Protein

• Proteins / polypeptides


  Chains formed by the condensation/combination of 20 different amino acids
• Polypeptides


  May be di-, tri-, etc; up to 10 amino acids
• Proteins


  Longer than 10 amino acid units; i.e. MW>10,000
• Amino acid


  A compound having both a carboxyl group (-COOH) and an amino group (-NH2)
• All amino acids from protein have the -NH2 attached at the Cα to the –COOH (as well as the H- & R-)
• All naturally occurring α-amino acids, except glycine (R=H), are chiral and the 'L' stereoisomer
• There are ~150 other physiologically important amino acids, GABA (a neurotransmitter)
• Amino acids


  • Contain both an acidic functional group (COOH) and a basic one (-NH2), NH or N
  • Reactions are highly pH dependent
• Condensation and Hydrolytic Reactions


  Figure 6.3
• Zwitterionic structures


  Contain both N-H+ and COO-
• At low pH, protonate COO-
• At higher pH, lose H on N
• Isoelectric pH


  Differs for each amino acid (due to structural differences)
• Essential amino acids


  Must be consumed in the diet
• Nonessential amino acids


  Can be synthesized in the body
• Conditionally essential amino acids


  Cannot be synthesized due to illness or lack of necessary precursors
• Premature infants lack sufficient enzymes needed to create arginine
• Dipeptides


  2 amino acids joined together
• Dipeptides from glycine (G) and alanine (A)


  • GG, AA, GA and AG
• Tripeptides


  3 amino acids joined together
• Possible tripeptides


  1. 3 choices for the N-terminal amino acid

  2. 2 choices for middle

  3. 1 choice for the C terminal amino acid

  4. Thus 3 x 2 x 1 = 6 choices if each aa must be present

  5. But total number possible is 3 x 3 x 3 = 27; includes AAA, PPP, GGG etc
• Levels of Protein Structure

  • Primary structure - sequence of amino acids and disulfide bridges
  • Secondary structure - conformation/shape of the backbone
  • Tertiary structure - 3D structure of the entire polypeptide
  • Quaternary structure - if the protein has more than one chain
• Insulin


  • 21 + 30 amino acids
• Secondary structure - sheets


  • Sheets/strands, e.g. fingernails, silk
  • H-bonding - intramolecular
• Tertiary structure of proteins

  • Arises from weaker attractive forces (non polar dispersion forces) between hydrophobic parts of the same chain that are widely separated in the primary structure, but close in space
  • Intramolecular
  • Results in chain twisting and folding
• Tertiary structure of protein

  • Collagen - a fibrous protein (precursor of gelatin) has a triple helix structure - some elasticity due to interchain interactions
  • Hemoglobin (a globular protein)
• Hemoglobin (H)


  Has 4 polypeptide chains: carries O2, CO2 and H+ in the blood, and possesses quaternary structure
• Myoglobin (M)


  Has a single chain of 153 amino acids: carries O2 from the blood vessels to the muscles and stores it until needed
• Both hemoglobin and myoglobin have Fe II containing heme unit in each chain that binds O2
• Myoglobin cannot have quaternary structure since it has only one polypeptide chain
• Hemoglobin has 4 polypeptide chains and possesses quaternary structure
• Enzymes


  • Many enzymes are proteins and their specific binding properties to a substrate depend on their overall molecular shape or "conformation"
  • Lock and key mechanism for activity
• Denaturation


  • Any physical or chemical process that changes the protein structure and makes it incapable of performing its normal function
  • Whether denaturation is reversible depends on the protein and the extent of denaturation
• Denaturation


  • Heating egg whites (irreversible)
  • 'Permanent' waving of hair (reversible)
• Proteins by Structure


  • Simple - insoluble 'reactive' (hair, horn)
  • Simple - soluble (enzymes)
  • Conjugated - 'structural' (HDL, interferon, hemoglobin)
  • Fibrous
  • Globular
  • Lipo-
  • Glyco-
  • Hemo-
• Fibrous Proteins


  • Collagens
  • Elastins
  • Keratins
  • Myosins
• Globular Proteins


  • Albumins (egg whites, enzymes)
  • Globulins (antibodies)
• Proteins by Function


  • Enzymes
  • Contractile (muscle)
  • Hormones (insulin, growth hormone)
  • Neurotransmitters (endorphins)
  • Storage (store nutrients, e.g. seeds, casein in milk)
  • Transport (hemoglobin)
  • Structural (collagen, keratins)
  • Protective (antibodies)
  • Toxins (snake venom, botulinum)
• Essential Amino Acids


  • histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine (and arginine in infants)
• Nonessential Amino Acids

  • Can be synthesized in our bodies from breakdown products of metabolism