Biological Oxidation and Reduction Reactions

Biological Oxidation Introduction

• Occurs in biological systems to produce energy
• Involves oxidation of energy-rich chemical substances like carbohydrates, lipids, and proteins

Oxidation Processes

• Oxidizing substances remove protons (H+) and electrons (e-)
• These protons and electrons are transported to acceptors via special transporters
• Energy is released and accumulates in ATP molecule during the transport of H+ and e-

Oxidation and Reduction Reactions

• Cells release energy from fuel molecules
• Oxidation reactions involve:
  • Removal of electrons (e-) or addition of oxygen or removal of H-atoms (H+ + e-)
• Reduction reactions always accompany oxidation reactions
• Reduction reactions involve:
  • Addition of H atoms or electrons or the removal of oxygen
• H-atoms are transferred initially to carrier molecules:
  • Nicotinamide adenine dinucleotide (NAD+)
  • Nicotinamide adenine dinucleotide phosphate (NADP+)
  • Flavin adenine dinucleotide (FAD)
• Oxidized forms of the molecules cannot be synthesized in the body
• Oxidized forms of the molecules must be supplied in the diet (as vitamins)
• Redox potential (E'0) defines the free energy change that occurs in oxidation/reduction reactions.

Stages of Oxidation of Food

• First stage: Primary metabolism (digestion) occurs in the Intestines
• Second stage: Secondary (intermediary) metabolism occurs in the TCA cycle
• Third stage: Tertiary metabolism occurs in the ETC

Redox Reactions

• Oxidation is always accompanied by reduction
• Redox reactions involve electron movement
• The electron donor is the reductant or reducing agent
• The electron acceptor is the oxidant or oxidizing agent
• The electron donor changes into oxidant form.
• The electron acceptor gets converted to the reductant form.
• Oxidoreduction reactions in living organisms are known as biological oxidation.

Types of Biological Oxidation

• Anaerobic oxidation: H+ and e- are transported to other substrates (acceptors) and they are reduced.
  • In anaerobic glycolysis, H+ and e- transported by NAD to pyruvate and reduce to lactate.
  • An energy is released in anaerobic glycolysis.
  • 2 ATP molecule is accumulated during anaerobic glycolysis.
• Aerobic oxidation: H+ and e- are transported to O2
  • Energy rich chemical substances (carbohydrates, lipids, proteins) get oxidized to CO2 and H2O

Electron Transport Chain

• Reducing equivalents H+ and e- are transferred to NAD and FAD
• NAD and FAD produce NADH and FADH2 reforms of coenzymes
• NADH and FADH2 pass through the electron transport chain (ETC) or respiratory chain
• Oxygen is reduced to water

Electron transport chain (ETC)

• Process in which electrons are transported to O2
• Special components include NAD, FMN, Iron-Sulfur protein, Coenzyme Q, Cytochromes b, c1, c2, a, a3
• All components of ETC are located on the inner mitochondrial membrane

How Energy Is Extracted From Food Molecules and Used To Synthesize ATP?

• Substrate-Level Phosphorylation:
  • Phosphate is transferred directly from substrate molecules to ADP
  • A small amount of ATP is formed in glycolysis and the citric acid cycle (Krebs' cycle)
• Oxidative Phosphorylation:
  • ATP synthase uses energy derived from a proton (H+) gradient
  • Oxidative Phosphorylation only occurs in the presence of O2
  • It accounts for almost 90% of the ATP generated by cellular respiration

Oxidation of Glucose

• Proceeds in stages
• In the presence of O2, 36-38 ATP are generated: Glycolysis, pyruvate oxidation, Krebs cycles,electron transport chain, chemiosmosis
• In the absence of O2, 2 ATP are generated: Glycolysis and Fermentation
• NADH and FADH2 reforms are collected during cellular respiration NADH = 3 ATP FADH2 = 2 АТР

Continuation of Glycolysis

• The fate of pyruvate depends on O2 availability:
  • when oxygen is present pyruvate is oxidized to acetyl CoA which enters the Krebs cycle
  • without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD+
• NADH must be recycled to NAD+ by:
  • Fermentation (occurs when oxygen is not available; an organic molecule is the final electron acceptor)
  • Aerobic respiration (occurs when oxygen is available as the final electron acceptor)

Mitochondria

• Power house of the cell
• Mitochondria structure:
  • Outer membrane: Freely permeable to most ions and small molecules
  • Inter-membrane space: Separates outer and inner mitochondrial membranes
  • Inner membrane: Impermeable to most small ions like protons, ATP, ADP, pyruvate.
    • Specialized carriers or transport systems are required to move ions or molecules across this membrane
    • The components of the ETC (except for cytochrome c which is found in the inter-membrane space) are located in the inner mitochondrial membrane
    • The inner membrane is highly convoluted with convolutions called cristae, which serve to greatly increase the surface area of the inner membrane
  • Matrix:
    • Contains NAD+ and FAD (the oxidized forms of the two coenzymes that are required as hydrogen acceptors)
    • It contains protons that can be pumped across the inner mitochondrial membrane to create a gradient that drives ATP synthesis
    • Consists of ADP and Pi that are used to produce ATP

Electron Transport Chain (ETC)

• Occurs in mitochondria
• Occurs across the inner mitochondrial membrane
• Organization of ETC:
  • Inner mitochondrial membrane can be disrupted into 5 separate protein complexes
  • Complex I: NADH dehydrogenase
  • Complex II: Succinate dehydrogenase
  • Complex III: Cytochrome b-c complex
  • Complex IV: Cytochrome oxidase (cytochrome a+a3)
  • Complex V: ATP synthase
  • Mobile electron carriers: Coenzyme Q and Cytochrome c

Enzymes of Biological Oxidation

• Enzymes of Biological Oxidation include oxidoreductases, oxidases, dehydrogenases, hydroperoxidases, oxygenases

Oxidases

• Oxidation of a metabolite catalyzed by an oxidase forms A) to H₂O as (B) forming H2O2-

A-Cytochrome Oxidase

-A hemoprotein that is widely distributed in many tissues.

