Amino acids: General properties and classification

  • Amino acids are the fundamental units of protein or polypeptides.
  • They are organic compounds having two functional groups; one acidic carboxylic (-COOH) group and the other basic amino (-NH2) group.
  • As many as 300 amino acids are found in nature but only 20 amino acids are standard as they are coded by genes (genetic codes).
  • Other non-standard amino acids are modified amino acids and called non-protein amino acids.
  • Amino acids are colorless, crystalline solid which are usually soluble in water but insoluble in organic solvent.

Structure of amino acid:

                                  Amino acid molecule
  • The amino acids are termed as α-amino acids, if both the carboxyl group and amino group are attached to the same carbon atom.
  • The α-carbon atom binds to a side chain represented by R which is different for each of the 20 amino acids found in proteins.
  • The amino acids (except glycine) possess four different groups (R, H, COO, NH3+) held by α-carbon.
  • Each amino acid is assigned a 3 letter or 1 letter symbol. E.g. Glycine:- Gly or G
  • Amino acids are ampholytes (having both negative and positive charge) and exist in Zwitterion form.
  • Only L- form of amino acids is found in proteins in human body. D-form if present, is converted into L-form by enzymes in liver. 

Classification of amino acids

  1. On the basis of R-group (structure and chemical nature):
  2. On the basis of polarity:
  3. On the basis of nutritional requirement:
  4. On the basis of Catabolism (metabolic fate):
                                                     20 Standard amino acids

On the basis of R-group:

1. Amino acids with aliphatic side chains:

  • Mono-amino, mono-carboxylic amino acids (simplest amino acids)
  • Glycine (G), Alanine (A), Valine (V), Leucine (L) and Isoleucine (I)
  • Valine, Leucine and Isoleucine are branched chain amino acids (have branched aliphatic side chains).

2. Hydroxyl group containing amino acids:

  • Contain –OH group in their side chain
  • Serine (S), Threonine (T) and Tyrosine (Y)
  • Tyrosine is usually considered under aromatic amino acids.

3. Sulphur containing amino acids:

  • Cysteine (with sulfhydryl group) and Methionine (with thioether group); (CM)
  • Cystine is formed by the condensation of two cysteine molecules.

4. Acidic amino acids and their amides:

  • Contain two carboxylic groups, hence called dicarboxylic monoamino acids
  • Highly acidic in nature
  • Glutamic acid (E) and Aspartic acid (D)
  • Glutamine (Q) and Asparagine (N) are their respective amide derivatives.

 5. Basic amino acids:

  • contain two amino groups, hence called dibasic monocarboxylic acids
  • Highly basic in nature
  • Lysine (K), Arginine (R) (with guanidine group) and Histidine (H)(with imidazole ring

6. Aromatic amino acids:

  • Contain aromatic, cyclic ring like structure in the side chain
  • Phenylalanine (F), Tyrosine (Y) and Tryptophan (w) (with indole ring)

7. Imino acids:

  • Contain imino (=NH) group instead of amino group
  • Proline (P) having pyrrolidine ring, is an α-imino acid.

On the basis of polarity:

1. Non-polar amino acids:

  • Also called hydrophobic amino acids and have no charge on the ‘R’ group
  • Alanine, Leucine, Isoleucine, Valine, Methionine, Phenyl alanine, Tryptophan and Proline; (ALIVMFWP)

2. Polar amino acids with no charge on ‘R’ group:

  • Carry no charge on R group but have polar groups like hydroxyl, sulfhydryl and amide groups in them
  • Glycine, Serine, Threonine, Cysteine, Glutamine, Asparagine and Tyrosine; (GSTCENY)

3. Polar amino acids with positive ‘R’ group:

  • Dibasic monocarboxylic acids
  • Lysine, Arginine and Histidine; (LRH)

4. Polar amino acids with negative ‘R’ group:

  • Dicarboxylic monoamino acids
  • Aspartic acid and Glutamic acid; (DE)

On the basis of nutritional requirement:

  • The 20 amino acids are required for the synthesis of variety of proteins, besides other biological functions. However, all these 20 amino acids need not be taken in the diet.

1. Essential amino acids:

  • These amino acids cannot be synthesized in our body, so these should be essentially present in diet.
  • Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine are essential amino acids; (PVTWIMHRLK)
  • Arginine, however is a conditional amino acid (Essential for infants, non-essential for adults)

2. Non-essential amino acids:

  • These amino acids can be synthesized in our body; hence, need not be included in diet.
  • Glycine, Alanine, Serine, Cysteine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Tyrosine, Proline; (GASCNQDEYP)

On the basis of Catabolism (metabolic fate):

  • The carbon skeleton of amino acids can serve as a precursor for the synthesis of glucose (glycogenic) or fat (ketogenic) or both.

 1. Glucogenic amino acids:

  • These amino acids serve as precursors of gluconeogenesis (for glucose or glycogen synthesis)
  • Glycine, Alanine, Methionine, Aspartic acid; (GAMD)

2. Ketogenic amino acids:

  • These amino acids breakdown to form ketone bodies for fat synthesis.
  • Leucine and Lysine; (LK)

3. Glucogenic and ketogenic amino acids:

  • These amino acids breakdown to form precursors for both ketone bodies (fats) and glucose.
  • Isoleucine, Phenylalanine, Tryptophan and Tyrosine; (IPWY)

Amino acids: General properties and classification