Amino acids, Peptides and Proteins

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Amino acids, Peptides and Proteins

• All amino acids have common structure except for
  proline.
• a-carbon is at the center, additional carbons in
  the side chains b, g, d, e

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Stereoisomers of a amino acids

• The two stereoisomers of alanine, L-, and D- are
  non superimposable mirror images (enantomers)
• In proteins only L- configuration is found
• Cell are able to synthesize L isomers in
  stereospecific reactions.

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Amino acids are classified by R-group
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Nonstandard amino acids have important functions

• Selenocysteine -derived from serine is found in
  handful of proteins.
• Ornithine and citrulline are not part of the
  proteins, but rather are key intermediates in the
  synthesis of arginine.

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Amino acids are weak acids and bases

• Zwitterion hybrid ion can act as acid (proton
  donor) or base (proton acceptor)
• COOH NH3 Net Charge 1
• COO- NH3 Net Charge 0
• COO - NH2 Net Charge -1

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Isoelectric point, (pI)

• At pH 5.97 glycine is fully ionized
• It has no NET electric charge
• Isoelectric point of glycine
• pI1/2 (pKa1 pKa2)1/2(2.349.60)5.97
• At pH5.97 glycine has no net charge!

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Biologically active peptides

• Oligomeric
• Protomeric
• Hormones
• Glutathione
• Ubiquitin

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