Lecture 10: Citric Acid Cycle and Fermentation

Acetyl-CoA Synthesis and Citric Acid Cycle Overview

• MOVES pyruvate molecules into mitochondria

• CONVERTED to acetyl-CoA

• BREAKS Acetyl-CoA down in citric acid cycle

• CREATES more NADH and FADH2 electron carriers

Pyruvate Molecule Overview

• MAJOR product of glycolysis

• LOTS of potential energy

Pyruvate and Citric Acid Cycle Products

• Pyruvate energy CONVERTED to ATP, NADH, and FADH₂

  • NADH and FADH₂ are the major products

Citric Acid Cycle Location

• Eukaryotes → Mitochondrial matrix

• Prokaryotes → Cytosol

Process of Pyruvate Entering Mitochondria

• Pyruvate → 3 CARBON molecule

• PROCESSED to acetyl-CoA

  • COMBINES with Coenzyme A (from B vitamin)

• RELEASES CO₂

• GENERATES one molecule of NADH (for each pyruvate)

Citric Acid Cycle Products

• 6 NADH

• 2 FADH₂

• 2 ATP

Energy Source of Citric Acid Cycle

• Pyruvate has LOTS of potential energy

• Citric acid cycle EXTRACTS ENERGY from pyruvate

Citric Acid Cycle Equation

• REACTANTS: 2 pyruvate + 8 NAD⁺ + 2FAD + 2 ADP + 2 P_i

• PROUCTS: 6 CO₂ + 8 (NADH + H⁺) + 2 FADH₂ + 2 ATP

How More ATP is Produced After Citric Acid Cycle

• PRODUCES high energy NADH and FADH₂ molecules

  • To make MORE ATP

Chemiosmosis

• COUPLING electron transport with ATP production

Electron Transport Chain

• SERIES of protein complexes

  • ACCEPTS HIGH energy electrons

  • ELECTRONS from NADH and FADH₂

• OCCURS in mitochondria inner membrane

Chemiosmosis Process (Step 1)

• NADH donates 2 electrons to complex 1

  • ENERY from electrons used

  • H⁺ ions AGAINST their concentration gradient

Chemiosmosis Process (Step 2)

• Electrons TRANSFERRED along chain from Complex I to Complex III

  • By QUINONE

• Complex II ACCEPTS electrons directly from FADH₂

  • FADH₂ DUMPS its high energy electrons off at Complex II

Chemiosmosis Process (Step 3)

• O₂ is FINAL electron acceptor

Proton Gradient Producing ATP

• Proton gradient DRIVES motor producing ATP

  • GRADIENT: Intermembrane Space → Mitochondrial Matrix

• CALLED oxidative phosphorylation

Substrate-Level Phosphoryation

• ATP PRODUCED by enzyme-catalyzed transfer of phosphate group

  • FROM intermediate substrate to ADP

  • PRODUCED in GLYCOLYSIS and CITRIC ACID CYCLE

Oxidative Phosphorylation BRIEF Overview

• Proton gradient PROVIDES energy for ATP production

• ATP SYNTHASE (membrane protein, “turbine”) uses energy to phosphorylate ADP to ATP

Methods of Producing ATP

• Substrate-Level Phosphorylation

• Oxidative Phosphorylation

Extracting Energy From Glucose (Cellular Respiration) Parts

• Glycolysis

• Pyruvate Processing

• Citric Acid Cycle

• Oxidative Phosphorylation

How ATP is made

• MADE by adding P group onto ADP

  • ATP → reduced nucleotide form

  • ADP → oxidized nucleotide form

How NADH is Made

• NADH (reduced form) made from NAD+ (oxidized form)

How FADH₂ is Made

• FADH₂ (reduced form) made from FAD (oxidized form)

Electrons and Potential Energy

• Electrons with HIGH potential energy move to electron transport chain

  • FROM NADH and FADH₂

  • Redox reaction

• PROGRESSIVE DECREASE in free energy as electrons move down ETC

Final Electron Acceptor

• Oxygen

O₂ Not Present in Electron Transport Chain

• Electron transport chain STOPS

  • Electrons DO NOT leave chain

  • BUILD UP (“like cars on freeway”)

• Reduced NADH and FADH₂ build up

Fermentation

• ALLOWS NAD+ to be regenerated in ABSENCE of O₂

  • NAD+ REQUIRED to continue glycolysis

    • Oxygen → NAD+ generated after electrons dumped at ETC

    • No-Oxygen → ETC does not work

  • Anaerobic Reaction

Alternative Pathways for Producing Energy

• Cellular Respiration

• Fermentation

Producing Energy With Oxygen Present

• CELLULAR RESPIRATION

• Generate ATP with electron transport chaon

Producing Energy Without Oxygen Present

• FERMENTATION

• Generate necessary molecules to continue glycolysis

Cellular Respiration Definition

• Glucose PRODUCES ATP

  • Glucose ENTERS cell

  • CATABOLIZED (broken down)

  • MAKES more ATP

Glycolysis Definition

• PRODUCES 2 net ATP, 2 NADH, and 2 pyruvate molecules

• Pyruvate have MOST chemical energy of glucose

  • When it entered cell

Oxidative Phosphorylation Definition

• NADH and FADH₂ (reduced electron carriers) DONATE high energy electrons to ETC

• Energy from electrons USED to move H+ ions

  • AGAINST concentration gradient

• H+ ions MOVE THROUGH ATP synthase

  • Enzyme ADDS P group to ADP

  • PRODUCES ATP

Regulating Catabolism of Glucose

• Produces ATP WHEN NEEDED

  • Cell with plenty ATP SLOWS/DELAYS glucose catabolism

• Phosphofructokinase (PFK) enzyme REGULATES early glycolysis step

  • Done ALLOSTERICALLY

Energy Investment Phase of Glycolysis

• NEED to add high energy P groups to glucose

  • PREPARES it to be broken down

• P’s COME FROM ATP

Phosphofructokinase Binding Sites

• ATP or ADP Regulatory Site

• ATP active site

Phosphofructokinase Activity Regulation

• Activity REGULATED by ATP

• Both ADP and ATP CAN BIND to PFK regulatory site

  • ADP → PFK activated (slows reaction)

  • ATP → PFK repressed

• RELATIVE LEVELS of ATP/ADP in cell ultimately regulate PFK activity