Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online





Learning Goals/Objectives for Chapter 8C: 

After class and this reading, students will be able to

  • explain reasons for the strongly exergonic hydrolysis of carboxylic acid anhydrides, phosphoric acid anhydrides, mixed anhydrides, and analogous structures and give approximate  values for the ΔG0 of hydrolysis of them;
  • identify from Lewis structures molecules whose hydrolytic cleavage are strongly exergonic;
  • explain how the exergonic cleavage of phophoanhydride bonds in ATP can be coupled to the endergonic synthesis of macromolecules like proteins;
  • draw mechanisms to show how oxidation and phosphorylation reactions are coupled in anaerobic metabolism through the productions of a mixed anhydride catalyzed by the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase;
  • explain how arsenate can double oxidation and phosphorlyation reactions in glycolysis
  • explain how NAD+ can be regenerated from NADH in anaeroboic condition to allow glycolysis to continue;
  • explain the general flow of electrons from NADH to dioxgen through a series of mobile and membrane protein bound electron acceptors in electron transport in the mitochondria inner member.
  • explain with picture diagrams how oxidation and phosphorylation reactions (to produce ATP) are coupled in aerobic metabolism through the generation and collapse of a proton gradient in the mitochondria;
  • draw pictures diagrams explaining the structure of F1F0ATPase in the inner mitochondria member and explain using the picture how ATP synthesis is coupled to protein gradient collapse
  • write an equation for the electrochemical potential and use it to calculate the available ΔG0 for ATP production on proton gradient collapse, given typical values for ΔpH and ΔE across the membrane 

C4.  An Overview of Mitochondrial Electron Transport

The main oxidizing agent used during aerobic metabolism is NAD+ (although FAD is used in one step) which get converted to NADH. Unless the NAD+ can be regenerated, glycolysis and the Kreb's cycle will grind to a halt. Luckily, under these conditions we are actually continually breathing one of the best oxidizing agents around, dioxygen. NADH is oxidized back to NAD+ not directly by dioxygen, but indirectly as electrons flow from NADH through a series of electron carriers to dioxygen, which gets reduced to water. This process is called electron transport.  No atoms of oxygen are incorporated into NADH or any intermediary electron carrier. Hence the enzymes involved in the terminal electron transport step, in which electrons pass to dioxygen, is an oxidase.  The enzymes of the Kreb's cycle and electron transport are localized in mitochondria.

Figure:  mitochondria

By analogy to the coupling mechanism under anaerobic conditions, it would be useful from a biological perspective if this electron transport from NADH to dioxygen, a thermodynamically favorable reaction (as you calculated in the last study guide - a value of about -55 kcal/mol), were coupled to ATP synthesis.  It is!  For years scientist tried to find a high energy phosphorylated intermediate, similar to that formed by glyceraldehyde-3-phosphate dehydrogenase in glycolysis, which could drive ATP synthesis (which likewise occurs in the mitochondria).  None could be found. A startling hypothesis was put forward by Peter Mitchell, which was proven correct and for which he was awarded the Nobel Prize in Chemistry in 1978. The immediate source of energy to drive ATP synthesis was shown to come not from a phosphorylated intermediate, but a proton gradient across the mitochondrial inner membrane. All the enzymes complexes in electron transport are in the inner membrane of the mitochondria, as opposed to the cytoplasmic enzymes of glycolysis.  A pH gradient is formed across the inner membrane occurs in respiring mitochondria. In electron transport, electrons are passed from mobile electron carriers through membrane complexes back to another mobile carrier. Initially, NADH shuttles electrons (2 electron oxidation, characteristic of NAD+/NADH), to a flavin derivative, FMN, covalently attached to Complex I. The reduced form of FMN then passes electrons in single electron steps (characteristic of FAD-like molecules, which can undergo 1 or 2 electrons transfers) through the complex to the lipophilic electron carrier, ubiquinone, UQ.

Figure:  lipophilic electron carrier, ubiquinone, UQ

This then passes electrons through Complex III to another mobile electron carrier, a small protein, cytochrome C. Then cytochrome C passes electrons through complex IV, cytochrome C oxidase, to dioxygen to form water. At each step electrons are passed to better and better oxidizing agents, as reflected in their increasing positive standard reduction potential. Hence the oxidation at each complex is thermodynamically favored.

 JmolUpdated Cytochrome C Oxidase   Jmol14 (Java) |  JSMol  (HTML5) 

Complex II (also called succinate:quinone oxidoreductase) is a Kreb cycle enzyme that catalyzes the oxidation of succinate to fumarate by bound FAD (hence its other name: succinate dehydrogenase).  It is not involved in flow of electrons from NADH to dioxygen described above but passes electrons from the reduced succinate to ubiquinone to form fumarate and reduced ubiquone which then can transfer electrons to cytochrome C through Complex III.  The crystal structure of this complex has recently been solved by Yankovskaya et al. who have shown that the arrangement of the redox-active sites in the complex minimizes potential oxidation of bound FADH2 by dioxygen, minimizing production of harmful reactive oxygen species like superoxide.

 Animation of electron transport in mitochondria


 Jmol: Updated Succinate Dehydrogenase (Complex II)    Jmol14 (Java) |  JSMol  (HTML5) 

At each complex, the energy released by the oxidative event is used to drive protons through each complex from the matrix to the intermembrane space of the mitochondria, and is not used to form a high energy mixed anhydride as we saw in the glyceraldehyde-3-phosphate dehydrogenase reaction. The actual mechanism of proton transfer is unclear.



Return to Chapter 8C:  ATP and Oxidative Phosphorylation

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Archived version of full Chapter 8C:  ATP and Oxidative Phosphorylation


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