Glycolysis Study Guide

Unit 3: Glycolysis

Course Information

• Course Code: CHEM 482

• Instructor: Michael Mingroni, PhD (he/him)

Glycolysis Overview

• Definition of Glycolysis: Glycolysis is an almost universal central pathway for glucose catabolism.

• Overall Process: Converts glucose into pyruvate, generating energy in the form of ATP and NADH.

Key Stages of Glycolysis

• Divided into Two Phases:

  • Preparatory Phase

  • Involves the preparation of glucose for catabolism through a series of phosphorylations and isomerizations.

  • Costs two ATP:

    • Key reactions involve the phosphorylation of glucose to form glucose-6-phosphate (G6P) using hexokinase.

    • G6P is converted to fructose-6-phosphate (F6P) by the action of phosphohexose isomerase.

    • F6P is phosphorylated to fructose-1,6-bisphosphate (F1,6BP) by phosphofructokinase-1.

  • Payoff Phase

  • When the 6-carbon molecule is split into two 3-carbon molecules, further oxidations and phosphorylations produce energy-rich intermediates yielding a total of four ATP and two NADH.

    • Glyceraldehyde-3-phosphate (G3P) is converted into 1,3-bisphosphoglycerate (1,3BPG) by glyceraldehyde-3-phosphate dehydrogenase.

    • 1,3BPG then transforms to 3-phosphoglycerate (3-PG), 2-phosphoglycerate (2-PG), and finally phosphoenolpyruvate (PEP) before leading to two molecules of pyruvate.

Phase One: Preparatory Phase

• Glucose to G6P

  • Hexokinase Function:

  • Transfers a phosphate group from ATP to glucose forming glucose-6-phosphate (G6P).

  • Note: ATP4- is not the substrate; MgATP2- acts as a co-factor.

  • Hexokinase exhibits induced fit preventing water from entering the active site to avoid ATP hydrolysis.

• G6P to F6P:

  • Catalyzed by phosphohexose isomerase.

  • This step is crucial as it transforms G6P into a more reactive form (F6P), facilitating further phosphorylation reactions.

• F6P to F1,6BP:

  • Catalyzed by phosphofructokinase-1 (PFK-1).

  • This is considered the first committed step in glycolysis, directing intermediates towards glycolysis instead of other metabolic pathways.

• Aldolase Reaction:

  • Fructose-1,6-bisphosphate is split into glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) via aldolase.

Phase Two: Payoff Phase

• G3P to 1,3BPG:

  • Conversion of G3P into 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase (involving the reduction of NAD+ to NADH).

  • This step is significant for ATP production in subsequent reactions under substrate-level phosphorylation.

• Energy Generation Steps:

• From 1,3BPG to 3-PG, 3-PG to 2-PG, and 2-PG to PEP, culminating in the production of pyruvate and the generation of ATP.

Energetics of Glycolysis

• Net Equation of Glycolysis:
  $ext{Glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 ext{Pi} <br>ightarrow 2 ext{pyruvate} + 2 ext{NADH} + 2 ext{H}^+ + 2 ext{ATP} + 2 ext{H}_2 ext{O}$

• Total Yield:

  • Two ATP consumed in the preparatory phase.

  • Four ATP produced in the payoff phase.

  • Net Gain: Two ATP and two NADH.

Phosphorylation of Intermediates

• Importance of Phosphorylation:

  • All intermediates between glucose and pyruvate are phosphorylated; this prevents them from crossing the cell membrane, thus controlling glycolytic flux and conserving energy.

  • Phosphate groups decrease the binding energy with enzymes via electrostatic repulsion, allowing for regulated enzymatic reactions.

Feeder Pathways of Glycolysis

• Dietary Sugars:

  • Disaccharides must be hydrolyzed before cell uptake, requiring specific enzymes:

  • Mannose via mannase

  • Sucrose via sucrase

  • Trehalose via trehalase

  • Lactose via lactase

  • Starches are digested by amylase in saliva.

Fate of Pyruvate - Anaerobic Metabolism

• To meet the cell's energetic demands in the absence of oxygen, pyruvate is converted to lactate in a process called lactate fermentation, which regenerates NAD+ for continued glycolysis.

Gluconeogenesis

• Definition: The process of synthesizing glucose from non-carbohydrate precursors (3- or 4-carbon molecules), primarily occurring in the liver and kidneys.

• Reverse Pathway: Involves 7 of the 10 glycolytic enzymes operating in reverse to regenerate glucose from pyruvate through key steps:

  • The conversion of pyruvate to phosphoenolpyruvate (PEP) requires two bypass steps involving pyruvate carboxylase and PEP carboxykinase.

• Energetic Costs: Gluconeogenesis is more energetically costly than glycolysis, requiring 4 ATP and 2 GTP to synthesize one glucose molecule:

  • Net equation for gluconeogenesis: $2 ext{pyruvate} + 2 ext{NADH} + 4 ext{ATP} + 2 ext{GTP} + 2 ext{H}^+ + 4 ext{H}_2 ext{O} <br>ightarrow ext{glucose} + 2 ext{NAD}^+ + 4 ext{ADP} + 2 ext{GDP} + 6 ext{Pi}$

Glycogen Metabolism

• Polysaccharides:

  • Store fuel in plants as starch and in animals as glycogen.

  • Starch consists of two forms: amylose (unbranched, (α1!4) linkages) and amylopectin (branched with both (α1!4) and (α1!6) linkages).

• Glycogenolysis:

  • Breakdown of glycogen into glucose-1-phosphate by the action of glycogen phosphorylase which cleaves the (α1!4) bonds.

• Glycogenesis:

  • The synthesis of glycogen utilizes uridine diphosphate glucose (UDP-glucose) as a precursor and involves the enzyme glycogen synthase that incorporates glucose units into the glycogen chain.

Hormonal Regulation of Glycogen Metabolism

• Insulin:

  • Promotes the synthesis of glycogen by facilitating glucose uptake from the blood; activates glycogen synthase (a).

• Glucagon and Epinephrine:

  • Stimulate glycogen breakdown through activation of phosphorylase (a), while inhibiting glycogen synthesis.

ATP Production in Cellular Respiration

• ATP Yield from 1 Glucose:

  • Glycolysis: 2

  • Citric Acid Cycle: 2 (per glucose)

  • Oxidative Phosphorylation: varies based on electron carriers.

• Pyruvate to Acetyl CoA: Each pyruvate yields additional ATP through subsequent aerobic respiration mechanisms.