Amino acids are a group of organic compounds containing two functional groups. They are amino group and carboxylic group.
Amino groups is basic while carboxyl group are acidic in nature.
Structure of amino acids:-
|Structure of amino acids|
The amino acids are termed as alpha amino acids if both the carbonyl and the amino group are attached to the same carbon atom.
Optical isomerism of amino acids:-
If a cabon atom is attached to 4 different groups it is asymmetric and therefore exhibit optical isomerism.
The amino acids except glycine processes four different groups [r1coo,h,nh3+] held by α carbon.
|Optical isomerism of amino acids|
The structure of l and d amino acids are written based on the configuration of l and d glyceraldehydes.
If NH2 is present in right side it is known as the d-amino acid.
And if NH2 is present in left side it is known as l-amino acid.
Classification of amino acid:-
Based on structure:-
That 20 amino acid found in protein are divided into seven distinct groups.
Amino acid with aliphatic side chain:-
These are mono amino carboxylic acids. This group consists of simple amino acid glycine, alanine, vialine,lucine and isolucine.
|Amino acid with aliphatic side chain|
Amino acids with hydroxyl group:-
Serine and threonine are hydroxyl group containing amino acids.
|Serine and threonine are hydroxyl group|
Amino acids with sulphur:-
cystine with sulphdryl group and methionine with tio ester group are two amino acid incorporated during the course of protein synthesis.
Cystine, another important sulphur containing amino acid is formed by condensation of two molecules of cystine. Cystine plays a unique role in protein synthesis.
|cystine with sulphdryl group and methionine|
Acidic Amino acids:-
Aspartic acid and glutamic acid are dicarboxylic mono amino acid. While aspargine and glutamine are the respective amide derivatives.
All these 4 amino acids processes distinict codons for their incorporation into proteins.
|acidic amino acids|
Basic amino acids:-
That 3 amino acids lysine, arginine [guanidine group] histidine [imidazol grouop]are dibasic monocarboxylic acid.
They are highly basic in character.
|lysine, arginine [guanidine group] histidine [imidazol grouop]|
Aromatic amino acids:-
Phenylalanine, tyrosine and tryptophan [indole group].
|Phenylalanine, tyrosine and tryptophan [indole group]|
Proline containing pyrolidine ring is a unique amino acid.
It has imino group [=nh] instead of an amino group.
Hydroxyproline is proline derivative found in proteins.
Nutritional classification of amino acids:-
That 20 amino acids are required for the synthesis of a variety of protein, besides other biological functions.
Based on the nutritional requirements amino acids are grouped into two classes.
Essential amino acids and nonessential amino acid.
Essential amino acids:-
The amino acid which cannot be synthesized by the body and therefore need to be supplied through the diet are called essential amino acids. The 10 amino acids listed below are essential for humans-
Argentine, valine, isolucine, lucine, lysine, methionine, phynyl alanine, threonine,tryptophan.
Argentine and histidine or semi essential amino acids.
Non essential amino acids:-
The body can synthesise about 10 amino acids to meet the biological needs hence they need not to be consumed in the diet.
These are glycine, alanine, serine, cystine, aspatate, aspargine, glutamate, glutamine, tyrosine and proline.
Properties of amino acids:-
The amino acids differ in their physic-chemical properties which ultimately determined the characteristics of proteins.
Most of the amino acids are usually soluble in water and insoluable in organic solvents.
Amino acids generally melt at higher temperature of an about 200o.
Amino acids maybe sweet [example glycine, alanine, valine]taste less [leucine],bitter in taste [argentine and isoleucine].
Sodium glutamate is a salt of glutamic acid. It is employed as the flavouring agent in food industry to taste and flavor.
All the amino acids except glycine processes optional isomers due to the presence of asymmetrical α-carbon atoms. Some amino acids also have a second asymmetric carbon [isoleucine,threoinne].
Zwitter ion [or] dipolar ion:-
It is a hybrid molecule containing positive and negative and ionic groups.
The amino acids rarely exist in a neutral forms with free carboxylic acid and amino groups.
In strongly acidic ph [low ph], the amino acid is positively charged [cation] while in strongly alkaline ph it is negatively charged [anion].
Each amino acid has a characteristics ph [leucine 6.o] at which it carries both positive and negative charges and exist as zwitter ion.
Iso-electric ph :-
It is defined as the ph at which a molecule exist as a zwitter ion [or] dipolar ion and carries no net charge. Thus the molecule is electrically neutral.
