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 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

       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

Leave a Comment