Amino Acids


Proteins constitute the structural  framework  of all components. Proteins are linear polymerase of long chain  made  up of amino acids. Thus, proteins have high molecular weight. Amino acids can be called as building blocks of proteins. The amino acids have general structure:

                      H                     NH2= Amino                                    |                                       Group 
        H2N___C___COOH    COOH= Carboxyl                             |                                     group 
                     R                      R = Alkyl group 

■ Amino acids are small molecules that are the building blocks of proteins. 

■ Chemically, an amino acid is a molecule that has a carboxylic acid group and an amine group that are each attached to a carbon  atom and an amino group that are each attached to a  carbon atom called the œ carbon.

   There are 20 types of Amino acids. These 20 amino acids can be classified as Essential and Non__ Essential amino Acids. 

■ Nonessential amino acids can be synthesized in the body, whereas essential amino acids must be obtained in the diet.

■ There are 9 Essential Amino acids and 11 Non__ essential Amino acids.

Number and Types:

Amino acids ( AA) are the building blocks of proteins. There are 20 amino acids that are normal components of cell proteins. These are called proteins amino acids. They are specified in the   genetic code. It may be noted that the genetic code is universal in  viruses , prokaryotes and eukaryotes The names of coded amino acids are written in an abbreviated form by using their first three letters, e.g., gly for glycine, ala for alanie. A protein may have amino acids not specified by the genetic code. These are termed rare amino acids. They are derived from the coded amino acids by modification, e.g., hydroxyproline formed from proline. Many¹ amino acids found in cells do not form proteins. These are called nonprotein.amino acids. Among these are ornithine, citrulline, game__ amino__ butyric acid  ( GABA) .The latter functions as a neurotransmitter. 

Draw the structure of an amino acid?


Composition of proteins Amino Acids: 


The protein amino acids are composed of carbon, hydrogen, oxygen and nitrogen.Some have sulphur also  Thus, the amino acids differ from carbohydrates and fats in having nitrogen and sometimes also sulphur.Eighteen of the 20 common amino acids have the same basic structure. They have a central four__ valence² carbon atom, called the alpha ( œ) carbon, to which are attached 

( i) an amino group ( ____ NH2) on one side, 

( ii) a carboxyl group ( ____ COOH) on the other side,

( iii) a hydrogen atom ( H) on the third side, and 

( iv) a variable group symbolised by R on the fourth side. The R group may be a carbon chain or ring. The side chain may be  polar or nonpolar. The carbon chains or rings differ in various amino acids and give the latter their unique properties. Generalized formulae for the amino acids. The amino acid glycine has a hydrogen atom joined to the fourth bond of the alpha carbon also. It is the simplest amino acid. All the 19 amino acids having œ carbon are called  œ amino acids. The amino acid proline has a ring structure including the central carbon. The carboxyl group imparts acidic properties and the amino group gives basic properties to the amino acid. Except glycine, the œ__ carbon is asymmetrically placed in the amino acid molecule. All the 20 amino acids found in the proteins are laevorotatory ³. Some nonprotein amino acids are dextrorotatory⁴. 


■ Each protein is a molecule made up of  different combinations of 20 types of  smaller, simpler amino acids.

■ protein molecules are long chains of amino acids that are folded into a three__ dimensional shape. 

■ Dietary proteins are the source of essential amino acids.

     ■ Proteins carry out  many functions in         living  organisms, some transport nutrients     across cell membranes, some fight        infectious organisms, some are hormones,    some are enzymes.

Collagen is the most abundant protein in the animal world and Ribulose bisphosphate   Carbohydrates__ Oxygenase ( RuBisCO) is the most abundant protein in the whole of the biosphere. 


Note: a) Over 170  amino acids are currently known to occur in cell

(b) Also written as valency.

( c) Rotate the ray of polarized light to the left.

(.d) Rotate the ray of polarized light to the right.

Properties and Functions 


Amino acids are colourless, crystalline, solid compounds. They are generally soluble in water, but insoluble in organic solvents. They are amphoteric, having both basic and acidic properties. 


The amino acids rise to a variety of compounds __ 

1): Formation of Proteins: The principal function of amino acids it to form proteins by polymerisation   involving  peptide linkages.

2): Formation of other Compounds: Some amino acids also give rise to other important compounds. For instance 

(i) Tyrosine produces the hormones thyroxine and adrenaline and the skin pigment melanin, 

( ii) Glycine forms heme, and 

( iii) Tryptophan produces vitamin nicotinamide and plant hormone indole__ 3__ acetic acid ( IAA). The coenzyme A, and vitamin pantothenic acid are derived from ß __ alanine. The coenzymes glutathione and alkaloids are some other compounds formed from amino acids.

