Module 6

• Proteins


  Chains of amino acids (AA) in different sequences

  
• Peptides


  • Dipeptide: 2 AA
  • Tripeptide: 3 AA
  • Oligopeptide: 4-9 AA
  • Polypeptides: 10+ (many peptide bonds)
• Protein


  More than 50 AA
• Proteins typically contain between 100-10,000 AA in a sequence
• Amino acids (AA)


  • Central carbon
  • Hydrogen
  • Acid group (COOH)
  • Amino group (NH2)
  • Unique side group (R)

  
• By combining the 20 AA into various sequences, the body is able to make 10,000-50,000 unique proteins
• Types of amino acids

  • Essential ("indispensable")
  • Non-Essential ("dispensable")
  • Conditionally Essential
• Essential amino acids


  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine
  • Histidine

  
• Non-essential amino acids


  • Alanine
  • Aspartic acid
  • Asparagine
  • Glutamic acid
  • Serine
• Conditionally essential amino acids

  • Arginine
  • Glutamine
  • Glycine
  • Proline
  • Tyrosine
  • Cyst(e)ine

  
• Essential amino acids cannot be synthesized in the body and must be supplied by the diet
• Non-essential amino acids can be synthesized in the body through transamination
• Conditionally essential amino acids are not synthesized at a rate to meet the body's needs
• Protein synthesis


  1. Transcription (DNA to mRNA)

  2. Translation (mRNA to protein)
• Protein structure


  • Primary (linear sequence of AA residues)
  • Secondary (3D elements - alpha & beta helices, sheets)
  • Tertiary (further folding)
  • Quaternary (bonds between subunits)

  
• Mutation in protein


  Alters primary structure, which in turn alters secondary, tertiary and quaternary structures, changing the protein's function
• Sickle-cell anemia


  • Single amino acid in hemoglobin is altered, causing protein molecules to bind together in long chains

  
• Cystic fibrosis


  • Involves the CFTR gene that codes for the CFTR protein, most common mutation is the ΔF508 deletion

  
• Denaturation


  Uncoiling of proteins when exposed to heat, acids, bases, heavy metals, alcohol, other damaging substances
• Denaturation of proteins


  • Stiffening of egg whites in making meringue
  • Marinating chicken breast / steak with salt or acid to make meat juicier and more tender
• Protein digestion


  1. Pepsin cleaves peptide bonds

  2. Pancreatic enzymes break down amino acids to residues

  3. Brush border enzymes complete absorption in small intestine
• Almost all (95-98%) protein is digested and absorbed in the small intestine
• Amino acid pool


  All amino acids in body tissues and fluids that are available for use
• Protein turnover


  Body proteins are continually degraded and replaced, body conserves protein by recycling amino acids
• Rate of protein turnover is higher in infants and older adults
• New growth depends on availability of amino acids in excess of maintenance needs
• Nitrogen balance


  Nitrogen (g) intake - Nitrogen (g) output
• Positive nitrogen balance


  Protein synthesis > breakdown
• Negative nitrogen balance


  Protein breakdown > synthesis
• Nitrogen balance is used to estimate protein requirements
• Nitrogen Balance


  • N is excreted [via urea], proteins are recycled or used
  • Can use the balance of N in body to evaluate if protein intake is sufficient
  • N balance: N(g) intake - N(g) output
• Positive N Balance


  Dietary N intake > N loss/excreted, can support growth and repair
• Negative N Balance


  Dietary N intake (g N2/day) < N loss/excreted, muscle wasting, weight loss
• Functions of Proteins


  • Structure
  • Enzymes
  • Hormones
  • Transport
  • Contraction
  • Water balance
  • Buffering
  • Protection & Defense
  • Detoxification
  • Source of energy & glucose
• Collagen


  • Most abundant protein in the body
  • Holds cells together and forms a protein framework of bones & teeth
  • Also forms tendons/ligaments
  • Strengthens arterial walls
  • Major constituent of scar tissue

  
• Enzymes


  Protein molecules that speed up metabolic reactions, but are not used up or destroyed in the process
• Hormones


  Chemical messengers that are secreted into blood by one tissue/organ and act on target cells in other parts of the body
• Transport


  Transport substances into and out of cells, usually highly specific
• Contraction


  Proteins in muscles allow us to move
• Water Balance


  Intercellular (also called interstitial fluid) balance, controlled by proteins & ions