Amino Acids

• Classifications of Amino Acids:

  1. Nonpolar Aliphatic
  2. Polar Uncharged
  3. Aromatic
  4. Positively Charged
  5. Negatively charged
• Nonpolar Aliphatic Amino Acids include:

  • Glycine, alanine, proline, valine, leucine, isoleucine, methionine

  They are hydrophobic and can be isolated by extraction with organic solvents
• Polar Uncharged Amino Acids:

  • Serine, threonine, cysteine asparagine, glutamine

  They can form Hydrogen bonds, and are weakly polar.
• Cysteine can be oxidised into Cystine, creating a disulfide bridge. This is important in stabilising many proteins
• Aromatic Amino Acids:

  • Phenylalanine, tyrosine, tryptophan

  They absorb light in the UV range (270-280), and can form hydrophobic interactions
• Positively Charged Amino Acids:

  • Lysine, arginine, histidine

  They are strongly polar.
• What is the least polar of the positively charged amino acids, and is often involved in catalysis?

  Histidine
• Negatively Charged Amino Acids:

  • Aspartate, glutamate

  They are strongly polar
• Alanine

  
• Valine

  
• Leucine

  
• Methionine

  
• Phenylalanine

  
• Tyrosine

  
• Tryptophan

  
• Serine

  
• Threonine

  
• Amino acid - Asparagine

  
• Glutamine

  
• Cysteine

  
• Glycine

  
• Proline

  
• Arginine

  
• Histidine

  
• Lysine

  
• Aspartic Acid

  
• Glutamic Acid

  
• Amino acids are amphoteric
• At acidic pH, the carboxyl group is protonated and the amino acid group is cationic
• At neutral pH, the carboxyl group is deprotonated but the amino group is protonated
• At alkaline pH, the amino group is neutral and the amino acid is in the anionic form
• The inflection point is the point at which the graph changes direction. This is whree the pKa = pH
• For amino acids without ionisable side chains, the Isoelectric Point is the pH at which the net electric charge is zero
• At the isoelectric Point:

  • Amino acids are least soluble in water
  • Amino acids do not migrate in an electric field
• Thyrotropin releasing factor is made of 3 amino acid residues
• Amantin is made of 8 amino acid residues
• Proteins may contain one or more polypeptide chains, and maybe also:

  • Cofactors
  • Prosthetic Groups
• Cofactors are metal ions or organic molecules
• Prosthetic groups covalently attached cofactors. one example is heme
• Peptide bonds are formed via condensation reactions between the amine group and the carboxylic acid groups of amino acids, eliminating H2O