Glossary

PROTEINS

Proteins are among the most complex organic compounds and are the fundamental constituents of all animal and plant cells. From a chemical point of view, proteins are polymers (also known as macromolecules) of amino acid residues. There are held together by a peptide bond, often with other molecules and/or other metallic ions (in which case they constitute a conjugated protein).

Proteins can be classified into two main families: globular proteins and proteins with an extended or fibrous structure. These two groups reflect the two broad functional separations that characterize them:

*Extended or fibrous proteins generally perform biomechanical functions, such as in the formation of nails, hair, of the corneous layer of the epidermis etc. providing a barrier against the outside world.

*Conversely, globular proteins perform a variety of critical biological functions. These include enzymes, respiratory pigments, numerous hormones and antibodies in the immune system.

The protein molecule consists of carbon, oxygen hydrogen and nitrogen atoms; sulphur is also often contained (such as in methionine, cysteine and cystine amino acids) and at times also phosphorous and/or metals such as iron, copper, zinc etc. may be present.

Protids are one of the fundamental components of cells. Their composition in terms of amino acids may vary and is genetically determined; hence their molecular weights may vary greatly, according to the number and type of amino acids (monomers) in each molecule (heteropolymer, of amino acids with an average molecular weight of 115). If the molecule has very few amino acid units (generally not more that 15-20) it is defined to be an oligopeptide. An oligopeptide generally lacks a well-defined conformation within a solution, changing continuously due to its rather high flexibility. Longer polymers are known as polypeptides. One or more polypeptides form a protein.

Living organisms contain a startling number of amino acids, yet only 20 are controlled genetically (as a result of evolutionary processes) and are thus contained in proteins:

 

1. aspartic acid (mono-amino di-carboxylic)

2. glutamic acid (mono-amino di-carboxylic)

3. alanine (mono-amino mono-carboxylic)

4. arginine (di-amino mono-carboxylic)

5. asparagine

6. cysteine (mono-amino mono-carboxylic)

7. phenylalanine (mono-amino mono-carboxylic)

8. glycine (or glycine)

9. glutamine

10. isoleucine

11. histidine

12. leucine

13. lysine (di-amino mono-carboxylic)

14. methionine

15. proline (iminoacid)

16. serine (mono-amino mono-carboxylic)

17. tyrosine

18. threonine

19. tryptophan (mono-amino mono-carboxylic)

20. valine

 

Non-proteic amino acids include GABA (amino butyric acid, a chemical mediator of the nervous system), DOPA (3,4-dehydroxy-l-phenylalanine, a precursor of adrenaline) and others that have important specific biological properties. There are 10 essential amino acids for the human body (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine). Some of these are “conditionally essential”, meaning that they only become essential under certain physiological or pathological conditions (e.g. cysteine, tyrosine, taurin, glycine, arginin, glutamine, proline).

The quantity of proteins that is present in serum is defined as total proteinemia, with normal values between 6 and 8 grams per deciliter.

Considered in isolation, total proteinemia can only provide generic indications regarding pathological states; despite undergoing modifications individually, the various fractions of the proteins tend to compensate, seeking a certain biological equilibrium. Thus, total proteinemia displays considerable variations (both positively and negatively) only in very rare cases, such as in the presence of serious diseases.

For example, a proteinemia increase can be observed in the context of excessive sweating  (prolonged physical exercise, intense physical activity) or dehydration whereby organic liquids are lost within a short period of time, such as cases of uncontrollable diarrhea or vomiting, uncompensated diabetes, shock or acute collapse.

A decrease in total proteinemia occurs upon a reduced protein intake, anorexia (eating disorder characterized by decreased appetite), intestinal absorption disorders, for failure or defective protein synthesis due to deficiency of vitamins and amino acids, for liver diseases of a chronic nature, for loss or destruction of proteins such as in hyperthyroidism, in certain kidney diseases, in extensive burns and haemorrhages.

 

 

See also:

Glossary

Presentation n. 3

Presentation n. 4