Glycogenesis Process Quiz

Carbohydrate Metabolism

• Carbohydrates, proteins, and lipids are degraded to release energy
• Carbohydrates are the most easily utilized energy source
• Digestion starts in the mouth with salivary amylase, breaking down starch to maltose and dextrins
• Salivary amylase is inactivated by stomach acid (pH 5.6-7)
• Lingual lipase begins fat digestion, active in both alkaline and acidic pH
• Digestion continues in the small intestine
• Different enzymes break down various disaccharides (e.g., maltase breaks down maltose, sucrase breaks down sucrose, lactase breaks down lactose)
• Monosaccharides (glucose, fructose, galactose) are absorbed passively or via hexose transporter (symport)
• Glucose is actively transported into the intestinal cells, driven by the high extracellular Na+ concentration

ATP - Cellular Energy Currency

• Catabolism degrades carbohydrates, proteins, and lipids
• Cells utilize an energy conversion strategy that oxidizes glucose
• Energy release occurs at various points in the pathway
• Energy is stored in adenosine triphosphate (ATP) bonds
• A mole of glucose yields 686 kcal
• ATP serves as a "go-between" molecule, coupling exergonic catabolic reactions with endergonic anabolic reactions
• ATP captures energy as phosphoanhydride bonds
• Hydrolysis of these bonds provides energy for cellular processes

ATP - Molecule Structure

• ATP is a nucleotide composed of:
  • Adenine (nitrogenous base)
  • Ribose (5-carbon sugar)
  • Three phosphoryl groups
• A phosphoester bond connects the first phosphoryl group to ribose
• Phosphoanhydride bonds link the second and third phosphoryl groups

Phosphoanhydride Bond Hydrolysis

• Hydrolysis of phosphoanhydride bonds releases large amounts of energy used for cellular processes

Overview of Catabolic Processes

• Carbohydrates, fats, and proteins are degraded to release energy
• This process occurs in three stages:
  • Stage 1: Breakdown of large molecules into simpler units (e.g., proteins to amino acids, carbohydrates to monosaccharides, fats to fatty acids and glycerol)
  • Stage 2: Conversion of monomers into usable forms for the citric acid cycle
  • Stage 3: Complete oxidation and ATP production

Saliva Composition and Function

• Carbohydrate digestion begins in the mouth
• Saliva consists of mucus, amylase, and lingual lipase:
  • Mucus (mucins and mucopolysaccharides): Lubricates food
  • Amylase: Initiates starch breakdown
  • Lingual lipase: Starts fat digestion
• Saliva functions include tasting, moistening, facilitating swallowing, speech, preventing calcium phosphate deposition, protecting teeth against bacterial acids, and providing a beneficial antiseptic function.

Salivary Amylase

• Attacks alpha(1-4) glycosidic bonds of starch (amylose and amylopectin)
• Breaks down starch into maltose and dextrins
• Dextrins are short glucose polymers linked by alpha(1-4) or alpha(1-6) bonds

Lingual Lipase

• Similar to gastric lipase, involved in the initial phase of fat digestion
• Active in both alkaline (mouth) and acidic (stomach) pH conditions
• Converts triglycerides into monoacylglycerol, glycerol, and fatty acids

Carbohydrate Digestion and Absorption

• a-amylase breaks down starch; maltase, sucrase and lactase finish breaking down disaccharides
• Glucose, fructose, and galactose are absorbed into the intestinal cells either via simple diffusion or assisted by the hexose transporter, which utilizes a symport with Na+.

Carbohydrate Absorption

• D-glucose, D-galactose, and D-fructose can diffuse passively through the intestinal villi
• Luminal membranes of small intestine mucosal cells contain a symport to transport glucose into the cell, requiring Na+ binding to the transport protein.

The Cori Cycle

• The Cori cycle is an important metabolic pathway that connects lactate produced in muscle tissue during anaerobic conditions with glucose production in the liver.
• The glucose produced by the liver is then returned to the muscle tissue to be used as fuel.

