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Amino Acids, Peptides and Proteins

The Proteins speak:

“We are the basis of structureand function of life;

Composed of twenty amino acids,the building blocks;

Organized into primary, secondary, tertiary

and quaternary structure;Classified as simple, conjugated

and derived proteins.”

AMINO ACIDS

- group of organic compounds containing two functional groups:

amino group (-NH2) basiccarboxyl group (-COOH) acidic

General Structure of Amino Acids

H H

R C COOH R C COOH

NH2 NH3

General Structure Exists as ion

⍺ - amino acids

aminogroups – attached to

thecarboxyl same carbon Atom

⍺ - carbon atom binds to a side chain represented by R (different for

each of the 20 amino acids found in proteins)

Ionized forms how they exist

Classification of Amino Acids based on polarity

of the R group

4 groups

Polarity reflects the functional role of AA in protein structure

1. Non-polar AA

hydrophobic (water hating)No charge on the ‘R’ groupExamples are:

Alanine MethionineLeucine

PhenylalanineIsoleucine

TryptophanValine Proline

3. Polar AA with (+) ‘R’ group

carries (+) charge Examples:

Histidine Arginine Lysine

4. Polar AA with (-) ‘R’ group

• carries (-) charge• Examples:

Glutamic Acid Aspartic Acid

2. Polar AA with no charge on ‘R’ group

no charge on the ‘R’ group possess groups hydroxyl

sulfhydrylamide

participate in hydrogen bonding of protein structure

Examples:Asparagine Glycine

Cysteine Tyrosine Serine Threonine Glutamine

A.Physical Properties

1. Solubility - soluble in water and insoluble in organic solvents

2. Melting Points - melt at higher temperatures often 200°C

3. Tastesweet (Gly, Ala, Val)tasteless (Leu)bitter (Arg, Ile)Sodium Glutamate – salt of Glutamic Acid – flavoring

agent

4. Optical Properties

- Assymetric a carbon atom is attached to 4 different groups

exhibiting optical isomerism

4 distinct groups R H - held by an

COOH ⍺-carbon NH3໋

All AA except Glycine possess optical isomers due to asymmetric ⍺-carbon atom

Some AA (Isoleucine, Threonine) 2nd asymmetric carbon

D- and L- forms of AA based onthe structure of glyceraldehyde

CHO CHO

H C OH OH C H

CH2OH CH2OH

D-Glyceraldehyde L-Glyceraldehyde

R R

H C NH2 H2N C H

COOH COOH

D-Amino Acid L-Amino Acid

The proteins are composed of L-⍺ amino acids

5. Amino acids as ampholytes

can donate a proton or accept a

proton

AA contain both acidic (-COOH) and basic (-NH2) groups

Zwitterion or dipolar ion:

Zwitter from German word – means

“hybrid”

Zwitter ion (or dipolar ion) a hybrid molecule containing

(+) and (-) ionic groups

AA rarely exist in a neutral form with free carboxylic (-COOH) and free Amino (-NH2) groups

Strongly acidic pH (low pH) AA (+) charged (cation)

Strongly alkaline pH (high pH) AA (-) charged (anion)

Each AA has a characteristic pH (e.g. Leucine, pH – 6.0), at which it carries both (+) and (-) charges and exist as zwitterion

Existence of an amino acid as Cation, Anion and Zwitterion

H

H໋ R C COOH H໋

NH2

H Amino Acid H

R C COOH R C COO ¯

NH3 ໋ H NH2

Cation H໋ H໋ Anion (low pH) R C COO ¯ (high pH)

NH3 ໋Zwitterion

(Isoelectric pH)

B. Chemical Properties

General Reactions mostly due to the 2 functional groups

Reactions due to - COOH group

1. AA from salts (-COONa) with bases and esters (-COOR’) with alcohols

2. Decarboxylation - AA undergo decarboxylation to produce corresponding amines

3. Reaction with Ammonia

- the carboxyl group of dicarboxylic AA reacts with NH3 to form amide

Asparatic Acid + NH3 AsparagineGlutamic Acid + NH3 Glutamine

Reactions due to -NH2 group

4. The Amino groups behave as bases and combine with acids (e.g. HCl) to form salts (-NH3 + Cl¯)

5. Reaction with NINHYDRIN- the ⍺-AMINO ACIDS react with

Ninhydrin to form a purple, blue or pink colour complex (Ruhemann’s purple)

Amino acid + Ninhydrin Keto acid + NH3 + CO2 + Hydrindantin

Hydrindantin + NH3 + Ninhydrin Ruhemman’s purple

Ninhydrin reaction – quantitative determination of AA and proteins

6. Colour reactions of Amino Acids

- AA can be identified by specific colour reactions

Color Reactions of proteins / AA

Reaction Specific group or AA

1. Buiret Reaction Two peptide linkages2. Ninhydrin Reaction ⍺-Amino acids3. Xanthoproteic Reaction Benzene ring of

aromatic AA (Phe, Tyr, Trp)

4. Million’s reaction Phenolic Group (Tyr)5. Hopkins – Cole Reaction Indole Ring (Trp)

6. Sakaguchi Reaction Guanidino Group (Arg)

7. Nitroprusside Reaction Sulfhydryl groups (Cys)

8. Paulys’ test Imidazole ring (His)

9. Sulfur test Sulfhydryl groups (Cys)

10. Folin – Coicalteau’s Phenolic groups test (Tyr)

7. Transamination

- important reaction in AA metabolism

- transfer of an amino group from an amino acid to a keto acid to form a new AA

8. Oxidative deamination

- AA undergo oxidative deamination to liberate free ammonia

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