Amino Acids.pdf

Amino Acids.pdf
Dr. Mausumi Adhya Associate Professor Supreme Knowledge Foundation, Hooghly Amino acid: Amino acid-definition, classification, function, properties
Amino acids Amine group –NH2 Acid group -COOH H N H C H R C OH O H N + H C H R C O -- O H amino acid (free) amino acid (zwitter ion) Although over 200 different amino acids occur in nature, only 20 of these form backbone of proteins.  L–α–amino acids  α-amino group, a α-carboxylic acid group
 Building block of peptides and proteins.  Source of sulfur: sulfur containing amino acids cysteine, methionine.  Source of energy: Amino acids converts to glucose by gluconeogenesis.  Precursor of biological molecules: tyrosine is the precursor of hormone thyroxine and skin pigment melanin, tryptophan is the precursor of vitamin 𝑩𝟑 (niacin). Functions amino acids in human body
Structure of L–α–amino acids C H COOH NH2 H C H COOH NH2 C H3 C H COOH NH2 CH C C C H COOH NH2 CH2 Glycine Alanine Valine Leucine C H COOH NH2 CH H2 CH3 Isoleucine C H COOH NH2 CH2 COOH Aspartic acid HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC C H COOH NH2 CH2 CO N H2 Asparagine C H COOH NH2 CH2 CH2 CO N H2 Glutamine Phenylalanine C H COOH NH2 CH2 C H COOH NH2 H C H COOH NH2 C H3 C H COOH NH2 CH C H3 C H3 C H COOH NH2 CH2 CH C H3 C H3 Glycine Alanine Valine Leucine C H COOH NH2 CH CH2 C H3 CH3 Isoleucine C COOH H NH CH2 C H2 C H2 Proline C H CH2 COOH HOOC C H CH2 CH2 C H3 HOOC C H CH2 CO N H2 C H CH2 CH2 CO N H2 C H CH2 C H CH2 O H C H NH2 C H3 C H COOH NH2 CH C H3 C H3 C H COOH NH2 CH2 CH C H3 C H3 Alanine Valine Leucine C H COOH NH2 CH CH2 C H3 CH3 Isoleucine C H COOH NH2 CH2 CH2 S C H3 Methionine C COOH H NH CH2 C H2 C H2 Proline 2 C H COOH NH2 CH C H3 C H3 C H COOH NH2 CH2 CH C H3 C H3 Valine Leucine C H COOH NH2 CH CH2 C H3 CH3 Isoleucine C H COOH NH2 CH2 CH2 S C H3 Methionine C COOH H NH CH2 C H2 C H2 Proline C H COOH CH2 O H Serine C H COOH NH2 CH2 CO N H2 C H COOH NH2 CH2 CH2 CO N H2 C H COOH NH2 CH2 C H COOH NH2 CH2 O H C H COOH NH2 CH2 CH N H C H COOH CH CH CH CH N H NH2 C H COOH NH2 C H3 C H COOH NH2 CH C H3 C H3 C H COOH NH2 CH2 CH C H3 C H3 Alanine Valine Leucine C H COOH NH2 CH CH2 C H3 CH3 Isoleucine C H COOH NH CH2 CH2 S C H3 Methionine C COOH H NH CH2 C H2 C H2 Proline C H COOH NH2 CH2 COOH Aspartic acid HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC COOH C H COOH NH2 H C H COOH NH2 C H3 C H COOH NH2 CH C H3 C H3 COOH C H3 Glycine Alanine Valine C H COOH NH2 CH2 COOH Aspartic acid HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC C H COOH NH2 CH2 CO N H2 Asparagine COOH C H COOH NH2 CH2 COOH Aspartic acid HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC C H COOH NH2 CH2 CO N H2 Asparagine COOH C H COOH NH2 H C H COOH NH2 C H3 C H COOH NH2 CH C H COOH NH2 CH2 Glycine Alanine Valine Leucine COOH CH3 C H COOH NH2 CH2 COOH Aspartic acid HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC C H COOH NH2 CH2 CO N H2 Asparagine C H COOH NH2 CH2 CH2 CO N H2 Glutamine COOH C H COOH NH2 H C H COOH NH2 C H3 C H COOH NH2 CH C H3 C H3 C H COOH NH2 CH2 CH C H3 C H3 Glycine Alanine Valine Leucine C H COOH NH2 CH CH2 C H3 CH3 Isoleucine COOH C COOH H NH CH2 C H2 C H2 Proline