Glycogenolysis

• The process of glycogen degradation to glucose, controlled by glucagon and epinephrine
• It involves two steps:
  • Step 1: Removal of terminal glucose as G-1P, utilizing glycogen phosphorylase
  • Step 2: Removal of the last glucose at the alpha(1-6) linkage by a debranching enzyme

Glycogen Phosphorylase

• Catalyzes the cleavage of alpha(1->4) glycosidic bonds in glycogen, releasing glucose-1-phosphate

Debranching Enzyme

• Transfers a trisaccharide unit to an exposed chain end using its transferase activity
• Hydrolyzes the alpha (1->6) linkage to release the last glucose molecule at a branch point.

Phosphoglucomutase

• Converts glucose-1-phosphate (G-1-P) into glucose-6-phosphate (G-6-P)

Glycogenesis

• The process of glycogen synthesis from glucose precursors.
• It occurs in two main stages:
  • Activation of glucose to form UDP-glucose
  • Glycogen synthesis by glycogen synthase and branching enzyme
• Insulin stimulates glycogenesis in case of high blood sugar
• Glucagon stimulates glycogen breakdown In cases of low blood sugar

Glycogenesis: Glycogen Synthase

• Adds UDP-glucose to the end of glycogen chains, resulting in an increase in a(1->4) bonds

Glycogenesis: Branching Enzyme

• Transfers oligosaccharide units from the non-reducing ends of glycogen chains to the interior of the glycogen, resulting in a-1-6-branching

Aerobic Respiration & Energy Production

• Breakdown of food in the presence of oxygen to produce ATP
• Oxidative phosphorylation generates ATP from energy in oxidative reactions
• Enzymes are located in the mitochondrial matrix, passing electrons from NAD+ or FAD to the electron transport system (ETC) and then to O₂
• Protons are transferred to the intermembrane space, creating a proton gradient that drives ATP synthesis

Mitochondria

• "Shoe-shaped" organelles with a dual membrane structure, including an outer membrane with pores, allowing small molecule passage, and an inner membrane with highly folded cristae increasing surface area

Conversion of Pyruvate to Acetyl CoA

• Pyruvate produced from glycolysis enters the mitochondria
• Pyruvate must be decarboxylated to acetyl CoA in order to enter the citric acid cycle
• This oxidation also results in NAD+ being reduced to NADH

Pyruvate Dehydrogenase Complex

• A complex of three enzymes responsible for the conversion of pyruvate to acetyl CoA, involving reactions of decarboxylation, oxidation, and high-energy coenzyme attachment

Role of Acetyl CoA in Metabolism

• Produced from glucose, fatty acids, and amino acids
• Acetyl CoA carries the acetyl group into the citric acid cycle for further oxidation
• Production of ATP from acetyl CoA is via the citric acid cycle and ETC
• Precursor for cholesterol and fatty acids, and the production of eicosanoids and phospholipids

Citric Acid Cycle (TCA cycle)

• The final stage in the breakdown of nutrients
• Acetyl CoA and oxaloacetate enter the cycle, where the acetyl group is completely oxidized to 2 CO2
• High-energy electrons are released and transferred to NAD+ and FAD to produce NADH and FADH2
• A molecule of GTP (or ATP) is produced per cycle

ETC & Oxidative Phosphorylation

• Electron transport chain (ETC) in the inner mitochondrial membrane involves electron carriers
• Enzyme ATP synthase makes ATP from the proton gradient
• NADH produces 3 ATPs

Hydrogen Ion Gradient

• Protons (H+) are pumped into the intermembrane space via ETC carriers
• The proton gradient drives ATP synthesis through ATP synthase

Uncoupling Proteins (UCPs)

• UCPs are inner mitochondrial membrane proteins
• These proteins allow protons to leak back across the membrane generating heat instead of ATP

Energy Outcome from One Glucose Molecule

• In aerobic respiration, one molecule of glucose can generate 36 ATPs with varying amounts from glycolysis, TCA cycle, and ETC (NADH, FADH2)