Amino Acid Structure

Amino Acid Structure
By GYassineMrabetTalk. This vector image was created with Inkscape. (Own work) [Public domain], via Wikimedia Commons

Figure 1 demonstrates the generic structure of amino acids. It is worth noting the following key points:

  • ALL 20 amino acids possess an amino group on the α carbon.
  • ALL 20 amino acids possess a carboxyl group on the α carbon.
  • Amino acids are Zwitterions: Neutral molecules that possess both a negative and positive electrical charge.
    • The negative charge is attributed to the carboxyl group.
    • The positive charge is attributed to the amino group.
  • The R side chain is variable between different amino acids which gives each amino acid its unique characteristics.

Classifications of Amino Acids

Furthermore, amino acids can be classified into acidic, basic, non-polar, and polar categories.

ACIDIC BASIC POLAR NON-POLAR
Aspartic Acid Lysine Serine Glycine
Glutamic Acid Arginine Cysteine Alanine
Histadine Tyrosine Valine
Threonine Leucine
Asparagine Isoleucine
Glutamine Phenylalanine
Tryptophan
Methionine
Proline

Hydrophilic vs Hydrophobic

Hydrophilic molecules are “water-loving” such that they are able to be dissolve in water whereas hydrophobic molecules are “water-hating”. It is important to recognize the hydrophilicity of the various amino acids. The simplest way to memorize this is to recognize the hydrophobic molecules.

Non-Polar Amino Acids are Hydrophobic

  • Glycine
  • Alanine
  • Valine
  • Leucine
  • Isoleucine
  • Phenylalanine
  • Tryptophan
  • Methionine
  • Proline

By remembering the hydrophobic amino acids, it is easy to identify the hydrophilic ones since all other amino acids are hydrophilic.

Essential vs Non-Essential

Our body is able to synthesize many different amino acids from other biological molecules. These are known as non-essential amino acids. However, there are a group of amino acids that absolutely must be obtained from diet called essential amino acids. The following list consists of the 9 essential amino acids:

  • Pheylalanine
  • Valine
  • Threonine
  • Tryptophan
  • Isoleucine
  • Methionine
  • Histadine
  • Lysine
  • Leucine

Chemical Structure and Nomenclature

The MCAT requires knowledge of each amino acids name, 3 letter abbreviation, single letter abbreviation, and chemical structure. The following figure provides a concise visual which covers the aforementioned topics.

Amino Acid Chart By Dancojocari [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
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