Free Energy and Gibbs Free Energy

• Energy is the capacity to do work.
• Heat energy maintains body temperature.
• Free energy is available for the performance of work.
• Changes in free energy (ΔG) predict reaction direction at constant temperature and pressure.

Negative ΔG

• A negative ΔG indicates a net loss of energy and the reaction proceeds spontaneously from A to B.
• This type of reaction is exergonic.

Positive ΔG

• A positive ΔG means there is a net gain of energy and the reaction will not go spontaneously from B to A.
• Energy must be added to the system to make the reaction proceed from B to A.
• This type of reaction is endergonic.

ΔG is Zero

• A ΔG of zero means the reactions are in equilibrium.
• A is converted to B as fast as B is converted to A.
• The ratio of [B] to [A] is constant regardless of the compound concentrations.

Additive Property

• Free energy changes are additive in biochemical pathways.
• Substrates must pass in a particular direction (A→B→C→D).
• If the sum of the ΔGs of individual reactions is negative, the pathway can proceed whether or not some reactions have a positive ΔG.

Coupled Reactions

• Endergonic reactions do not occur independently; they are part of a coupled exergonic-endergonic system.
• The overall net change in these systems is exergonic.
• Exergonic reactions term is catabolism, the breakdown/oxidation of fuel molecules.
• Anabolism is the synthesis of substances
• Metabolism is the combination of catabolic and anabolic processes.
• Adenosine triphosphate (ATP) is the principal high-energy intermediate carrier compound.

Stages of Food Degradation (Catabolism)

• Stage I: Complex macromolecules of carbohydrates, proteins, and lipids are broken down into monosaccharides, amino acids, glycerol, and fatty acids, respectively; No free energy is obtained.
• Stage II: Products of Stage I are catabolized to form Acetyl CoA or other citric acid cycle intermediates, yielding some free energy.
• Stage III: Most free energy is obtained.
• Acetyl CoA is oxidized by the citric acid cycle to produce reduced coenzymes (NADHH+ and FADH2) and CO2.
• Reduced coenzymes are oxidized in the respiratory chain (presence of oxygen) to form water and release energy.
• Some energy is captured as a high-energy phosphate bond (ATP), and the rest is released as heat.

Types of Hydrolyzable Linkages

• Hydrolyzable linkages are divided into low energy bond and high-energy bond groups.
• low energy" bond: Upon hydrolysis, it liberates 2-4 Kcal/mole and is represented by an ordinary dash (-): Phosphate ester, Carboxyl ester, Glycosidic and Peptide linkages
• High-energy bond: Upon hydrolysis, it will liberate 7-15 Kcal/mole and is represented by a curved dash (~).
  • High energy phosphate bond: Pyrophosphate linkage between 2 phosphates such that connects the terminal 2 phosphate in ATP (anhydride bond). High-energy bonds are between α-β and β -y phosphate groups in ATP.
  • Thioester linkage: Between a carboxyl group and a thiol group such as that connects fatty acids to coenzyme A.

Energy Collection Mechanism

• Free energy that is liberated during the degradation of foodstuffs is collected in the form of high-energy phosphate bonds (ATP).
• This happens on two levels; substrate level and the respiratory chain level.
  • Substrate-level phosphorylation
  • Oxidation chain phosphorylation.

Substrate-Level Phosphorylation

• ATP formation uses energy resulted from hydrolysis of high energy bond in the substrate while being oxidized.
  • 1,3 bisphosphoglycerate + ADP → 3 phosphoglycerate + ATP (Phosphoglycerate kinase)
  • Phosphoenolpyruvate + ADP → Enolpyruvate + ATP (Pyruvate kinase)
  • Succinyl-CoA + ADP + Pi → Succinic acid + ATP (Succinate thiokinase)
• The first two above reactions occur in the process of glucose breakdown in the glycolysis process
• The third reaction occurs in the citric acid cycle.

Oxidative Phosphorylation

• ATP formation uses energy released from a series of oxidation-reduction reactions in the electron transport chain.
• Carbohydrates, fats, and proteins oxidation reactions donate hydrogen atoms or (electrons) to hydrogen carriers (NAD of FAD) to form NADH and FADH2
• The reduced coenzymes (NADH or FADH2) give their hydrogen (or electrons) to the electron transport chain (respiratory chain).
• This process is a series of oxidation-reduction reactions that produce energy used in ATP formation.

