Digestion

• How does pepsin help with the digestion of proteins? Also mention how pepsin is formed.

  In the stomach where pH is low, pepsinogen can be cleaved by another pepsinogen, and form the active form pepsin. Pepsin cleave proteins into protein fragments that will be further degraded by proteases of the intestine.
• How does the low pH help with protein digestion?

  The pH of 1 to 2 denatures all proteins except for pepsin and the denatured proteins makes the protein a better substrate for degradation by proteases.
• What enzymes are released as protein digestion continues into the intestine from the stomach?

  The low pH of the food triggers the release of secretin which promotes the release of sodium bicarbonate from pancreatic cells to neutralize the pH of the food as it exits the stomach.

  The polypeptide products from pepsin degradation will stimulate the release of CCK, which triggers the pancreas to release a host of digestive enzymes into the intestine.
• What happens to proteins as it reaches the intestines?

  The pancreatic proteases hydrolyze the proteins into oligopeptides and amino acids in the lumen (amino acids can go straight into the cell by a transporter). Oligopeptides are further cleaved by peptidases attached to the outside of the intestinal cell into di and tripeptides that can be transported into the cell by transporters. Antiporters then pass the amino acids and di and tripeptides out into the blood stream.
• How does α-amylase help with carbohydrate digestion and what must be done after α-amylase has degraded the carbs?

  The enzyme cleaves α- 1,4 bonds but not α- 1,6 or α- 1,4 bonds too close together. The products are maltose, maltotriose, and limit dextrin which cannot be digested by α- amylase because of the α- 1,6 bonds.

  • Maltase, α- glucosidase, and α- dextrinase complete the hydrolysis of starch
  • Other carbohydrate-cleaving enzymes such as sucrase and lactase digest disaccharides
• How are lipids digested in the stomach and prepped for the intestine?

  • Most lipids are in the form of triglycerols which must be broken down into fatty acids but lipids are not soluble in water where most enzymes are. Instead, the lipids are converted into an emulsion in the stomach and bile salts (amphpathic) are secreted by the gallbladder to enhance the emulsion.
  • Lipases from the pancreas attach to the surface of the lipid droplets and cleave off two fatty acids.
  • The resulting fatty acids and monoacylglycerols form globular structures in aqueous solutions called micelles.
• How are the fatty acids and monoacylglycerols digested in the intestine?

  They are transported into intestinal cells by fatty-acid bind protein (FABP), before being ferried by fatty-acid transport proteins (FATP) to the smooth endoplasmic reticulum.

  • At the SER, triglycerides are made from fatty acids and monoacylglyerols.
  • Triglycerols combine with proteins, phospholipids and cholesterol to form lipoprotein transport particles called chylomicrons. Proteins and phospholipids remain on the surface.
  • Chylomicrons are released into the lymph system and then into the blood
• Why are proteins added to the surface of chylomicrons?

  They serve as recognition sites for target tissues
• Catabolism: fuel molecules → CO2 + H20 + useful energy

  Anabolism: simple molecules + energy → complex molecules

  Amphibolic= both catabolic and anabolic

  • degradative and biosynthetic pathways are generally separate, allowing for more control of metabolism
• What general processes hydrolyze ATP to ADP?

  Motion, active transport, biosynthesis, and signal amplification
• What general processes convert ADP to ATP?

  Oxidation of fuel molecules or photosynthesis
• ATP hydrolysis formula

  A) H20

  B) ADP

  
• What are the factors for ATP hydrolysis?

  • electrostatic repulsion of the 4 neg charges making to prone to hydrolysis
  • better resonance stabilization of ADP and Pi than ATP making the hydrolysis more favourable
  • Water binds to ADP and Pi, stabilizing them and making the reverse reaction of ATP synthesis less favourable
  • Increase in entropy: since there 2 products made instead of only having the single ATP molecule
• ATP hydrolysis coupling reactions can shift the equilibrium of otherwise non-sponetaneous reactions to be sponetaneous
• The most reduced molecule has the most energy that can be liberated by oxidation
• As a compound gets oxidized, the energy is used to either generate a compound with a high phosphoryl-transfer potential or an ion gradient. The common end point is the formation of ATP
• Phosphoryl transfer can be used to drive otherwise thermodynamically unfavourable reactions, alter the energy or conformation of a protein. The phosphoryl-group donor in all of these reactions is ATP.

  • Thus ATP is an activated carrier of phosphoryl groups because phosphoryl transfer from ATP is energetically favourable (an exergonic process)
• Regulation of metabolism

  1. control of amounts of enzyme: how fast they are synthesized and degraded
  2. control of enyzmes' catalytic activities: allosteric control or reversible covalent modification, and energy status of cell (energy charge)
  3. regulation of accessibility of substrates: compartmentalization (different processes take place in different organelles) and control of the flux of substrates into a compartment