Aminoacids

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Aminoacids

• By
• Dr. Sumbul Fatma
• 29th September, 2010

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Types of proteins

• Type Examples
• Structural tendons, cartilage, hair, nails
• Contractile muscles
• Transport hemoglobin
• Storage milk
• Hormonal insulin, growth hormone
• Enzyme catalyzes reactions in cells
• Protection immune response

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Amino acids

• Building blocks of proteins
• 20 are present in mammalian proteins

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Structure of amino acids

• Groups attached to a- carbon
• a carboxyl group
• an amino group
• a side chain

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Examples

• H
• I
• H2NC COOH
• I
• H glycine
• CH3
• I
• H2NC COOH
• I
• H alanine

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Zwitterions
Net charge is zero on the molecule
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Isoelectric point

• The pH at which the molecule carries no net
  charge
• In acidic solution-cationic
• In alkaline solution- anionic

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Titration curve of glycine

• pK1- pH at which 50 of molecules are in cation
  form and 50 are in zwitterion form
• pK2- pH at which 50 of molecules are in anion
  form and 50 are in zwitterion form
• Buffering action is maximum around pK values and
  minimum at pI

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• What if amino acid has more than two ionizable
  groups?

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Titration curve of aspartic acid
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Learning check-1

• CH3 CH3
• H3NCHCOOH H2NCH2COO
• (1) (2)
• Select from the above structures
• A. Alanine in base.
• B. Alanine in acid.

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Solution-1

• CH3 CH3
• H3NCHCOOH H2NCH2COO
• (1) (2)
• Select from the above structures
• A. Alanine in base- (2)
• B. Alanine in acid (1)

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Classification on the basis of side chain

• Non-polar
• Side chain does not bind or give off protons
• Promote hydrophobic interaction
• In aqueous solution- clustered in the interior
• Helps giving a 3-d shape

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Proline

• imino acid
• Has a secondary amino group

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Classification on the basis of side chain

• Polar
• uncharged
• Acids
• Bases
• Hydrophilic

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Polar uncharged amino acids

• Have zero net charge at neutral pH
• Hydrophillic

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Polar acidic amino acids

• Have a negative charge on the R-group

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Polar basic amino acids

• Have a positive charge on the R-group

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Learning check-2

• Identify each as (1) polar or (2) nonpolar
• A. NH2CH2COOH (Glycine)
• CH3
• CHOH
• B. NH2CHCOOH (Serine)

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Solution-2

• Identify each as (1) polar or (2) nonpolar
• A.(2) NH2CH2COOH (Glycine)
• CH3
• CHOH
• B. (1) NH2CHCOOH (Serine)

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Optical properties of amino acids

• Amino acids having asymmetric carbon atom exhibit
  optical activity
• The mirror image forms produced with reference to
  the a-C atom are called D and L isomers
• L-amino acids natural amino acids
• D-amino acids are found in antibiotics (like
  Gramicidin-S, Actinomycin-D and Valinomycin) and
  in plant and bacterial cell walls
• Glycine an exception- no optical activity

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Stereoisomers/Enantiomers
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Learning check-3

• All 20 amino acids exist in nature equally as
  both D and L stereoisomers
• True/False

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Solution-3

• All 20 amino acids exist in nature equally as
  both D and L stereoisomers
• False

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Amino acid derivatives of importance

• Gamma amino butyric acid (GABA, a derivative of
  glutamic acid) and dopamine (from tyrosine) are
  neurotransmitters
• Histamine (Histidine) is the mediator of allergic
  reactions
• Thyroxine (Tyrosine) is an important thyroid
  hormone

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Peptide bond

• Proteins are made by polymerization of amino
  acids through CO-NH bridge
• a-carboxyl group of one amino acid reacts with
  a-amino group of another amino acid

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Proteins are made by controlled polymerization of
amino acids
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Peptides

• 2 aa- dipeptide
• 3-?
• 4- ?
• Upto 10- oligo peptide
• 10-50- polypeptide
• More than 50 - proteins

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Learning check-4

• What are the possible tripeptides formed from
  one each of leucine, glycine, and alanine?

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Solution-4
Tripeptides possible from one each of leucine,
glycine, and alanine Leu-Gly-Ala Leu-Ala-Gly
Ala-Leu-Gly Ala-Gly-Leu Gly-Ala-Leu Gly-Leu
-Ala
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References

• Lippincotts Illustrated reviews Biochemistry
  4th edition unit 1