). The precise sequence of amino acids in the chains determines the shape and function of the protein. There are 20 protein amino acids; since the body is unable to synthesize some of them (therefore called essential), it is necessary to take them every day through the diet.

The genetic code (DNA) contained in the cell nucleus contains the information to indicate how the amino acids must follow each other; special molecules called RNA then transport the information contained in the DNA outside the nucleus where the production of proteins takes place (protein synthesis).

Proteins can be formed from a single chain, or from two or more chains closely associated with each other, and often joined by cross-links.Each chain has a three-dimensional structure and arrangement that represents its specific shape: like a "ribbon" it assumes certain "folds" or conformations (helix, globe, folded sheet) and, combining with other chains, can generate more processed. The overall shape is just as important as the correct sequence of the individual amino acids to ensure the functioning of a protein; losing its shape (for example due to the effect of heat), the protein also loses its functions.

In fact, a small change, even of a single amino acid or loss of the three-dimensional structure in a minimal portion of the molecule, is enough to have different proteins, sometimes anomalous and causing diseases.

The "importance of the function of proteins in the body" is included in their name: once discovered in the first half of the nineteenth century, they were named after the Greek term pròtos, what does it mean first or which occupies the first position.

Proteins perform important functions in our body including:

  • plastic / structural or support (for example the collagen that makes up the connective tissue or the keratin of the nails or hair)
  • protective (such as the fibrinogen of the blood that allows it to clot or the antibodies that the body produces to defend itself against bacteria and viruses)
  • of transport (for example the "hemoglobin of red blood cells, which is used to fix the oxygen in the breathed air and distribute it to the cells to carry out their metabolic functions)
  • of deposit (for example ferritin, a protein that capture, so that it is not eliminated, all the iron that the spleen recovers from demolition of old red blood cells)
  • contractile (for example actin and myosin, which allow the muscles to shorten and lengthen doing their job)

But perhaps the most important function of proteins is the regulatory and energy function performed by enzymes, which act by accelerating biological reactions and transforming slow reactions into faster processes, with lower energy requirements: they therefore act as catalysts. Enzymes combine with a specific substance (called substrate) which has a form exactly complementary to the part of the enzyme in which the reactions take place (active site). This allows a perfect fit, like a key in its lock, which allows the substrate to be transformed into a different substance (or product) which is removed by the enzyme at the end of the reaction.

A typical example of enzymatic activity is offered by the digestion process: enzymes such as amylases and proteases are able to transform starch and proteins (which due to their size could not be absorbed) by 'breaking them up' into simple units (maltose and amino acids) absorbable from the intestine.

The enzyme is neither modified nor consumed by the reaction, so at the end of a reaction it is ready for the next substrate molecule. Therefore, small quantities are enough to control reactions of a large number of molecules. Usually, an enzyme is specific for one or more substances (substrate specificity) and is able to catalyze only one or a limited number of similar reactions.

Enzymatic activity can be influenced by other molecules. There are, in fact, substances capable of inhibiting it and also molecules capable of activating an enzyme, increasing its activity (many drugs and toxic substances are enzyme inhibitors or activators). A typical example are nerve gases, which inhibit an enzyme, acetylcholinesterase, which no longer removes acetylcholine, a substance that allows the transmission of impulses from the nervous system to the muscle. The acetylcholine which accumulates in this way determines a stimulus continuous nervousness of the muscles that from initial cramps leads in a short time to muscle collapse and death.

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