Introduction to Proteins and Amino acids

Introduction to Proteins and Amino acids

Contents

–      Introduction to proteins

–      Classification of aminoacids

Objective

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

Proteins

•       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

Denaturing

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

–      Heat

–      Acids

–      Bases

–      Salts

–      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

–      Meats

–      Poultry

–      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 R^l group. e.g.: Alanine,  Leucine, Isoleucine, Valine etc

b. Polar amino acid with no charge on R^l group: amino acid with no charge on R^l 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 R^l group: e.g. Leucine, Argenine, Histidine

d. Polar amino acid with – ve charge on R^l 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 200^oc)
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 P^H (P^I): is defined as the P^H 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 (-COOR^I)

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

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

Functions

•       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

Summary

•       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