proteins

• Proteins


  Serve many functions including structure, catalysts, movement, transport, hormones, protection, storage, and regulation
• Functions of proteins


  • Structure
  • Catalysts
  • Movement
  • Transport
  • Hormones
  • Protection
  • Storage
  • Regulation
• Fibrous proteins


  One type of protein
• Globular proteins


  One type of protein
• Amino acid


  A compound that contains both an amino group and a carboxyl group
• α-Amino acid


  An amino acid in which the amino group is on the carbon adjacent to the carboxyl group
• Amino acids are more properly written in the zwitterion (internal salt) form

  
• Amino acids


  • With the exception of glycine, all protein-derived amino acids have at least one stereocenter (the α-carbon) and are chiral
  • The vast majority of α-amino acids have the L-configuration at the α-carbon
• Nonpolar side chains of amino acids (at pH 7.0)


  • Alanine
  • Valine
  • Leucine
  • Isoleucine
  • Proline
  • Phenylalanine
  • Tryptophan
  • Methionine
• Polar side chains of amino acids (at pH 7.0)


  • Serine
  • Threonine
  • Cysteine
  • Tyrosine
  • Asparagine
  • Glutamine
• Acidic and basic side chains of amino acids (at pH 7.0)


  • Aspartic acid
  • Glutamic acid
  • Histidine
  • Lysine
  • Arginine
• Amino acids


  • For 19 of the 20, the α-amino group is primary; for proline, it is secondary
  • With the exception of glycine, the α-carbon of each is a stereocenter
  • Isoleucine and threonine contain a second stereocenter
• Ionization vs. pH


  The net charge on an amino acid depends on the pH of the solution in which it is dissolved
• Isoelectric point (pI)


  The pH at which the majority of molecules of a compound in solution have no net charge
• Isoelectric points (pI) of amino acids


  • Alanine: 5.97
  • Arginine: 10.76
  • Asparagine: 5.41
  • Aspartic acid: 2.77
  • Cysteine: 5.07
  • Glutamic acid: 3.22
  • Glutamine: 5.65
  • Glycine: 5.97
  • Histidine: 7.59
  • Isoleucine: 6.02
  • Leucine: 5.98
  • Lysine: 9.74
  • Methionine: 5.74
  • Phenylalanine: 5.48
  • Proline: 6.48
  • Serine: 5.68
  • Threonine: 5.87
  • Tryptophan: 5.89
  • Tyrosine: 5.66
  • Valine: 6.01
• Cysteine


  The -SH (sulfhydryl) group of cysteine is easily oxidized to an -S-S- (disulfide)
• Phenylalanine, tryptophan, and tyrosine

  Have aromatic rings on their side chains
• Tryptophan


  Is the precursor to the neurotransmitter serotonin
• Phenylalanine and tyrosine


  Are precursors to norepinephrine and epinephrine, both of which are stimulatory neurotransmitters
• Uncommon amino acids


  • Hydroxylated proline, lysine, and tyrosine
  • Iodinated tyrosine
• Peptide bond (peptide linkage)


  The special name given to the amide bond between the α-carboxyl group of one amino acid and the α-amino group of another
• Peptide


  A short polymer of amino acids joined by peptide bonds
• Types of peptides


  • Dipeptide: Two amino acids joined by a peptide bond
  • Tripeptide: Three amino acids joined by peptide bonds
  • Polypeptide: Many amino acids joined by peptide bonds
  • Protein: At least 30 to 50 amino acids joined by peptide bonds
• Peptide bond


  Has about 40% double bond character and is planar, which is explained by the concept of resonance
• Writing peptides


  Conventionally from the N-terminal amino acid (free -NH3+ group) to the C-terminal amino acid (free -COO- group)
• Proteins


  • Behave as zwitterions and have an isoelectric point (pI)
  • At the pI, the protein has no net charge
  • Above the pI, the protein has a net negative charge
  • Below the pI, the protein has a net positive charge
  • Proteins are least soluble in water at their pI and can be precipitated from solution when pH = pI
• Levels of protein structure


  • Primary: Sequence of amino acids
  • Secondary: Conformations of amino acids in localized regions
  • Tertiary: Complete 3D arrangement of atoms
  • Quaternary: Spatial relationship and interactions between subunits
• Primary structure


  The sequence of amino acids in a polypeptide chain, read from the N-terminal to the C-terminal
• The number of possible peptides from 20 amino acids is enormous, e.g. 2060 = 1078 for a 60 amino acid protein
• Human insulin consists of two polypeptide chains with a total of 51 amino acids, the sequence of which is critical for its function
• Quaternary structure


  The spatial relationship and interactions between subunits in a protein that has more than one polypeptide chain
• Primary structure


  The sequence of amino acids in a polypeptide chain
• The number of peptides possible from the 20 protein-derived amino acids is enormous
• There are 20 x 20 = 400 dipeptides possible
• There are 20 x 20 x 20 = 8000 tripeptides possible
• The number of peptides possible for a chain of n amino acids is 20n
• For a small protein of 60 amino acids, the number of proteins possible is 2060 = 1078, which is possibly greater than the number of atoms in the universe
• Insulin


  Consists of two polypeptide chains, A and B, held together by two disulfide bonds
• Differences between four types of insulin


  • Human
  • Cow
  • Hog
  • Sheep
• Vasopressin and oxytocin are both nonapeptides but have quite different biological functions