EAMCET QR Chemistry Sr Chem 15.Biomolecules PROTEINS

8/8/2019 EAMCET QR Chemistry Sr Chem 15.Biomolecules PROTEINS

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• Amino acids contains both amine group (–NH2) and carboxylic acid group (–COOH).

• Amino acid molecules are linked by forming an amide bond

NH C

O

• Carboxylic acid of one molecule reacts with the amino group of another molecule to form amide

bond.

2H O2NH CH COOH H NH CH COOH

| |

R R

−− − + − − − ⎯⎯⎯→

2Amide bond(Peptide bond)

O H|| |H N CH C NH C COOH

| |RR

− − − − −

• Linkages between amino acids are known a peptide linkages or peptide bonds.

• The product obtained from two amino acid molecule through peptide bond is called dipeptide.

• Based on number of amino acid molecule in peptide they are called tri, tetra and polypeptides.

• Protein from a Greek word proteios mean prime importance.

• Proteins are naturally occurring, polypeptides containing 100 to 300 amino acid units.

• Silk, hair, skin, connective tissues most of the enzymes, hormone etc are examples for proteins.• In carboxylic acid chain based on the location of NH2 group on the carbon amino acids are

named as α, β, γ, δ.

H2N – CH2 – COOH α-amino acid

H2 – N – CH2 – CH2 – COOH β-amino acid

H2N – CH2 – CH2 – CH2 – COOH γ-amino acid

• Naturally occurring amino acids are more than 700 but important amino acids are 20.

• Protein forming amino acids are α-amino acid containing a primary amine group except protein a

secondary amine.

• Simplex amine acid is glycine Greek meaning sweet.• IUPAC name of glycine is 2-amino ethanoic acid.

• Amino acid containing equal number of –NH2 and –COOH groups are neutral.

• Amino acid contains more number of –NH2 group it is basic, if it containing more COOH groups it

is acedic.

• Amino acids that cannot be synthesized in the body must be supplied through diet are called

essential amino acids.

• Non-essential amino acid can be synthesized in the body.

• Amino acids are colourless crystalline solids.

• Amino acids are highly polar in aqueous solution.

• Proton transfers from acid group to amine group to give Zwitter ion or inner salt.

15. BIOMOLECULES

ii) PROTEINS



Biomolecules

2

• In Zwitter ion acedic nature is due to –NH3+

group and basic nature is due to COO−− group.

H2N CH COOH

R

H2O

H3N CH COO

ROH

H+

(Cation)H3N CH COOH

R

(Anion)H2N CH COO

R

• Zwitter ion in acid medium becomes positive ion and in basic medium becomes negative ion.

• Dipolar Zwitter ion act as a neutral ion and does not migrate towards anode or cathod at a

particular pH called isoelectric point of the aminoacid.

• Isoelectric point of the amino acid depends on the groups present in the amino acids.

• For neutral amino acids isoelectric point is in the range 5.5 to 6.3.

• Least solubility of amino acid at isoelectric point helps in the separation of different amino acids

obtained from the hydrolysis of protein.

• Due to asymmetric (chiral) α-carbon all amino acids are optically active except glycine.

• In fisher projection D-form of amino acid –NH2 group on the right and in L-form – NH2 group is on

the left – COOH group is on the top in both forms.

COOH

HH2N

RL - form

COOH

NH2H

RD - form

• Most of the naturally occurring amino acids are with L-configuration.

• In a polypeptides free amino group (NH2)

N-terminal residue to the left and acid group is to the right.

• Alanyl glycylalanine can be represented as

Ala - Gly - Ala

Alanyle glycyl alanine

NH2 CH C NH CH2 C NH CH COOH

CH3 CH3

OO

N-terminal C-terminal

• Shorter peptides are called oligopeptides longer peptides are polypeptides.

• Proteins are polypeptides containing many amino acids molecular mass is more than 10,000.

• Polypeptides are amphoteric.

• Most of the toxins which poisonous substances present in animal and plant venoms are proteins.

• Oligopeptides are effective hormones.

• Aspertame is a dipeptides which is 160 times sweeter than sucrose.

• Aspertame is aspartyl phenylalanine methyl ester.



Biomolecules

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H2N CH CO NH CH COOCH3

CH COOH CH2C6H5

STRUCTURE OF PROTEINS :

• Proteins are biopolymers of large number of amino acid linked through peptide bonds or

disulphide bond.

• In disulphide linkage S S| |

H H

− = −

• Primary structure of amino acid gives specific sequence of amino acids in polypeptide.

• 100 amino acid units having 20 different amino acids can combine (20)100

different ways.