The pi value can be calculated by taking the average pka values corresponding the ionizable groups. For instant, leucine has two ionizable groups and its pi value is calculated as follows.
Leucine exist as caution at ph below 6 and anion at ph above 6.
At the isoelectric ph=[pi=6.0] leucine is found as zwitter ion.
The general reaction of the amino acids are mostly due to in the presence of two groups namely carboxyl (-cooh) and amino(-nh2) group.
Amino acids undergo decarboxylation to produce corresponding amines.
Reaction with nh3:-
The carboxyl group of dicarboxylic amino acid react with nh3 to form amide.
Aspartic acid + nh3 à aspargine.
Glutamic acid + nh3 à glutamine.
Reaction with ninhydrine:-
The α- amino acid reacts with ninhydrin to form a purple, blue or pink colour complex [ruhemann’s purple].
Amino acid + ninhydrin à keto acid + nh3+co2 + hydrindatin.
Hydrindatin + nh3 + ninhydrin àruhemann’s purple
Transfer of an amino group from an amino acid to keto acid to form a new amino acid is a very important reaction in amino acid metabolism.
All transminases require pyrindoxal phosphate [plp] a co enzyme derived from vitamin b6.
It is reversible it is very important for redistribution of amino group and production of non essential amino acids.
Transamination divert the excess amino acids towards energy generation.
Glutamate is the only amino acid that undergo oxidation. Deamination to a significant extent to liberate free nh3 for urea synthesis.
All amino acids except lysine, threonine, proline and hydroxyproline participate in transamination.
The removal of amino group from the amino acids as nh3 is called deamination.
It is either oxidative [or] non oxidative.
It is the liberation of free amino ammonia from the amino group of amino acids coupled with oxidation.
It is to provide nh3 for urea synthesis.
L-amino acid oxidase and d-amino acid oxidase are proteins processing fmn and f a d respectively.
The act on the corresponding amino acids [l or d] to produce α-keto acids and nh3.
In this reaction oxygen is reduced to h2o2, which is later decomposed by catalase.
Non oxidative deamination:-
Some of the amino acids can be dominated to liberate nh3 without undergoing oxidization.
Amino acid desulfhydrases:-
The sulphur containing amino acid namely cystine under deamination coupled with desulfhydration to give keto acids.
Structural organisation of proteins:-
The structure of proteins can be divided into four levels of organization.
Each protein has a unique sequence of amino acids.
The amino acids are held together in a protein by covalent peptide bond.
Peptide bond is formed in amino groups of one amino acid and carbonyl group of another amino acid. The peptide bond is rigid and plannar peptide bond are polar and are involved in hydrogen bond formation.
The proteolytic enzyme such as trypsin and pepsin and chymotrypsin exhibit specific in cleaning the peptide bond.
Cyanozen bromide is commonly used to split polypeptide in a smaller fragments.
The confirmation of polypeptide chain be twisting or holding is referred as secondary structure.
Two types of 20 structures α-helix and β-sheet indian scientists ramachandran made a significant in understanding the special arrangement of polypeptide, chains.
It is the most common spiral structure of proteins α-helix structure was proposed by pauling and corey in 1951.
The α-helix is a tightly packed coiled structure with amino acid side chains extending outward from the central axis.
The α-helix is stabilized dubai extensive hydrogen bonding. It is formed between hydrogen atom attached to peptide nitrogen, oxygen atom attach it to peptide carbon.
The hydrogen bonds are individually week but collectively they are strong enough to stabilize the helix.
Each turn of α-helix contains 3.6 amino acid and travel a distance of 0.54 nm the spacing of each amino acid is 0.15 nm.
The right handed α-helix is more stable then left handed helix.
This is a second type of structure proposed by pauling and corey β-pleated sheet are composed of two or more segments are fully extended peptide chains.
In the β-sheets the hydrogen bonds are found between the neighbouring segment of polypeptide chains.
Tertiory structure of proteins:-
The 3-dimensional arrangement of protein structure is refered to as tertiory structure.
It is a compact structure with a hydrophobic side chains held interior while the hydrophilic groups are on the surface of the protein molecules.
This type of arrangement ensures stability of the molecule.
Besides the hydrogen bond disulphide bonds ionic interactions and hydrophobic interactions also contribute to the territory structure of the proteins.
Quaternary structure of proteins:-
Some of the proteins consists of two or more polypeptides which may be identical or unrelated. Such proteins are termed as oligomers and processes quaternary structure.