Amino Acids - Biology Ease

Amino Acids- A functional Group

Amino acids are a group of organic compounds containing two functional groups- amino and carboxyl.
The amino group (-NH2) is basic while the carboxyl group (COOH) is acidic in nature.
The general structure of amino acids The amino acids are termed as a-amino acids.
If both the carboxyl and amino groups are attached to the same carbon atom, as depicted below The a-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 mostly exist in the ionized form in the biological system.
Optical isomers of amino acids If a carbon atom is attached to four different groups, it is asymmetric and therefore exhibits optical isomerism.
The amino acids (except glycine) possess four distinct groups (R, H, COO, NH) held by o-carbon.
Thus all the amino acids (except glycine where R = H) have optical isomers.
The structures of L- and D-amino acids are written based on the configuration of L- and D-glyceraldehyde.
The proteins are composed of L-a-amino acids

Amino Acid- A functional Group

CLASSIFICATION OF AMINO ACIDS

There are different ways of classifying the amino acids based on the structure and chemical nature, nutritional requirement, metabolic fate etc.

Amino acid classification based on the structure :

A comprehensive classification of amino acids is based on their structure and chemical nature. Each amino acid is assigned a 3 letter or 1 letter symbol.
These symbols are commonly used to represent the amino acids in protein structure.
The 20 amino acids found in proteins are divided into seven distinct groups.

These are monoamino monocarboxylic acids. This group consists of the most simple amino acids glycine, alanine, valine, leucine and isoleucine.
The last three amino acids (Leu, lle, Val) contain branched aliphatic side chains, hence they are referred to as branched-chain amino acids.

Serine, threonine and tyrosine are hydroxyl group-containing amino acids.
Tyrosine-being aromatic in nature—is usually considered under aromatic amino acids.

Cysteine with sulfhydryl group and methionine with thioester group are the two amino acids incorporated during protein synthesis.
Cystine, another important sulfur-containing amino acid, is formed by condensation of two molecules of cysteine.

Aspartic acid and glutamic acids are dicarboxylic monoamine acids while asparagine and glutamine are their respective amide derivatives.
All these four amino acids possess distinct codons for their incorporation into proteins.

The three amino acids lysine, arginine (with guanidine group) and histidine (with imidazole ring) are dibasic monocarboxylic acids. They are highly basic in character.

Phenylalanine, tyrosine and tryptophan (with indole ring)are aromatic amino acids. Besides these, histidine may also be considered under this category.

Proline containing pyrrolidine ring is a unique amino acid. It has an amino group (=NH), instead of an amino group (-NH2) found in other amino acids. Therefore, proline is an a-amino acid.
Classification of amino acids based on polarity: Amino acids are classified into 4 groups based on their polarity.
The polarity, in turn, reflects the functional role of amino acids in protein structure.
Non-polar amino acids: These amino acids are also referred to as hydrophobic (water-hating).
They have no charge on the ‘R’ group. The amino acids included in this group are — alanine, leucine, isoleucine, valine, methionine, phenylalanine, tryptophan and proline.
Polar amino acids with no charge on the ‘R’ group: These amino acids, as such, carry no charge on the ‘R’ group.
They, however, possess groups such as hydroxyl, sulfhydryl and amide and participate in hydrogen bonding of protein structure.
The simple amino acid glycine (where R= H) is also considered in this category.
The amino acids in this group are- glycine, serine, threonine, cysteine, glutamine, asparagine and tyrosine.
Polar amino acids with positive ‘R’ group: The three amino acids lysine, arginine and histidine are included in this group.

Polar amino acids with negative ‘R’ group :

The dicarboxylic monoamine acids aspartic acid and glutamic acid are considered in this group.

Nutritional classification of amino acids :

The twenty amino acids are required for the synthesis of a variety of proteins, besides other biological functions.
However, all these 20 amino acids need not be taken in the diet.
Based on the nutritional requirements, amino acids are grouped into two classes essential and non-essential.

Essential or indispensable amino acids :

The amino acids cannot be synthesized by the body and, therefore,
They need to be supplied through the diet are called essential amino acids.
They are required for the proper growth and maintenance of the individual.
The ten amino acids listed below are essential for humans (and also rats): Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine, phenylalanine, Threonine, Tryptophan.
The two amino acids namely arginine and histidine can be synthesized by adults and not by growing children, hence these are considered as semi-essential amino acids (remember Ah, to recall).
Thus, 8 amino acids are essential while semi-essential.
Non-essential or dispensable amino acids: The body can synthesize about 10 amino acids to meet the biological needs,
hence they need not be consumed in the diet. These are-glycine, alanine, serine, cysteine, aspartate, asparagine, glutamate, glutamine, tyrosine and proline.

PROPERTIES OF AMINO ACIDS

The amino acids differ in their physicochemical properties which ultimately determine the characteristics of proteins.

PHYSICAL PROPERTIES

Solubility: Most of the amino acids are usually soluble in water and insoluble in organic solvents.
Melting points: Amino acids generally melt at higher temperatures, often above 200°C.
Taste: Amino acids may be sweet (Gly, Ala, Val), tasteless (Leu) or bitter (Arg, lle).
Monosodium glutamate (MSG; Ajinomoto) is used as a flavouring agent in the food industry, and Chinese foods to increase taste and flavour.
In some individuals intolerant to MSG, Chinese restaurant syndrome (brief and reversible flu-like symptoms) is observed.
Optical properties: All the amino acids except glycine possess optical isomers due to the presence of asymmetric carbon atom.
Some amino acids also have a second asymmetric- carbon e.g. isoleucine, threonine. The structure 5 of L- and D-amino acids in comparison with glyceraldehyde.
Amino acids as ampholytes: Amino acids contain both acidic (-COOH) and basic (-NH2) groups.
They can donate a proton or accept a proton, hence amino acids are regarded as ampholytes.
Zwitterion or dipolar ion: The name zwitter is derived from the German word which means hybrid.
Zwitter ion (or dipolar ion) is a hybrid molecule containing positive and negative ionic groups. The amino acids rarely exist in a neutral form.

CHEMICAL PROPERTIES

The general reactions of amino acids are mostly due to the presence of two functions.
The amino groups behave as bases and combine with acids (e.g. HCI) to form salts (-NHZCH.
Reaction with ninhydrin: The a-amino acids react with ninhydrin to form a purple, blue or pink colour complex (Ruheman’s purple).
Colour reactions of amino acids: Amino acids can be identified by specific colour reactions.
Transamination: Transfer of an amino. The group from an amino acid to a keto acid to form a new amino acid is a very important reaction in amino acid metabolism
Oxidative deamination: The amino acids undergo oxidative deamination to liberate free ammonia.

References

https://www.ncbi.nlm.nih.gov/books/NBK234922/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3042786/