• A component of the respiratory chain. -It is inhibited by CO, cyanide and hydrogen sulfide. -Contains two molecules of heme as a prosthetic group.

B-Flavoprotein Enzymes

• Contain FMN or FAD as prosthetic group.
• L-amino acid oxidase (in kidney)
• Contain molybdenum.
• Xanthine oxidase is important in the uric acid synthesis.

Dehydrogenases

• Hydrogen transports in order to oxidize a metabolite through coupled dehydrogenases.
  • Coenzymes (hydrogen carriers): -Nicotinamides (NAD/NADP)- involved in glyolysis, citric acid cycle & respiratory chain/ Fatty acid synthesis, steroid synthesis & in pentose phosphate pathway
    • Riboflavin (FMN/FAD) Concerned with the respiratory chain and oxidative decarboxylation of pyruvate α-ketoglutarate
  • Cytochromes are classified as dehydrogenases excluding cytochrome oxidase

Hydroperoxidases

• Protect body from harmful effects of peroxides
• Include peroxidases and catalase
  • Peroxidases: Present in milk & in various tissues as leukocytes & platelets -Catalase: Hemoprotein that contains four heme groups

Oxygenases

• Catalyze the direct transfer and incorporation of oxygen into a substrate molecule.
• Two groups include dioxygeases and monooxygenases A)- Dioxygenases -A+O2→AO2 - Includes Homogentisate dioxygenase; 3-hydroxyanthranilate dioxygenase; L-tryptophan dioxygenase (tryptophan pyrrolase) B)- Monooxygenases:
  • Cytochromes P450 A-H+O2+ZH2 → A - OH+H2O+Z
  • Heme-containing monooxygenases, 11000, located mainly in the endoplasmic reticulum of liver & intestine, also found in the mitochondria.
• Cytochromes P450 usages:
  • Detoxification of drugs in the liver microsomes, Found in the mitochondrial cytochrome and takes part in steroidogenic tissues to share in steroid hormone biosynthesis, In kidney, the 25-hydroxycholecalciferol can be hydroxylated; and aides in the formation of bile acid in the liver

Superoxide Dismutase & ATP

• A single electron is transferred to generate the potentially damaging superoxide anion free radical (O2.=).
• 1 molecule of ATP energy: -7.3 kcal/mol
  • ATP's function as mobility, membrane transport, signals transduction and to synthesize of nucleotides

Oxidative Phosphorylation

• Enables aerobic organisms to capture a greater portion of available free energy of the oxidizing substrates in the form of ATP.

Members of the electron transport chain

• Complex I: NADH dehydrogenase, also called NADH coenzyme Q reductase, is located in the inner mitochondrial membrane and also contains non-heme iron atoms

• Dehydrogenase enzymes cannot directly react with oxygen, rather instead are indirectly related to the metabolite and next member in the chain through an electron complex.

• The enzymes compose of a protein and coenzyme.

• Complex II: Is composed of the Coenzyme Q/ubiquinone & cytochrome c reductase

• it's in the inner membrane in the free form or protein bound form

• coenzyme occupies the position between metalloflavoproteins

• at the point of coenzyme , the H+ ions dissociate and go into solution, releasing electrons to the cytochromes Complex III, Cytochrome c oxidase

  • Is similar to the structure of myoglobin/hemoglobin. The significant feature is its the heme structure containing the irons +3 to +2 by the addition of an electron

complex IV (cytochrome oxidase), catalyses the transport of electrons from cytochrome c to molecular O, and thus the reduction of O, to H,O, as it contains cytochrome a and o

Complex V: ATP Synthase contains Fo and F1 and helps the protruding F1 to bind to the ATP molecules, so a removal of this component (experimentally) leads to impairment in ATP production.

Respiratory Chain Inhibitors:

• Respiratory chain inhibitors are classified as:
  • Inhibitors of respiratory chain
  • Inhibitors of oxidative phosphorylation,
  • Uncouplers of phosphorylation.

Respiratory Chain Inhibitors:

• Arrest respiration where barbiturates like amobarbital antibiotic such as piericidin A, antimycin A, retinone and CO & CN inhibit cytochrome oxidase and reduces O transfer.
• Halting process and stopping ATP/ life and blocking the pass of electrons as well inhibits ATP generation.

Inhibitors of oxidative phosphorylation

• Inhibitors include Oligomycins ( Fo &CFo unit) and Venturicidin + Dicyclohexylcarbodiimide (DCCD); Inhibition of proton efflux from Fo and Cfo

Inhibitors of Uncouplers of phosphorylation

• The substance carriers dissolve into the membrane and acts functions of carriers that block oxidative and transport and reduce ATp synthesis, where this includes 2,4 dinitro phenol, dinitrocresol, as well, and ionophores
• Ionophores are lipid soluble and helps transport cations
• Valiomycin ions complex through it for a smooth pass and they all induce and act as uncouplers for oxidative phosphorylation.