CHEMISTRY OF AMINO ACIDS

Amino acids are the monomer units of proteins. they are the group of compounds which contain an amino (-NH2) and carboxylic (COOH) groups. These compounds have the basic structure of

Figure 1 Basic amino acid structure. Image found at www.whitetigernaturalmedicine.com

over 300 naturally occurring amino acids are known, however only 20 of these amino acids participate in protein synthesis. Amino acids are usually represented by the first 3 letters of their name (usually referred to as the 3 letter amino acid code)

Amino acids can be classified based on several criteria including
1.   based on the the structure/ chemical property of amino acids
2.    based on the polarity of the amino acid
3.   based on the nutritional requirement of the amino acid
4.   based on the metabolic fate of the amino acid

for the purpose of this course we would limit our classification to the first 3 criteria listed above

Classification of Amino Acids Based on the Structure and Chemical Properties of the Amino Acid

1. Aliphatic amino acids:
    Glycine
    Alanine
    Valine
    Leucine
    Iso-leucine
all contain an aliphatic uncharged R- group as shown in the structures above.

2. Hydroxyl Group Containing Amino Acids
 
   Serine
   Threonine
   Tyrosine

3. Sulfur Containing Amino Acid
   Cysteine
   Methionine

4. Acidic Amino Acids and their Amides
  Aspartic Acid
  Asparagine
  Glutamic Acid
  Glutamine

5.  Basic Amino Acid
   Arginine
   Lysine
   Histidine

6. Aromatic Amino Acids
 Tryptophan
 Tyrosine
 Phenylalanine

7. Imino Acid
  Proline

Classification of Amino Acids based on Polarity

the R- group of amino acids can exist as either a [polar hydrophilic group which can form hydrogen bonds with water such as in

amino acids with OH group (Serine, Threonine and Tyrosine)

amino acids with SH group (Cysteine)


amino acids with amide group (Glutamine and Asparagine)

amino acid with +ve charge on R- group (Lysine, Arginine, Histidine)

amino acid with -ve charge on R-group (Aspartic acid and Glutamic Acid)]


or R-group is hydrophobic and unable to form Hydrogen bonds

Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Proline and Methionine

Classification of Amino Acids Based on Nutritional Requirement

The body utilizes all the 20 amino acids for protein synthesis, however the body cannot synthesize all the 20 amino acids required for protein synthesis, Hence the basis for the nutritional classification of amino acids.

Amino acids can be either Essential (those required by the body in the diet for protein synthesis) or Non-Essential (those that can be synthesized by the body hence not required in the diet)

Essential Amino Acids incude;Valine, Phenyalanine, Lysine, Methionine,Isoleucine, Leucine, Threonine, Tryptophan, Argine and Histidine.

Non-Essential Amino Acids include; Alanine, Aspartic Acid, Cysteine, Glutamic acid, Glutamine, Glycine,mAsparagine, Serine, Proline and Tyrosine

Arginine and Histidine are sometimes considered semi-essential amino acids because they can be synthesized by adults but not children

Properties of Amino Acids

Amino Acids contain both basic (NH2) and acidic (COOH) groups, as such they can act as either an acid or a base.

amino acids can carry both positive and negative charges in solution. hence depending on the pH of the solution an amino acid would carry a net negative or positive charge. However at certain pH an amino acid can exist as a ZWITTERION carrying no net charge. The pH at which an amino acid carries no net charge is referred to as the pI (isoelectric pH) of the amino acid. The isoelectric pH varies for different amino acids. At the pI the amino acid is electrically neutral.




amino acids have the capacity to form salts with alkali, amides with ammonia and also decarboxylation of amino acids leads to the generation of the corresponding amines

Non Standard Amino Acids 

apart from the 20 amino acids, there are several other amino acids which are biologically important; these include 

amino acid derivatives (synthesized by post-transcriptional modification of the 20 standard amino acids). they are important for protein structure and function(examples include 5-hydroxylysine found in collagene protein, gamma-carboxyglutamic acid found in coagulation proteins, methylated and acetylated amino acids found in histones) and 

non-protein amino acids; These are not usually found in proteins but however carry out several important biologic functions. examples include; Ornithine, Citrulline, Arginosuccinic acid, Thyroxine, Tri-iodothyronine, Homocysteine, Dopamine, Creatinine etc.