Glycolysis

BIOL 430: Biological Chemistry - Lecture 9: Glycolysis

Overview of Digestion

• Components of a meal: proteins, lipids, and polysaccharides must be degraded for absorption and transport.

• Process of Digestion:

  • Proteins are broken down into amino acids by proteases from the stomach and pancreas.

  • Lipids are converted to fatty acids and glycerol by lipases from the pancreas.

  • Carbohydrates are broken down into monosaccharides by alpha-amylase in saliva and enzymes from the pancreas.

• The digestion begins in the mouth and proceeds through the stomach.

Digestion Process

• Mouth:

  • Food is mechanically degraded by chewing, converting it into a slurry, aiding hydrolytic enzyme activity.

  • Saliva contains alpha-amylase, which initiates carbohydrate digestion by converting polysaccharides into mono- and disaccharides.

• Common Disaccharides:

  • Sucrose: Cleaved by sucrase into glucose and fructose.

  • Lactose: Cleaved by lactase into glucose and galactose.

  • Maltose: Cleaved by maltase into glucose.

Carbohydrate Metabolism

• Polysaccharides and disaccharides are converted into monosaccharides, which are absorbed by intestinal epithelial cells.

• Transport Mechanisms:

  • Glucose and galactose: Transported via sodium-glucose linked transporter.

  • Fructose: Enters via GLUT5 transporter.

• Glycogen: Internal storage form of glucose within cells.

  • Metabolism of glycogen to glucose units will be discussed in subsequent units.

Glycolysis: General Overview

• Glycolysis is a nearly universal, 10-step metabolic pathway utilized by prokaryotes and eukaryotes.

• It produces ATP through glucose oxidation.

• Key Inputs and Outputs:

  • Inputs: 1 glucose, 2 NAD⁺, 2 ADP, 2 inorganic phosphate ($P_i$).

  • Outputs: 2 pyruvate, 2 NADH, 2H⁺, 2 ATP, 2 H₂O.

• Notably, glycolysis requires an initial investment of 2 ATP, yielding a net gain of 2 ATP.

Chemical Transformations in Glycolysis

1. Degradation of glucose (6C) into two 3C pyruvate molecules.

2. Phosphorylation of ADP to ATP through compounds formed during glycolysis.

3. Reduction of NAD⁺ to NADH.

4. Overall Reaction: $ext{glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 P<em>i ightarrow 2 ext{pyruvate} + 2 ext{NADH} + 2 ext{H}^+ + 2 ext{ATP} + 2 ext{H}</em>2 ext{O}$

   • Net gain of 2 ATP after accounting for the 2 ATP used at the start.

Phases of Glycolysis

1. Preparatory Phase

• Uses 2 ATP to convert glucose into fructose 1,6-bisphosphate.

• First step catalyzed by hexokinase, which phosphorylates glucose at C-6, producing glucose 6-phosphate, which is then trapped within the cell.

  • Reaction: $ext{Glucose} + ext{ATP} <br>ightarrow ext{Glucose 6-phosphate} + ext{ADP}$

• Isomerization: Glucose 6-phosphate $
  ightarrow$ Fructose 6-phosphate (catalyzed by phosphoglucose isomerase).

• Key Steps:

  • Phosphofructokinase (PFK) catalyzes the addition of a second phosphate to form fructose 1,6-bisphosphate, which is the rate-limiting step of glycolysis.

2. Payoff Phase

• Both glyceraldehyde 3-phosphate (GAP) molecules undergo oxidative conversion to pyruvate, yielding 2 NADH and ATP.

• Key reactions:

  • Formation of 1,3-bisphosphoglycerate: GAP is oxidized by glyceraldehyde 3-phosphate dehydrogenase, producing NADH and phosphorylating GAP to 1,3-bisphosphoglycerate.

  • Substrate-level phosphorylation: Transfer of phosphate from 1,3-bisphosphoglycerate to ADP to form ATP (catalyzed by phosphoglycerate kinase).

• Net Outputs of Payoff Phase: 2 ATP and 2 NADH per glucose.

Importance of Phosphorylated Intermediates

• All nine intermediates in glycolysis are phosphorylated, ensuring they remain in the cell and conserve metabolic energy while lowering activation energy of reactions.

Regulation of Glycolysis

• Enzymes catalyzing irreversible reactions serve as regulation points:

  • Hexokinase, Phosphofructokinase, Pyruvate kinase.

• Hexokinase is inhibited by glucose 6-phosphate (feedback inhibition).

• Phosphofructokinase is allosterically inhibited by ATP and allosterically activated by AMP, adjusting its activity based on cellular energy status.

• Pyruvate kinase is inhibited by ATP and alanine and stimulated by fructose 1,6-bisphosphate (feed-forward).

Glycolysis in Muscles and Liver

• Muscles: Regulated primarily by energy demand; low energy charge stimulates glycolysis, while high energy charge inhibits it.

• Liver: More complex regulation due to diverse roles; negatively regulated by ATP and citrate, positively by fructose 2,6-bisphosphate after carbohydrate-rich meals.

Entry of Other Dietary Carbohydrates into Glycolysis

• Humans typically consume glucose in polymeric forms such as glycogen or starch and polymers must be broken down into monosaccharides.

• Enzymes for carbohydrate conversion:

  • Alpha-amylase for starch.

  • Various enzymes for disaccharides (lactase, sucrase, maltase).

Clinical Insights

• Lactose Intolerance: Affects adults lacking lactase, leading to gastrointestinal disturbances.

• Fructose Metabolism: Excessive fructose can bypass phosphofructokinase, leading to unregulated metabolism and potential health issues like fatty liver disease and type 2 diabetes.

Pyruvate Fates and Fermentation

• Fates of Pyruvate:

  • Under aerobic conditions, converted into acetyl-CoA.

  • Under anaerobic conditions, reduced to lactate (lactic acid fermentation) or converted to ethanol and CO₂ (alcoholic fermentation).

• Key Enzyme in Fermentation: Lactate dehydrogenase (catalyzes the conversion of pyruvate to lactate, regenerating NAD+).

Summary of Glycolysis

• Overview: Glycolysis converts glucose into pyruvate while producing ATP and NADH. It is pivotal for cellular energy production and is tightly regulated at key enzymatic steps.