BIOL203 Lesson 6

• Protein


  Energy-yielding nutrient, chemically similar to carbohydrates and lipids
• Amino acids


  The building block for proteins; has 3 parts: amino group, acid group, and side group (functional group, different for every AA)
• Amino acids


  • Non-polar: glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, cysteine, methionine
  • Negatively charged (polar, acidic): aspartic acid, glutamic acid
  • Positively charged (polar, basic): histidine, lysine, arginine
  • Uncharged polar: serine, threonine, asparagine, glutamine, tyrosine
• There are 20 different amino acid groups
• 9 essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine), other 11 can be synthesized de novo in the liver
• Proteins

  Formed of a chain of amino acids linked together by peptide bonds; a longer chain is called a polypeptide
• Proteins


  • Hemoglobin (a set of 4 polypeptides that deliver oxygen to cells)
• Protein shape

  • Determined by the sequence of amino acids
  • Protein environment can break apart hydrogen bonds, causing the protein to unfold and become denatured, no longer functional
• Collagen

  • When intact, increases resistance to chewing
  • The older the animal, the more collagen, which is why veal meat is more tender when slow-cooked, turning into gelatin
• Protein functions

  • Structural support and movement (collagen and keratin are key structural proteins)
  • Enzymes to digest food
  • Hormones (e.g. insulin)
  • Maintain fluid balance: plasma proteins are needed to maintain the right amount of fluid in your tissues
  • Channels and pumps on cell membrane: allow passage of substances across the lipid bilayer
  • Immune system (antibodies); proteins are shapeshifters, which means they can change form to bind to pathogens
• DNA provides instructions for making proteins, with over 90% of genes coding for proteins
• Making a protein


  1. DNA sequence tells the cell the order to connect different amino acids

  2. If a missing essential amino acid, protein synthesis comes to a halt and the partially made polypeptide chain is dismantled
• Protein digestion and absorption

  1. Stomach acid and pepsin denature and partially digest proteins

  2. Small intestine further digests proteins with enzymes like trypsin and chymotrypsin

  3. Final digestion and absorption occurs in small intestinal cells

  4. Absorbed amino acids enter hepatic portal vein and travel to liver
• Very little dietary protein is excreted in feces
• Protein turnover


  Degradation and synthesis of protein (recycling); without it, we would need enormous amounts of protein from food every day
• Protein turnover

  • Hemoglobin in red blood cells is recycled when the red blood cells are destroyed
• Nitrogen balance


  Intake of nitrogen (from food) + Retention (protein turnover) = Losses (in urine, feces, sweat)
• Nitrogen balance states

  • Positive (intake + retention > losses) during pregnancy, lactation, exercise, recovery from illness, growing children
  • Negative (intake + retention < losses) during starvation, severe trauma
• Deamination

  Body uses fat and carbs for energy, but protein can also be used by removing the amino group and using the carbon skeleton to make glucose or lipids
• Diseases involving proteins

  • Food allergies
  • Celiac disease
  • Non-celiac gluten sensitivity
• Food allergies

  Hypersensitive immune responses to proteins in foods like peanuts, tree nuts, milk, eggs, fish, shellfish, and wheat

  • Most allergic reactions are mild, but severe reaction can cause anaphylaxis
  • Prevalence of allergies is rising due to genetics and food preparation
• Celiac disease


  Autoimmune disorder that damages the small intestine's villi due to a hypersensitive immune response to gluten, leading to malabsorption of nutrients
• Non-celiac gluten sensitivity


  Clinical state of individuals who develop symptoms when consuming gluten-containing foods, but do not have celiac disease
• Dietary Reference Intakes (DRIs) for protein

  • EAR: 0.66 g/kg of body weight
  • RDA: 0.8 g/kg of body weight
• Protein intake increases during pregnancy, breastfeeding, infancy, childhood, and recovery from serious illness, blood losses, and burns
• High-quality protein

  • Influenced by amino acid composition and protein digestibility
  • Contains all essential amino acids and is well absorbed
• Low-quality incomplete protein


  Lacks one or more essential amino acids and is poorly absorbed
• High-quality protein sources

  • Animal protein (meat, fish, poultry, eggs, milk products)
  • Plant protein (legumes), but lacks at least 2 essential amino acids
• Protein complementation


  Mixing incomplete plant-based protein sources to provide all essential amino acids without adding animal proteins
• Protein synthesis in the body

  • Food is digested into AAs, they are absorbed and delivered to cells
  • Cells have all the material required to build specific proteins (90% of genes code for protein)
  • Proteins give cells their shape and function
  • If diet is missing essential AAs, protein synthesis comes to a halt and the partially made polypeptide chain is dismantled and AAs are returned to pool
• Protein digestion and absorption

  • Stomach: proteins undergo denaturation by stomach acid and partial digestion by pepsin
  • Small intestine: further digestion as pancreas secretes protein-splitting enzymes (trypsin and chymotrypsin)
  • Small intestine cells: final digestion occurs
  • Liver: after being absorbed, AAs enter the hepatic portal vein and travel to the liver
  • Rectum: very little dietary protein is excreted in feces