Week 10: Protein digestion

Week 10: Protein digestion

1. Protein digestion

   • Proteins enter the stomach
   • Gastric mucosa secretes the hormone gastrin
   • gastrin stimulates secretion of HCl
   • HCl denatures the proteins
   • Pepsin hydrolyses peptide bonds on amino-terminal side of Phe, Trp, Tyr residues
   • Resulting peptides enter small intestine
   • Trypsin, chymotrypsin, carboxypeptidases A and B and aminopeptidase hydrolyse the peptides to amino acids
   • Amino acids are absorbed into blood ad travel to liver for degradation

2. Amino Acid degradation

   • Two types of amino acids: ketogenic and glucogenic
   • Ketogenic amino acids: degraded to acetyl CoA, used to form ketone bodies
   • Glucogenic amino acids
     • Degraded to pyruvate or intermediates of citric acid cycle
     • Can be used to form glucose
   • Amino acids are alphta-keto acids with an amino group attached
     • Glutamate is derived from alpha-ketoglutarate
     • asparate is derived from oxaloacetate
     • Alanine is derived from pyruvate
   • Involves the separation of carbon skeleton from amino groups and their separate metabolism
   • Amino group is released as ammonium which toxic
   • Amino acids are converted to glutamate
   • Glutamate is oxidized in liver mitochondria
   • Urea synthesis also begins in liver mitochondria

3. Gluco-alanine cycle is use for muscle proteins to produce glucose via gluconeogenesis during starvation

4. Urea excretion

   • Urea passes from liver into the blood
   • In blood, it travels to the kidneys
   • Kidneys, it is excreted in urine
   • Amino acid metabolism involves many amino-transferase enzymes

5. Aminotransferase enzymes have same reaction scheme. All aminotransferase enzymes have pyridoxal phosphate (PLP) as prosthetic group

6. Major function of liver is to maintain the blood glucose concentration constant about 5mM

   • Glucogenic amino acids can be used to produce glucose
     • Uncontrolled type 1 diabetes
     • During starvation
     • Low carb/high protein
     • Removal of oxaloacetate from citric acid cycle leads to accumulation of acetyl CoA

7. Fats

   • referred to tri-acyl-glycerols or tri-glycerides or neural fats
   • Fats are made up glycerol with three fatty acids attached via ester linkages
   • Long hydrocarbon tails make fats hydrophobic
   • Saturated - no double bonds
   • Monounsaturated - one double bond
   • Polyunsaturated - more than one double bond
   • Unsaturated fatty acids don't pack as well, fats containing higher proportions of unsaturated fatty acids have lower melting temperatures
   • EPA has 20 carbons and 5 double bonds

8. Omega-3-fatty acids

   • n-3 fatty acids
   • Seafood is richest natural source of n-3: EPA and DHA
   • DHA has range of human benefits
   • Naturally occuring fatty acids have cis double bonds which causes kinks in hydrocarbon tail making fat more fluid
   • Trans fats
     • Fatty acids go rancid when oxygen attacks double bonds in their fatty acids
     • Partial hydrogenation converts some of double bonds to single bonds
     • Increases shelf life of fatty foods and also makes oils into solids
     • Partial hydrogenation also converts some cis-double bonds to trans-double bonds

9. Energy Storage

   • Gram for gram, fats store more than twice as much energy as carbohydrates because
     • Fats are more reduced than carbohydrates
     • Fats don't have a hydration shell to carry around
     • Glycogen stores are depleted within 24 hours whereas fat stores can supply energy for months

10. Lipids

    • Storage lipids i.e. fats and oils
    • Structural lipids in cellular membranes
    • Lipids that act as signal molecules, enzyme cofactors and pigments
    • Cellular membranes consists of lipid bilayer with hydrophilic (polar) on surface and hydrophobic fatty acids on inside

11. Eicosanoids

    • paracrine hormones derived from arachidonic acid
    • Involved in inflammatory responses
    • Synthesis is inhibited by NSAIDS

12. Other lipids

    • D-regulates the metabolism of calcium and prevents rickets
    • A - derived from beta- carotene is precuror of retinal which is involved in vision
    • E is antioxidant
    • K - involved in blood clotting
    • Ubiquinone is mitochondrial electron carrier