3): Formation of Glucose: Many amino acids change into glucose by losing amino group ( gluconeogenesis).

4) : Formation of Amines:  Amino acids also form amino by losing carboxyl group. For instance, histamine derived from the amino acid histidine. Histamine is a powerful stimulant of gastric secretion,  constrictor of bronchial smooth muscle, and a vasodilator that causes a fall in blood pressure.

5): Nitrogen Storage:The amino acid derivatives called amides serve as a storage of nitrogen. 

6): Biosynthetic pathways: Some nonprotein amino acids participate in certain important biosynthetic pathways. For example, the amino acids ornithine and citrulline play a role in urea cycle in the liver cells.

7): Biological Buffers: Amino acids, being amphotrric, act as buffers in solutions, resisting changes in pH. They do so by donating H+ ions as pH increases and accepting H+ ions as pH decreases. 

8): Antibiotics: Nonprotein amino acids are components of  antibiotics .

9): Prokaryotic Cell Wall:Small peptides are a component of peptidoglycan,  a compound that forms prokaryotic cell wall. 

10): Enkephalins: These are pentapeptides formed in certain parts of the brain. They are related to the perception of pain and pleasure. 

Classification of protein Amino Acids: 

The protein amino acids are classified from three different points of view.

A): On the basis of their reaction in solution: The amino acids may be divide into three groups on the basis of their reaction in solution: acidic, basic and neutral.

(i) Acide Amino Acids:  These have two carboxyl groups and one amino group molecule. They are called monoamino__ dicarboxylic amino acids. They include aspartic acid and glutamic acid.

( ii) Basic Amino Acids:  These have two amino groups and one carboxyl group per molecule. They are termed diamino__ monocarboxylic amino acids. They include lysine and arginine.

( iii)  Neutral Amino Acids:  These have one amino group and one carboxyl group per molecule. They are known as monoamino__ monocarboxylic amino acids. They include alanine, glycine, serine and others except the above four.

    A particular property of amino acids is the  ionizable nature of __ NH2 and __ COOH groups. Therefore,  in solutions of different pHs,the structure of amino acids changes.

( B) is called zwitterionic form.

B): On the basis of charges: The amino acids may also be classified into three groups on the basis of charges.

( i) Nonoplar having no charges ;
( ii) Polar uncharged,  charges balance, making the molecule as a whole uncharged  ;

(iii) Polar charged, may be negatively charged ( acidic ) or positively charged ( basic).

   The nonpolar amino acids are insoluble in water, the polar charged amino acids are soluble in water.

 C)  On the basis of special structure:  Amino acids are of three types regarding special structure __ 

( i) Sulphur__ containing Amino Acids, e.g., cysteine, methionine. 

 ( ii) Alcoholic Amino Acids:  These have alcoholic or hydroxyl group, e.g., serine,thronine. 

( iii) Heterocyclic Amino Acids:  These have nitrogen in the ring structure, e.g., histidine, proline.

Essential Amino Acids:

Seven amino acids are the essential amino acids for man. They include leucine, isoleucine, lysine, metheonine, phenylalanine, tryptophan and valine. They must be ingested in food for survival since they are not synthesized in the body. These are taken from the plants directly by herbivores and indirectly by carnivores. The other amino acids are synthesized by the animals from their existing or ingested precursors. Plants and some microorganisms can synthesize all the amino acids from inorganic nitrogen sources. 

How are amino acids bones together? Describe how these bonds are formed?

Linkage of Amino Acids: 

Amino acids link up in such a way that the amino group of one amino acid is joined with the help of an  enzyme to the carboxyl group of the adjacent one with  a loss of water molecule for each linkage . The amino group contributes  hydrogen( H) and the carboxyl group contributes hydroxyl group ( __ OH) to water. The covalent bond joining the two amino acids by__ NH__ CO linkage is called peptide bond. The linking of amino acids, called protein synthesis, takes place on the ribosome.

       A combination of a few amino acids forms a peptide molecule. A chain of two amino acids is called a dipeptide; of three amino acids, a tripeptide, and of any ( 100__ 300) amino acids, a polypeptide. A polypeptide is formed by one__ by__ one addition of amino acid units. A polypeptide breaks down to its constituent amino acids by hydrolysis. 


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