www.Examville.com Online practice tests, live classes, tutoring, study guides Q&A, premium content and more. Amino Acids, Peptides and Proteins The Proteins speak: “We are the basis of structure and 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) basic carboxyl 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 amino groups – attached to the carboxyl 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’ group Examples are: Alanine Methionine Leucine Phenylalanine Isoleucine Tryptophan Valine 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 sulfhydryl amide 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. Taste sweet (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 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 on the structure of glyceraldehyde CHO CHO H C OH OH C H CH2OH CH2OH D-Glyceraldehyde L- R R H C NH2 H 2N C H COOH COOH D-Amino Acid L-Amino Acid The proteins are composed of L-⍺ amino 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 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 ¯ NH3໋ H NH2 Cation H໋ H໋ Anion (low pH) R C 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 3. Reaction with Ammonia - the carboxyl group of dicarboxylic AA reacts with NH3 to form amide Asparatic Acid + NH3 Asparagine Glutamic 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 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 linkages 2. Ninhydrin Reaction ⍺-Amino acids 3. Xanthoproteic Reaction Benzene ring of aromatic AA (Phe, Tyr, Trp) 4. Million’s reaction Phenolic Group (Tyr) 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 www.Examville.com Online practice tests Live classes Tutoring Study guides Q&A Premium content and more.
Pages to are hidden for
"Amino Acids, Peptides, and Proteins"Please download to view full document