Proteins are an one of 4 major types of biological molecules and made up of amino acids.

An importance of proteins can be seen through the vast range of functions that they perform.

Some examples of proteins are:

  • Immunoglobins, which protect animals from foreign microbes and cancer cells.
  • Hemoglobin, which shuttles oxygen from place to place in mammals.
  • Protein carriers, which help move sucrose through phloem tissue in plants.
  • Keratin, the most common protein in vertebrates, is found in hair and fingernails.
  • Fibrin, a protein that helps a blood clot.
  • Collagen, which forms the protein component of bones, skin, ligaments and tendons.
  • Enzymes are catalysts in biochemical reactions.
  • Hormones are chemical-signaling molecules.

Types of 20 amino acids and their structures

Each of all 20 amino acids has the same basic fundamental structure, which consists of a central carbon atom, also known as the alpha (α) carbon, bonded to an amino group (-NH2), a carboxyl group (-COOH), and to a hydrogen atom.

Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group.

Types of 20 amino acids and their structures

In eukaryotes, there are multiple copies of the 21 proteinogenic amino acids (the 20 of the standard genetic code, plus selenocysteine) dissolved in a cell's cytoplasm and these amino acids are constructed by the cell from simpler compounds or obtained from the diet.

Essential amino acids for the human body

There are 9 essential amino acids that can not be produced by the human body and must be obtained from the diet.

A list of these 9 essential amino acids includes such amino acids as:

  1. Tryptophan
  2. Methionine
  3. Valine
  4. Threonine
  5. Phenylalanine
  6. Leucine
  7. Isoleucine
  8. Lysine
  9. Histidine

Different types of 20 amino acids

The chemical nature of the side chain determines the nature of the amino acid (that is, whether it is acidic, basic, polar, or nonpolar).

20 amino acids can be divided in the next groups:

  • Aliphatic amino acids: Alanine, glycine, isoleucine, leucine, Proline and Valine.
  • Aromatic amino acids: phenylalanine, tryptophan and tyrosine
  • Acidic amino acids: aspartic acid and glutamic acid.
  • Basic amino acids: arginine, histidine and lysine.
  • Hydroxylic amino acids: serine and threonine.
  • Sulphur containing amino acids: cytosine and methionine.
  • Amidic amino acids: asparagines and glutamine.

Amino acid peptide bond

Amino acids combine to form large organic polymers known as peptides.

Each amino acid is attached to another amino acid by a covalent bond, known as a peptide bond or an amide bond, which is formed by a dehydration reaction.

Amino acid peptide bond

The carboxyl group of one amino acid and the amino group of the incoming amino acid combine, releasing a molecule of water and forming a natural condensation polymer.

The resulting bond is the peptide bond and the products formed by such linkages are peptides.

4 structures of proteins

There are four levels of structure in globular proteins:

Primary structure of protein

The primary structure of a protein is the unique sequence of amino acids in a polypeptide chain.

Secondary structure of protein

The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein.

The most common are the α-helix and β-pleated sheet structures.

Both structures are the α-helix structure.

The helix of the secondary structure of the protein held in shape by hydrogen bonds.

Tertiary structure of protein

The tertiary structure of a polypeptide refers to the unique three-dimensional structure of a polypeptide.

This structure is in part due to chemical interactions at work on the polypeptide chain.

Primarily, the interactions among R groups creates the complex three-dimensional tertiary structure of a protein.

Quaternary structure of protein

Primary, secondary and tertiary apply to individual polypeptide chains of a protein and Quaternary applies to the interactions that occur between polypeptide strands in proteins composed of two or more polypeptides.

Weak interactions between the subunits help to stabilize the overall structure.

Four levels of protein structure