2 C H3 C H COOH NH2 CH CH2 C H3 CH3 Isoleucine C H COOH NH2 CH2 CH2 S C H3 Methionine C COOH H NH CH2 C H2 C H2 Proline C H COOH NH2 CH2 O H Serine C H COOH NH2 CH2 S H Cysteine C H COOH NH2 CH C H3 OH Threonine NH2 Glutamine Phenylalanine C H COOH NH2 CH2 Tyrosine C H COOH NH2 CH2 O H Tryptophan C H COOH NH2 CH2 CH N H C H COOH NH2 CH2 CH2 CH2 CH2 N H2 Lysine C H COOH NH2 CH2 CH2 CH2 NH CH N H2 NH Arginine C H COOH NH2 CH2 C HC N NH CH Histidine Structure of amino acids C H NH2 CH2 CH2 S C H3 Methionine C H COOH NH2 CH2 O H Serine C H COOH NH2 CH2 S H Cysteine C H COOH NH2 CH C H3 OH Threonine N H2 Structure of amino acids C H NH2 CH2 O H Serine C H COOH NH2 CH2 S H Cysteine C H COOH NH2 CH C H3 OH Threonine C H COOH NH2 CH2 CH2 CH2 CH2 N H2 Lysine C H COOH NH2 CH2 CH2 CH2 NH CH N H2 NH Arginine C H COOH NH2 CH2 C HC N NH CH Histidine Structure of amino acids C H COOH NH2 CH2 S H Cysteine C H NH2 CH C H3 Threonine C H COOH NH2 CH2 CH2 CH2 NH CH N H2 NH Arginine C H COOH NH2 CH2 C HC N NH CH Histidin Structure of amino acids C H COOH NH2 CH2 CH2 S C H3 Methionine C COOH H NH CH2 C H2 C H2 Proline C H COOH NH2 CH2 O H Serine C H COOH NH2 CH2 S H Cysteine C H COOH NH2 CH C H3 OH Threonine Tyrosine C H COOH NH2 CH2 O H Tryptophan C H COOH NH2 CH2 CH N H C H COOH NH2 CH2 CH2 CH2 CH2 N H2 Lysine C H COOH NH2 CH2 CH2 CH2 NH CH N H2 NH Arginine C H COOH NH2 CH2 C HC N NH CH Histidine Structure of amino acids ine ucine hionine ne ne teine eonine NH2 2 2 2 Glutamine Phenylalanine C H COOH NH2 CH2 Tyrosine C H COOH NH2 CH2 O H Tryptophan C H COOH NH2 CH2 CH N H C H COOH NH2 CH2 CH2 CH2 CH2 N H2 Lysine C H COOH NH2 CH2 CH2 CH2 NH CH N H2 NH Arginine C H COOH NH2 CH2 C HC N NH Histidine amino acids C H COOH NH2 H C H COOH NH2 C H3 C H COOH NH2 CH C H3 C H3 C H COOH NH2 CH2 CH C H3 C H3 Glycine Alanine Valine Leucine C H COOH NH2 CH CH2 C H3 CH3 Isoleucine C H COOH CH2 CH2 S C H3 C COOH H NH CH2 C H2 C H2 Proline C H COOH NH2 CH2 COOH Aspartic ac HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic a HOOC C H COOH NH2 CH2 CO N H2 Asparagine C H COOH NH2 CH2 CH2 CO N H2 Glutamine Phenylalan C H COOH NH2 CH2 Tyrosine C H COOH NH2 CH2 O H C H COOH CH2 CH C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC C H COOH NH2 CH2 CO N H2 Asparagine C H COOH NH2 CH2 CH2 CO N H2 Glutamine Phenylalanine C H COOH NH2 CH2 Tyrosine C H COOH NH2 CH2 O H Tryptophan C H COOH NH2 CH2 CH N H C H COOH CH2 CH2 CH2 CH2 NH2 Serine C H COOH NH2 CH2 S H Cysteine C H COOH NH2 CH C H3 OH Threonine C H NH2 CH2 CH2 CH2 CH2 N H2 Lysine C H COOH NH2 CH2 CH2 CH2 NH CH N H2 NH Arginine C H COOH NH2 CH2 C HC N NH CH Histidine Structure of amino acids COOH NH2 COOH NH2 COOH NH2 COOH NH2 Glycine Alanine Valine Leucine COOH NH2 Isoleucine COOH C H COOH NH2 CH2 COOH Aspartic acid HOOC C H COOH NH2 CH2 CH2 C H3 Glutamic acid HOOC C H COOH NH2 CH2 CO N H2 Asparagine C H COOH NH2 CH2 CH2 CO N H2 Glutamine Phenylalanine C H COOH NH2 CH2 C H COOH CH Structure of L–α–amino acids
