Classification of amino acids

Amino acids are organic compounds that serve as the building blocks of proteins. They consist of a central carbon atom (the alpha carbon) bonded to four groups: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain or R group that determines the specific characteristics of each amino acid.

Chemical Structure

General Formula: H₂N-CHR-COOH
- Amino Group (-NH₂): A basic group that can accept a proton.
- Carboxyl Group (-COOH): An acidic group that can donate a proton.
- Side Chain (R group): This varies among different amino acids and determines the properties and function of the amino acid.

Classification of Amino Acids

Amino acids can be classified based on several criteria, including the nature of their side chains, their polarity, and their nutritional necessity.

1. Based on the Nature of Side Chains (R groups)

Nonpolar (Hydrophobic) Amino Acids: These amino acids have side chains that do not interact favorably with water. They tend to be found in the interior of proteins, away from the aqueous environment.
- Examples:
  - Glycine (Gly, G)
  - Alanine (Ala, A)
  - Valine (Val, V)
  - Leucine (Leu, L)
  - Isoleucine (Ile, I)
  - Methionine (Met, M)
  - Phenylalanine (Phe, F)
  - Tryptophan (Trp, W)
  - Proline (Pro, P)
Polar (Hydrophilic) Amino Acids: These amino acids have side chains that can form hydrogen bonds with water, making them soluble in aqueous environments.
- Examples:
  - Serine (Ser, S)
  - Threonine (Thr, T)
  - Cysteine (Cys, C)
  - Tyrosine (Tyr, Y)
  - Asparagine (Asn, N)
  - Glutamine (Gln, Q)
Acidic Amino Acids: These amino acids have side chains with a carboxyl group, which can donate a proton, giving them a negative charge at physiological pH.
- Examples:
  - Aspartic acid (Asp, D)
  - Glutamic acid (Glu, E)
Basic Amino Acids: These amino acids have side chains with an amino group, which can accept a proton, giving them a positive charge at physiological pH.
- Examples:
  - Lysine (Lys, K)
  - Arginine (Arg, R)
  - Histidine (His, H)

2. Based on Polarity

Polar Amino Acids:
- Examples: Serine, Threonine, Asparagine, Glutamine, Tyrosine
Nonpolar Amino Acids:
- Examples: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline

3. Based on Nutritional Necessity

Essential Amino Acids: These cannot be synthesized by the human body and must be obtained through the diet.
- Examples:
  - Histidine
  - Isoleucine
  - Leucine
  - Lysine
  - Methionine
  - Phenylalanine
  - Threonine
  - Tryptophan
  - Valine
Non-Essential Amino Acids: These can be synthesized by the human body.
- Examples:
  - Alanine
  - Asparagine
  - Aspartic acid
  - Glutamic acid
Conditionally Essential Amino Acids: These are typically non-essential but may become essential under certain conditions, such as illness or stress.
- Examples:
  - Arginine
  - Cysteine
  - Glutamine
  - Tyrosine
  - Glycine
  - Proline
  - Serine