• Primary structure tells us about peptide linkages and sulphide linkages.• Primary structure is only due to covalent bond linkage.

• Secondary structure of protein or polypeptide explains shape and describes conformation of

segments.

• Peptide chain folds to limit the possible conformation, to minimise number of hydrogen bonds

and to avoid steric hindrance between R groups.

• Secondary structure is due to hydrogen bonds between N and O.

• The segment of the protein back bone fold either α-helix or β pleated sheet or coil conformation.

• Tertiary structure is three-dimensional arrangement of atoms in the protein.

• It explains extensive coiling or folding to produce a complex.

• Quaternary structure defines the structure resulting from the inhalations between the subunits of

polypeptide chains.

• The interactions between subunits to give quarlernary structure are

i) Hydrogen bonding

ii) Electrostatic attraction

iii) Hydrophobic interactions

• Sub units arrangement in space is given by quarlernary structure.

• Protein denaturation involves breaking of tertiary structure of protein.

• Proteins with weak interactive bonds can be easily denatured.

• Denaturation can be by

i) Changing pH to disrupts hydrogen bonds.

ii) By adding reagent like urea to form strong hydrogen bonds with urea.

iii) Adding detergents like sodium dodecyl sulphate.

iv) Organic solvents associates with non-polar groups to interfere with hydrophobic interactions.

v) Heating or agitation which causes disruption of attractive forces.

AMINO ACIDS DERIVED FROM PROTEINS :

A. Neutral Amino Acids :1. Glycine (Gly)



Biomolecules

4

(Aminoacetic acid)

C

H

NH2

COOHH

2. Alanine (Ala)

(α-Aminopropinoic acid)

C

H

NH2

COOHCH3

3. Valine (Val)

(α-Aminiosovaleric acid)

C

H

NH2

COOHCH3

CH3

H

C

4. Leucine (Leu) (α-Aminoisocaproic acid)

C

H

NH2

COOHCH3

CH3

H

C C

H

H

5. Isoleucine (Ileu) (α-Amino-β-methylvaleric)

C

H

NH2

COOHCH3

CH3

H

CCH2

6. Serine(Ser) (α-Amino-β-hydroxypropionic acid)

C

H

NH2

COOHHOC

H

H

7. Threonine (Thre) (α-Amino-β-hydroxybutyric acid)

CH3 C

H

OH

C

H

NH2

COOH

8. Phenylalanine (Phe) (α-Amino-β-phenylpropionic acid)



Biomolecules

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C

H

H

C

H

NH2

COOH

9. Tyrosine (Tyr) (α-Amino-p-hydroxyhydrocinnamic acid)

C

H

H

HO C

H

NH2

COOH

10. Tryptophan (Try) (α-Amino-β-(3-indolyl) propionic acid)

N

H

C

H

H

C

H

NH2

COOH

11. Proline (Pro)1

(2-Pyrrolidine carboxylic acid)

CHCOOH

CH2H2C

H2C

N

H

12. Hydroxyproline (Hpro)1

(4-Hydroxy-2-pyrrolidine carboxylic acid)

CHCOOH

CH2CH

H2C

N

H

HO

1Proline and hydroxyproline are imino acids . The nitrogen atom, although joined to the α-carbon,is part of a ring. An imino nitrogen bears only one hydrogen atom but can still take part in theformation of proteins.

B. Basic Amino Acids: 13. Histidine (His)

(α-Amino-β-4-imidazolylpropionic acid)

C

H

NH2

COOH

NH

C

C

H

H CH2C

N



Biomolecules

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14. Lysine (Lys) (α, ε-Diaminocaproic acid)

C

H

NH2

COOHCH2

NH2

CH2 CH2 CH2

15. Arginine (Arg) (α-Amino-δ-guanidinovaleric acid)

C

H

NH2

COOHH2N C N

HN

H

CH2CH2CH2

C. Acidic Amino Acids :

16. Aspartic acid (Asp) (Aminosuccinic acid)

H

NH2

COOHHOC

O

CH2 C

17. Glutamic (Glu)

(α-Aminoglutaric acid)

HOC

O

CH2 CH2

H

NH2

COOHC

D. Sulfur-Containing Amino Acids :

18. Methionine (Met) (α-Amino- γ-methylthiobutyric acid)

CH3 S CH2 CH2

H

NH2

COOHC

19. Cysteine (Cys) (α-Amino-β-mercaptopropionic acid)

CH2

H

NH2

COOHCHS

20. Cystine (Cys-Scy)



Biomolecules

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•

CH2

H

NH2

COOHCS

CH2

H

NH2

COOHCS

EAMCET QR Chemistry Sr Chem 15.Biomolecules PROTEINS

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