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Amino Acids: Protein basics and more

Published: June 01, 2018

Amino Acid Basic Structure
Amino Acid Basic Structure

The twenty amino acids required for the synthesis of thousands of different proteins in your body belong to a group of more than 200 amino acids found in living organisms.

In addition to the 20 standard amino acids, this large group of amino acids also includes non-protein amino acids which have a variety of functions.  
Amino acids are organic chemical compounds with the same fundamental structure of a central carbon (C) to which an acid group, an amino group, a hydrogen atom and a side group are attached.
The acid group is comprised of 1 carbon, 2 oxygen and 1 hydrogen atoms, and the amino group is comprised of 1 nitrogen and 2 hydrogen atoms. 
Under normal physiological condition in cells, the acid group is ionized: that is, the hydrogen atom is removed, and the amino group is protonated: that is, it gains a hydrogen atom.
The side group, which may also be referred to as a side chain or R-group, varies between amino acids and gives each amino acid its unique features.
19 of the 20 standard amino acids can exist in two forms which are mirror images of each other or stereoisomers identified as D-amino acids or L-amino acids.
The majority of amino acids found in nature are in the L-amino acid conformation. In a 2-dimensional drawing the amino group will be on the left side of the central (or alpha) carbon.
Side groups
Side groups provide each amino acid with its unique identity and range in complexity from a single hydrogen atom (glycine) to a bicyclic indole composed of 18 atoms (Tryptophan).
Amino acids are grouped according to the properties of their side chains:

  • Aliphatic R- groups: The side groups are composed of hydrogen and carbon atoms, Glycine, Alanine, Proline, Valine and Leucine.
  • Aromatic R-groups: the side groups contain an aromatic group such as benzyl or indole: Phenylalanine, Tyrosine, and Tryptophan.
  • Sulphur R-groups: Contain a sulphur molecule: Methionine and Cysteine.
  • Alcohol R-groups: Contain a beta-hydroxyl group: Serine and Threonine
  • Basic R-groups: The nitrogen in these side groups is positively charged at a pH 7: Histidine, Lysine, and Arginine.
  • Acid R-group: The side groups contain a beta-carboxyl group (acid group): Aspartate, Glutamate, Asparagine, and Glutamine.

Alanine, Isoleucine, Leucine, and Valine can also be identified by their branched side chains which vary in their carbon and hydrogen composition.

The side groups affect the hydropathy, relative hydrophobicity, or hydrophilicity of the amino acid.
Isoleucine, Phenylalanine, Valine, Leucine and Methionine are highly hydrophobic or water hating, while Histidine, Glutamine, Asparagine, Glutamate, Aspartate, Lysine and Arginine are highly hydrophilic or water loving.
Tryptophan, Alanine, Glycine, Cysteine Tyrosine, Proline, Threonine, and Serine are mildly hydrophobic.
Hydrophathy is an important property with respect to protein folding and formation.
Essential amino acids
With respect to human growth and development, there are nine essential amino acids which the body does not...link to the full article to learn more.

Related Topics

Diet  Digestion  Metabolism  Protein  Your Body 


Gropper, S.S., Smith, J.L. & Groff, J.L. (2005). Advanced Nutrition and Human Metabolism (4thEd.). Belmont, CA: Thomson Wadsworth.
Horton et al. (2002). Principles of Biochemistry (3rd Ed.). Upper Saddle River, NJ. Prentice Hall
Whitney, E. & Rady Rolfes, S. (2005). Understanding Nutrition. Belmont, CA: Thomson Wadsworth