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.
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
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:
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.