Explaining Proteins: Structure and Function

Proteins are one of the most important nutrients. But how are they exactly constituted and what do they do? Find out here...

Structure

A protein is a long chain of polypeptides, which is constituted of amino acids that are bound to each other by peptide bonds. Amino acids are organic bonds that are made of an amino-group (NH2), a carboxylgroup (COOH) and a certain side chain. These side chains are specific combinations of atoms with different chemical properties. There are twenty possible side chains, which means that twenty different amino acids can be build.

The difference between animal and plant protein is the fact that animal protein is called complete; animals ingest essential amino acids and make their own non-essential ones. Plants, however, contain incomplete protein, meaning that plant proteins do not possess enough of some amino acids people need. This is the reason why vegetarians should combine certain foods. For example, beans contain small amounts of the amino acid methionine. Rice possesses small amounts of the amino acid lysine. By combining beans and rice, the amount of amino acids in your food is improved, which leads to more complete proteins.

Functions

Proteins regulate almost every metabolic process in your body and are an important part of the cellular structure. They’re made of amino acids. There are twenty amino acids of which nine are essential, which means that they have to be ingested. The remaining eleven can be constituted in the human body.

The protein keratin forms the outer layers of the skin (epidermis), the nails and the hair. One of the reasons why protein needs to be ingested daily, is to ensure the growth of these external structures. Since the human body stores fat and glucose, there is no excess protein present. When the human body suffers from a protein shortage, protein will be allocated from those parts of the body where it is used. That is why the body of anorexic people will start breaking down muscle tissue, for example the heart.

Inside the body, muscle and bone contain a lot of protein. Red blood cells contain hemoglobin, a structure consisting of a ‘heme group’ (contains iron and transports oxygen) and a ‘globin’ (a protein). Immunoglobulins are protein structures that are fabricated by the immune system and serve as antibodies to fight bacteria and viruses that enter the body.

In order to perform certain functions, proteins are able to form combinations with other substances in the body:

  • Lipoproteins are a combination of lipids and proteins that transport cholesterol.
  • Glycoproteins are a combination of carbohydrates and proteins that are present in the cellular membranes and the mucus of the digestive tract.
  • Phosphoproteins are a combination of phosphoric acid and proteins that are the most important protein in milk, casein.

One of the most important functions of proteins is to act as enzymes. Enzymes function as catalysts, which means that they speed up a chemical reaction without being depleted or changed during that reaction. There are six important groups of enzymes:

  • Ligases, which bind two molecules together.
  • Lyases, which split up two molecules.
  • Hydrolases, which split up two molecules when water is added.
  • Isomerases, which are used to build isomers (different chemical structures with the same formula).
  • Oxidoreductases, which catalyze oxidation reactions (where an electron is given away) and reduction reactions (where an electron is accepted).
  • Transferases, which transport chemical groups from one substance to another.

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