Biochemistry Quiz: Carbohydrate Metabolism

Chapter Twenty Two: Metabolic Pathways for Carbohydrates

• This chapter discusses metabolic pathways for carbohydrates.
• Homework assignments are included (numbers 1-7, 15, 16, 20-31, 33, 35, 37-44, 47, 49, 51-78, 85, 89, 125, 126, 127, 129, 131, 133, and 135).

Metabolism

• Metabolism encompasses all chemical reactions supplying energy and substances for cell growth.
• Two types of metabolic reactions exist:
  • Catabolic reactions: Break down complex molecules into smaller ones, releasing energy.
  • Anabolic reactions: Use energy to build larger molecules.

Stages of Catabolism

• Catabolic reactions are divided into stages:
  • Stage 1: Digestion and hydrolysis break down polymers into monomers, which enter the bloodstream.
  • Stage 2: Cellular degradation breaks down molecules into two and three-carbon compounds.
  • Stage 3: Oxidation of all small molecules within the citric acid cycle and electron transport to produce ATP energy.

Stages of Catabolism: From Digestion to Cell

• This diagram illustrates the process of catabolism, outlining the breakdown of proteins, polysaccharides, and lipids into smaller molecules within the mitochondria.
• The process ultimately leads to the production of ATP, the primary energy currency of the cell.

ATP, Adenosine Triphosphate

• ATP is composed of adenine, ribose sugar, and three phosphate groups.
• Hydrolysis of ATP to ADP and AMP releases energy; more phosphates equate to more stored energy.

ATP Drives Reactions

• Catabolic reactions release energy, storing it as ATP.
• Anabolic reactions use ATP energy to build molecules.
• ATP hydrolysis releases 7.3 kcal/mole (-31 kJ/mole) of energy.

Metabolic Reactions

• Coenzymes can gain or lose hydrogen ions and electrons.
• Reduced coenzymes store energy, while oxidized ones release it.

Structure of Coenzyme NAD+

• NAD+ is nicotinamide adenine dinucleotide.
• It has ADP attached to a niacin derivative (vitamin B3).
• It collects H⁺ and 2 electrons.

Coenzyme NAD+ or NADP+

• NAD+ and its phosphorylated form, NADP+, are coenzymes essential for metabolic reactions.
• NADH is the reduced form, while NAD+ is the oxidized form.

Coenzyme FAD

• FAD is flavin adenine dinucleotide.
• It contains ADP and riboflavin (vitamin B2).
• It collects 2H⁺ atoms (+2e¯) to reduce to FADH₂.

Structure of Coenzyme A

• Coenzyme A (CoA) has three components: pantothenic acid (vitamin B5), phosphorylated ADP, and aminoethanethiol.

Function, Coenzyme A

• CoA prepares acyl groups (e.g., acetyl) for reactions.
• CoA produces the energy-rich thioester acetyl CoA.

Digestion of Carbohydrates

• Stage 1 of catabolism is digestion, breaking down food into small molecules.
• In the mouth, physical and chemical digestion occurs.
• Salivary α-amylase hydrolyzes glycosidic bonds in carbohydrates, continuing to act in the stomach until denatured by stomach acid.

Digestion: In Small Intestine

• Pancreatic α-amylase breaks down polysaccharides to glucose.
• Sucrase, maltase, and lactase break disaccharides.
• Monosaccharides are absorbed into the bloodstream.

Digestion of Carbohydrates (Diagram)

• This diagram illustrates the breakdown of polysaccharides, maltose, lactose, and sucrose into monosaccharides (glucose, fructose, and galactose) by specific enzymes in the digestive system and absorption into the bloodstream and cells.

Lactose Intolerance

• Lactase production can cease after childhood.
• Undigested lactose in the large intestine may cause discomfort due to bacterial fermentation.
• Lactose-free products are available as a solution for intolerance.

In The Cell Stage 2: Glycolysis

• Glucose from the bloodstream enters cells and undergoes degradation (Stage 2 catabolism).
• Glycolysis converts 1 glucose (6C) into 2 pyruvate (3C).
• ATP is initially used but ultimately gained in glycolysis.
• Glycolysis happens in the cell's cytosol.

Glycolysis: Reaction 1

• Phosphorylation occurs, transferring a phosphate group from ATP to glucose.
• Glucose-6-phosphate and ADP are produced.
• Hexokinase catalyzes the reaction.

Glycolysis: Reaction 2

• Isomerization converts glucose-6-phosphate to fructose-6-phosphate.
• Phosphoglucose isomerase catalyzes the reaction.