Respiratory Chain or Electron Transport Chain (ETC) and Oxidative Phosphorylation

• Five separate protein-lipid enzyme complexes span the inner mitochondrial membrane.
• These termed complexes 1, II, III, IV and V. Complexes I to IV are part of the respiratory chain.
• Complexes I and II each donate hydrogen or electrons to CoQ.
• Complex III donates electrons to cytochrome c.
• Complex IV gives electrons to oxygen.
• Complex V catalyzes ATP formation.

ETC Complexes

• Complex I: flavoprotein containing FMN (called NADH dehydrogenase). It receives hydrogen from NADH and gives it to CoQ. NAD may receive hydrogen directly from substrate like malate.
• Complex II: flavoprotein containing FAD and receives hydrogen from substrate (e.g. succinate) and gives it to CoQ. - Complex III: hemoprotein composed of cytochrome b and cytochrome c1. Receives elections from CoQ and gives it to hemoprotein called cytochrome c. At this site hydrogen ions are released and only electrons flow along successive cytochromes til they meet oxygen. - Complex IV: hemoprotein composed of cytochrome aa3 and copper. It receives elections from cytochromes c and gives them to molecular oxygen. Electrons and hydrogen ions (previously released from CoQ) combine to form water here.
• Hydrogen atom (or electron) passes from more electronegative (NADH) to the more electropositive (oxygen) through H carriers of the respiratory chain.
• As they do, their energy progressively lowers until all potential energy is drained.
• At each stage of hydrogen (or electron) transfer, liberate certain amount of energy.
• If energy is sufficiently high it can be utilized for connecting an inorganic phosphate (Pi) to ADP to form ATP. meaning oxidation is coupled with phosphorylation (oxidative phosphorylation). Some energy is liberated as heat.
• 3 ATP Molecules form because passage of one pair of hydrogen atoms from NAD to Oxygen.
• 3 Pi are utilized for each one atom of oxygen consumed by the respiratory chain, The P/O ratio of NADH is 3/1 so 1 ATP equivalent is formed in each of complexes I, III, and IV.
• 2 ATP molecules are formed with passage of hydrogen from FAD to oxygen.
  • 2 Pi are utilized for each oxygen atom consumed in the chain, with a P/O ratio for FADH2 is 2/1.

Electron Transport Chain Regulation

• The coupling of oxidation with phosphorylation along the respiratory chain is tight.
• If ADP and Pi are unavailable e.g. when all are converted to ATP, oxidation in respiratory chain will stop.
• H carriers (NADH and FADH) of respiratory chain will remain reduced inhibiting oxidation of substrates in other metabolic reactions.
• Inhibition will continue until ATP utilization, freeing sufficient ADP and Pi to allow the cycle to work again.
• This is how energy yielding oxidative reactions self regulate according to the cell's needs.

Inhibitors of the Respiratory Chain

• Complex I, II, III, and IV can be inhibited by barbiturates/rotenone, carboxin, antimycin, and CO/H2S/cyanide, respectively.
• The inhibitor binds the complex and prevents the passage of electrons beyond this block.
• All carriers will be kept before the block in reduced state and after the block in the oxidized state.
• Since phosphorylation is coupled with oxidation, ATP Formation will be inhibited.
• Complex V is inhibited by the antibiotic oligomycin blocking phophorylation and ATP formation.
• Atractyloside will inhibit the transport of ADP into and ATP out of the mitochondria subsequently inhibiting the oxidation.

Uncouplers

• Are substances that can disturb the phosphorylative side of the chain without affecting oxidative side.
• This leads to permitting oxidative reactions of the respiratory chain to proceed without ATP generation, liberating energy as heat. -The respiratory chain will loss self-regulation and cellular respiration procceds at fast rate and increases metabolic rate.
• Large doses of these subtances are toxic and cause ATP formation.
• Examples of these substances are 2,4 dinitrophenol, dinitrocresol, high doses of aspirin and thyroxine

Transporting Reducing Equivalents: (NADH)

• Inner mitochondrial membrane lacks an NADH transport protein, in turn causing NADH produced in the cytosol to not directly penetrate into mitochondria.
• Meaning, 2 electrons of NADH (reducing equivalents) are transported from the cytosol into the mitochondria using shuttle mechanisms
  • The glycerophosphate shuttle: Skeletal muscle and brain
  • Malate-aspartate shuttle: Liver, kidney and heart