Amino acids

 Amino acids

Amino acids are organic compounds that serve as the building blocks of proteins, which are essential molecules in all living organisms. Chemically, an amino acid consists of a central carbon atom, known as the alpha (α) carbon, to which four groups are attached: an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom (H), and a distinctive side chain (R group) that varies for each amino acid. The side chain determines the chemical nature and properties of the amino acid, such as whether it is acidic, basic, polar, non-polar, or aromatic. Amino acids are linked together through peptide bonds to form proteins, which carry out a vast array of biological functions, including acting as enzymes, hormones, structural components, and signaling molecules.

Standard amino acids are the twenty amino acids that are universally found in proteins and are incorporated into proteins during ribosomal protein synthesis in all living organisms. These amino acids are called “standard” because they are the common set that cells use to build proteins, as specified by the genetic code. Each of these amino acids has a specific three-letter and one-letter code used to represent them in protein sequences.

Optical isomers of amino acids are forms of amino acids that exist as non-superimposable mirror images of each other due to the presence of an asymmetric (chiral) carbon atom. Most amino acids have a central alpha (α) carbon attached to four different groups: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom (H), and a side chain (R group). This asymmetry at the alpha carbon makes the molecule chiral, allowing it to exist in two optical forms, called enantiomers, which are mirror images of each other.

These optical isomers are designated as D- (dextro) and L- (levo) forms, based on their relationship to the reference compound glyceraldehyde. In the L-form, which is the one commonly found in proteins in living organisms, the amino group is on the left when drawn in a standard Fischer projection. In contrast, the D-form has the amino group on the right. Glycine is an exception because its side chain is a hydrogen atom, so its alpha carbon is not asymmetric, and therefore glycine does not have optical isomers.

The existence of optical isomers is biologically significant because enzymes and ribosomes are stereospecific, meaning they recognize and utilize only the L-forms of amino acids to synthesize proteins. D-amino acids are rare in nature but can be found in some bacterial cell walls and certain antibiotics.

Based on their chemical structure

1.  Amino acids with aliphatic side chains- Structurally, amino acids are distinguished by the chemical nature of their side chains, known as R-groups. The simplest group is the aliphatic amino acids, which are monoamino monocarboxylic acids with non-aromatic side chains. This group includes glycine, alanine, valine, leucine, and isoleucine, with valine, leucine, and isoleucine also recognized as branched-chain amino acids due to their branched side chains.

2.     Hydroxyl group-containing amino acids- Another important structural group is the hydroxyl-containing amino acids, such as serine, threonine, and tyrosine, which possess hydroxyl (-OH) groups in their side chains and participate in hydrogen bonding; tyrosine is also aromatic.

3.     Sulfur-containing amino acids- They include cysteine, which has a sulfhydryl (-SH) group, and methionine, which contains a thioether (-S-CH3) group, while cystine, formed by the condensation of two cysteine molecules, is another key sulfur amino acid.

4.     Acidic amino acids -like aspartic acid and glutamic acid have two carboxyl groups, whereas their respective amides, asparagine and glutamine, are amide derivatives.

5.     Basic amino acids- Lysine, arginine, and histidine possess highly basic side chains with functional groups like guanidino in arginine and imidazole in histidine.

6.     Aromatic amino acids- Phenylalanine, tyrosine, and tryptophan have aromatic rings in their side chains, and histidine may also be considered aromatic due to its imidazole ring.

7.    Imino acids- Proline is unique as an imino acid, containing a cyclic pyrrolidine ring and an imino group instead of the typical amino group found in other amino acids.

Based on their polarity

Amino acids can also be classified based on their polarity, which is important for protein structure and folding.

1.     Non-polar amino acids are hydrophobic and have R groups that do not carry a charge. These amino acids tend to avoid water and are usually located in the interior of proteins. Examples of non-polar amino acids include alanine, leucine, isoleucine, valine, methionine, phenylalanine, tryptophan, and proline.

2.     Polar amino acids with no charge on the R group have side chains that can form hydrogen bonds but are not ionized under physiological conditions. They are generally hydrophilic and often participate in interactions with water or other polar molecules. This group includes glycine, serine, threonine, cysteine, asparagine, glutamine, and tyrosine.

3.     Polar amino acids with a positive R group are also called basic amino acids because their side chains contain groups that can accept protons, giving them a positive charge at physiological pH. The amino acids in this category are lysine, arginine, and histidine.

4.     Polar amino acids with a negative R group are termed acidic amino acids because their side chains contain carboxyl groups that can donate protons, giving them a negative charge at physiological pH. The acidic amino acids are aspartic acid (aspartate) and glutamic acid (glutamate).

Thus, the classification of amino acids based on R group polarity and charge helps explain their behavior in proteins, including how they interact with water, form hydrogen bonds, stabilize protein structures, and participate in enzyme catalysis.

Based on nutritional perspective

Amino acids are categorized into essential and non-essential types.

1.     Essential amino acids cannot be synthesized by the human body and must be obtained through diet. These include valine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, arginine, and histidine.

2.     Among these, arginine and histidine are semi-essential because they can be synthesized in adults but are required in the diet of growing children.

3.     Non-essential amino acids, which the body can synthesize, include glycine, alanine, serine, cysteine, aspartate, asparagine, glutamate, glutamine, tyrosine, and proline.

Based on their metabolic fate

This classification depends on whether their carbon skeletons are used for glucose or fat synthesis.

1.    Glycogenic amino acids, such as alanine, aspartate, glycine, and methionine, serve as precursors for glucose or glycogen.

2.    Ketogenic amino acids, including leucine and lysine, are metabolized into ketone bodies or fatty acids. Some amino acids have dual roles and can contribute to both glucose and fat metabolism.