Introduction to Proteins and Amino acids

Introduction to Proteins and Amino acids


      Introduction to proteins

      Classification of aminoacids


At the end of this lecture, student will be able to

      Explain general concept of proteins

      Classify amino acid

      Explain properties of amino acid 

      Explain the significance of amino acid


       Proteios: holding the first place

       10 – 12kg in adult body proteins

       Proteins are the most abundant organic molecules of living system and forms the fundamental basis of structure and function of life   

       Large molecules

       Made up of chains of amino acids

       Are found in every cell in the body

       Functions are broadly grouped as structural and dynamic function

       Proteins are polymers of amino acid 

Elements of proteins

       Carbon – 50%

       Hydrogen – 6%

       Oxygen – 19%

       Nitrogen – 13%

       Sulfur – 5%

       Other elements are P, Fe, Cu, I, Mg, Mn, Zn etc

Classification of proteins

Structure of Proteins

       Proteins on complete hydrolysis with conc. HCl yield L-α-amino acid and is the common properties of all proteins

       As many as 300 amino acid occur in nature, out of these only 20 are known as standard amino acid and are repeatedly found in structure of proteins 

       Made up of chains of amino acids; classified by number of amino acids in a chain

      Peptides: fewer than 50 amino acids

       Dipeptides: 2 amino acids

       Tripeptides: 3 amino acids

       Polypeptides: more than 10 amino acids

      Proteins: more than 50 amino acids

       Typically 100 to 10,000 amino acids linked together

       Chains are synthesizes based on specific bodily DNA

       An adult has about 100g of free amino acid, which represents amino acid pool of the body

       Four levels of structure

      Primary structure

      Secondary structure

      Tertiary structure

      Quaternary structure

Any alteration in the structure or sequencing changes the shape and function of the protein


       Alteration of the protein’s shape and thus functions through the use of





      Mechanical agitation

       Primary structure is unchanged by denaturing

Functions of protein

      Provide structural and mechanical support

      Maintain body tissues

      Functions as enzymes and hormones

      Help maintain acid base balance

      Transport nutrients

      Assist the immune system

      Serve as a source of energy when necessary

Best Sources of Protein

       Proteins are abundant in

      Dairy foods



      Meat alternatives such as dried beans, peanut butter, nuts, and soy

       Cooked meat, poultry, or fish

      Provides 21–25 grams of protein

      About 7 g

      About the size of a deck of cards

      Adequate amount for one meal

Adults should consume 0.8 g/kg/d of protein

Amino acid

       Amino acid are the group of organic compounds containing two functional groups ie. Amino and carboxyl groups

       Amino group is basic and carboxyl group is acidic in nature

       Amino acid mostly exist in ionized form in biological system

       α- amino acid: If both –COOH and – NH2 is attached to same carbon atom

Amino acid structure

Peptide Bonds Link Amino Acids

       Form when the acid group (COOH) of one amino acid joins with the amine group (NH2) of a second amino acid

       Formed through condensation

       Broken through hydrolysis

Condensation and Hydrolytic Reactions

Classification of Amino acid

       Amino acids are classified into different ways based on structure, Polarity, nutritional requirement etc..

1. Classification based on structure:

       Each amino acid is assigned with 3 letter or 1 letter symbol and are commonly used to represents the amino acid in proten structure

       20 amino acid found I proteins are divided into seven groups

I. Amino acid with aliphatic side chain

                a. Glycine – Gly or G                        

                b. Alanine – Ala - A                                           

                c. Valine – Val - V 

                d. Leucine – Leu – L    

                e. Isoleucine – Ile - I

II. Amino acid containing hydroxyl group

                f. Serine – Ser - S                                             

                g. Threonine – Thr – T

III. Sulfur containing amino acid

                h. Cysteine – Cys- C                        Cystine – Cys- C

                i. Methonine – Met- M

IV. Acidic amino acid and their amides

                j. Asparitc acid – Asp – D              

                k. Asparagine – Asn – N

                l. Glutamic acid – Glu – E              

                m. Glutamine – Gln - Q    

V. Basic amino acid

                n. Lysine – Lys – k                                           

                o. Arginine – Arg – R

                p. Histidine – His – H                      

IV. Aromatic amino acid

                q. Phenylanaline – Phe – F          

                r. Tyrosine – Tyr – Y

                s. Tryptophane – Trp – W

VII. Imino acid  

                t. Proline – Pro –P

2. Classification based on polarity: 4 types

a. Non Polar amino Acid: Amino acid are also referred as hydrophobic (water heating). They have no charge in Rl group. e.g.: Alanine,  Leucine, Isoleucine, Valine etc

b. Polar amino acid with no charge on Rl group: amino acid with no charge on Rl group. However they possess other groups such as hydroxyl, sulfhydryl & amide and precipitate in hydrogen bonding of protein synthesis. e.g.: glycine, serine, threonine, cystine, tyrosine

c. Polar amino acid with + ve charge on Rl group: e.g. Leucine, Argenine, Histidine

d. Polar amino acid with – ve charge on Rl group: e.g. Glutamic acid, Aspartic acid