Glycolysis: Reaction 3

• Phosphorylation adds another phosphate group to fructose-6-phosphate (via ATP), producing fructose-1,6-bisphosphate.
• Phosphofructokinase catalyzes the reaction.

Glycolysis: Reaction 4

• Cleavage splits fructose-1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
• Aldolase catalyzes this reaction.

Glycolysis: Reaction 5

• Isomerization converts DHAP to G3P.
• Triose phosphate isomerase catalyzes the reaction.

Glycolysis: Reaction 6

• Oxidation and phosphorylation of G3P, producing 1,3-bisphosphoglycerate, reducing NAD⁺ to NADH and H⁺ (x2).
• Glyceraldehyde-3-phosphate dehydrogenase catalyzes this reaction.

Glycolysis: Reaction 7

• Phosphate transfer from 1,3-bisphosphoglycerate to ADP produces ATP (x 2).
• Phosphoglycerate kinase catalyzes the reaction.

Glycolysis: Reaction 8

• Isomerization converts 3-phosphoglycerate to 2-phosphoglycerate by phosphoglycerate mutase.

Glycolysis: Reaction 9

• Dehydration converts 2-phosphoglycerate to phosphoenolpyruvate by enolase.

Glycolysis: Reaction 10

• Phosphate transfer by pyruvate kinase converts phosphoenolpyruvate to pyruvate with ATP production (x 2).

Glycolysis: Overall Reaction

• Two ATP initialize phosphorylation of glucose and fructose-6-phosphate.
• Four ATP are formed through direct phosphate group transfers to ADP.
• A net gain of 2 ATP and 2 NADH molecules from glucose.

Fructose and Galactose

• Galactose and fructose undergo intermediary reactions to enter the glycolysis pathway.

Regulation of Glycolysis

• Hexokinase activity is inhibited by high glucose-6-phosphate amounts.
• Phosphofructokinase activity is inhibited by high ATP levels, but activated by high AMP levels.
• Pyruvate kinase is inhibited by high ATP levels and acetyl CoA.

Fates of Pyruvate: Fermentation or Oxidation

• Pyruvate's fate depends on oxygen availability.
• In anaerobic conditions, it ferments to lactate or ethanol and CO₂.
• In aerobic conditions, it's oxidized into Acetyl-CoA and CO₂.

Pyruvate: Anaerobic Conditions

• High NADH and low NAD⁺ prevent glycolysis continuation.
• Reduction of pyruvate to lactate regenerates NAD⁺.
• Production of a small amount of ATP in glycolysis continues.

Pyruvate: Aerobic Conditions

• Pyruvate moves from the cytosol to the mitochondria where it's oxidized, converting to acetyl-CoA, CO₂, and reducing NAD⁺ to NADH.
• Acetyl-CoA enters the citric acid cycle and NADH moves to electron transport for ATP production.

Gluconeogenesis

• Glucose is a primary energy source for the brain and other tissues.
• Gluconeogenesis is crucial during fasting and starvation, producing glucose from pyruvate, amino acids, and glycerol when glycogen stores are depleted.

Glycolysis and Gluconeogenesis (Diagram)

• Diagram illustrates the relationship between glycolysis and gluconeogenesis, highlighting the key enzymes and reaction steps.
• Bypassing regulated reactions in glycolysis (i.e. 1,3,10), gluconeogenesis uses different enzymes.

Energy Cost of Gluconeogenesis

• Glucose synthesis during gluconeogenesis costs 4 ATP, 2GTP, and 2 NADH.
• This process is necessary for maintaining glucose levels in the body.

Regulation of Blood Glucose

• Insulin and glucagon, hormones released by the pancreas, regulate blood glucose.
• Insulin promotes glucose uptake by cells after meals.
• Glucagon signals the liver to release glucose during fasting.

The Biochemistry of Extreme Activity

• Intense activity initially uses available ATP and creatine phosphate and then glycolysis.
• Anaerobic glycolysis occurs until lactate buildup causes muscle fatigue.

The Biochemistry of Longer Exercise

• Maintaining oxygen supply is crucial for long-duration exercise.
• Aerobic glycolysis produces ATP.
• ATP is generated solely through glycolysis during the anaerobic threshold.

Lactate and the Cori Cycle

• Lactate from muscles travels to the liver.
• The liver converts pyruvate to glucose, which returns to the muscles.

Test your knowledge on carbohydrate digestion and metabolism, focusing on the roles of various enzymes like salivary α-amylase and glycolysis. This quiz covers reactions, hormones involved, and consequences of disorders such as lactose intolerance. Challenge yourself with these key concepts of biochemistry!