Classification of amino acids  based on the structure of the side chain 𝐑  based on their acid and base nature  based on the polarity of the side chain 𝐑  based on their nutritional value  based on their metabolic fate H N H C H R C OH O amino acid (free)
 Phenylalanine Based on the structure of the side chain 𝑹  Aliphatic amino acids: having aliphatic groups in their side chains  Glycine  Alanine  Valine  Leucine  Methionine  Aspartic acid  Asparagine  Glutamic acid  Isoleucine  Serine  Threonine  Cysteine  Glutamine  Arginine  Proline  Lysine  Aromatic amino acids: having aromatic groups in their side chains.  Histidine  Tryptophan  Tyrosine
Based on their acid and base nature  Acidic amino acids: Side chain 𝐑 contains a carboxylic acid functional group.  Glutamic acid  Aspartic acid  Basic amino acids: Side chain 𝐑 contains an amine functional group.  Arginine  Histidine.  Lysine  Neutral amino acids: Dont have any amino group or carboxylic acid group in side chain R.  Glycine  Alanine  Valine  Leucine  Methionine  Asparagine  Phenylalanine  Tyrosine  Isoleucine  Serine  Threonine  Cysteine  Glutamine  Proline  Tryptophan
Based on the polarity of the side chain 𝐑  Polar amino acids: Various functional groups like acid, amide, alcohol, amine in side chain R  Serine  Threonine  Tyrosine  Asparagines  Cysteine  Glutamine  Aspartic acid  Glutamic acid  Histidine  Lysine  Arginine  Non-polar amino acids: Pure hydrocarbon groups (aliphatic and aromatic) in side chain R  Glycine  Alanine  Valine  Leucine  Isoleucine  Phenylalanine  Tryptophan  Methionine  Proline
Based On Their Nutritional Value  Essential amino acids: Can not be synthesized in human body and must be taken in diet.  Valine  Leucine *Arginine and histidine are essential in children, but not in adults.  Isoleucine  Phenylalanine  Methionine  Tryptophan  Threonine  Histidine  Lysine  Arginine  Non-essential amino acids: Can be synthesized in human body and hence they are non- essential in the diet. Alanine asparagine  Aspartic Acid  Glutamic Acid  Glutamine  Glycine  Proline  Serine  Cysteine  Tyrosine
Based On Their Metabolic Fate  Glucogenic amino acids: Used for glucose synthesis only.  Glycine  Alanine  Methionine  Aspartic acid  Cysteine  Histidine  Glutamine  Arginine  Proline  Valine  Serine  Asparagine  Glutamic acid  Ketogenic amino acids: Used for ketone body synthesis only.  Lysine  Leucine  Ketogenic and glucogenic amino acids: Used for both ketone body synthesis and glucose synthesis.  Tryptophan  Tyrosine  Threonine  Isoleucine  Phenylalanine
General properties of α-amino acids 1. Optical activity 2. Acid-Base properties of amino acids 3. Amphoteric properties of amino acids 4. Zwitterions form 5. Isoelectric point pI 6. Buffering capacity of amino acids 7. Physical properties of amino acids :
Buffering capacity of amino acids Physical properties of amino acids  Amino acids are mainly water soluble.  They are soluble in polar solvents and not soluble in non-polar solvents.  They have high melting point.  A buffer is a solution which resists changes in 𝒑𝑯 when limited amount of acid or base is added to it.  A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.