3. Nutritional classification of amino acid

20 amino acid are required for the synthesis of various proteins

Based on nutritional requirement. They are 2 types

a. Essential amino acid: Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methonine, Phenylalanine, Threonine and Tryptophan

b. Non-essential amino acid: Remaining all

4. Amino acid classification based on their metabolic fate

Carbon skeleton of amino acid can serve as a precursor for the synthesis of glucose, fats or both

a. Glycogenic Amino acid: amino acid are precursor for the formation of glucose or glycogen e.g.: Alanine, Aspartate, Glycine

b. Ketogenic amino acid: amino acid are precursor for the formation of fats. e.g.: Leucine and Lysine

c. Glycogenic & ketogenic amino acid: amino acid are precursor for the formation of glucose and fats e.g.: Isoleucine, Phenylalanine, Tryptophan, Tyrosine etc.

Properties of Amino acid

Amino acid differ in their physiochemical properties which determine the characteristics of proteins

I.            Physical properties

  1. Solubility:  Most are soluble in water and insoluble in organic solvent
  2. Melting point: melts at high temperature (above 200oc)
  3. Taste: may be sweet, tasteless, or bitter
  4. Optical properties: All amino acid except glycine possess optical isomers due to presence of asymmetric carbon atom
  5. Amino acid as ampholytes: it contains both functional group i.e. acidic & basic group. They can donate a proton or accept a proton. Hence regarded as ampholytes  
  6. Zwitterion or Dipolar ion: Zwitter means hybrid.
  7. zwitterion is a hybrid molecule contain both +ve and –ve ionic group.
  8. Isoelectric PH (PI): is defined as the PH at which molecules exists as Zwitterion, thus molecules are electrically neutral 

II. Chemical properties

Based on two functional group

  1. Reaction due to –COOH group

1.       Amino acid forms salt (-COONa) with base & esters with alcohol (-COORI)

2.       Decarboxylation: Amino acid undergoes decarboxylation to form amines

3.       Reaction with ammonia: the carboxylic group of dicarboxylic amino acid react with NH3 to form amide

                Aspartic acid + NH3                                          Asparagine

                Glutamic acid + NH3                                        Glutamine

b. Reaction due to - NH2 group:

  1. Amino acid group behaves as bas and combined with acid to form salts
  2. Reaction with Ninhydrin: Amino acid react with ninhydrin to form a purple, blue or pink colour complex
  3. Transamination: transfer of amino group from one amino acid to a ketoacid to form a new amino acid. Imp in amino acid metabolism
  4. Oxidative deamination: Amino acid undergoes oxidative deamination to liberate free ammonia

       Amino acid are chiral molecules. Only L-amino acid are found in proteins, D form occurs in bacterial peptides


       Peptides (from Greek word meaning "digested") are biologically occurring short chains of amino acid monomers linked by peptide (amide) bonds


       Act as biological catalyst called enzymes

       Provide structural frame work of cells and tissues

       Act as transport media in blood stream

       Act as hormones or regulatory proteins for controlling biological process

       Perform mechanical work (skeletal muscle contraction, pumping of heart)

       Serve as essential nutrient

       Act as antibodies in the blood stream

       Act in clotting mechanism

Some important biologically active peptides in human body

Protein Turnover

       Protein turnover is the balance between protein synthesis and protein degradation.

       More synthesis than breakdown indicates an anabolic state that builds lean tissues, more breakdown than synthesis indicates a catabolic state that burns lean tissues.

Nitrogen Balance

       Nitrogen balance is the measure of nitrogen input with the nitrogen output subtracted from it

       Nitrogen Balance = Nitrogen intake - Nitrogen loss

       Blood urea nitrogen can be used in estimating nitrogen balance, as can the urea concentration in urine

       A positive value is often found during periods of growth, tissue repair or pregnancy.

       A negative value can be associated with burns, fevers, wasting diseases and other serious injuries and during periods of fasting.

       This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested

       A negative Nitrogen balance can be used as part of a clinical evaluation of malnutrition


       Protein are polymers of amino acid

       Amino acid are the group of organic compounds containing two functional group i.e. amino and carboxyl group

       Amino acid exhibit zwitterion

       Nitrogen balance is the measure of nitrogen input with the nitrogen output subtracted from it

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