Buffering capacity of amino acids  An amino acid has both an acidic group (carboxyl ) and a basic group (amine). Carboxyl group acts as conjugate acid (proton donor) and amino group acts as proton acceptor. Therefore together carboxyl group and amino group resists change in pH when small amount of acid or base is added.  For example glycine, glutamic acid and histidine use for preparation of buffer.
Optical activity of amino acids  The ability of a substance to rotate plane-polarized monochromatic light is called optical activity.  The compound having carbon atom with four different groups is generally optically active.  All amino acids except glycine contain a tetrahedral carbon attached to four different groups (-COOH, -NH2, -R and –H).  In glycine, the tetrahedral carbon is attached to three different groups (-COOH, -NH2, and –H).  Therefore all amino acids except glycine is optically active.
Acid-Base properties of amino acids  All amino acids contain at least two ionizable groups; an amine group (-NH2) and a carboxylic acid group (-CO2H).  Some amino acids contain an additional acidic (-CO2H) or basic group (-NH2) in their side chain, which are responsible for acid-base behaviour of amino acids.  Aspartic acid, glutamic acid are acidic amino acids.  Arginine, histidine, lysine are basic amino acids.  Other 15 amino acids are neutral.
Zwitterions form of amino acids  A zwitterion is a neutral molecule with a positive and a negative electrical charge. All neutral amino acids are present in zwitterions form at physiological 𝐩𝐇 (around 7.4). H N H C H R C OH O H N + H C H R C O -- O H amino acid (free) amino acid (zwitter ion)  But acidic and basic amino acids form zwitter ions at different pH. R C COOH NH3 H R C COO -- NH3 H R C COO -- NH2 H pKa1 pKa2 H + HO -- + + at acidic pH (cation) at neutral pH (zwitterion) at basic pH (anion)
Isoelectric point (𝐩𝐈) of amino acids  All amino acids have specific pH in which they form zwitter ion.  It is that 𝐩𝐇 at which net electrical charge of an amino acid is equal to zero ,and thus at this 𝐩𝐇 the amino acid can not move in an electric field.  𝐩𝐈 value varies from one amino acid to other. It is the average of two 𝒑𝑲𝒂 values. 𝐩𝐈 = 𝟏 𝟐 (𝒑𝑲𝒂𝟏 + 𝒑𝑲𝒂𝟐)
R C COOH NH3 H R C COO -- NH3 H R C COO -- NH2 H pKa1 pKa2 H + HO -- + + at acidic pH (cation) at neutral pH (zwitterion) at basic pH (anion) Amphoteric properties An amphoteric species is a molecule or ion that can react as an acid as well as a base. Due to presence of ionizable amine and carboxylic acid groups, amino acids can act sometimes as acids and sometimes as bases depending on the 𝐩𝐇 of their media and isoelectric point. H+ donate: Acid H+ accept: Base OH- donate: base OH- accept: acid Electrons donate: Base Electrons accept: acid

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