Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online

CHAPTER 8 - OXIDATION/PHOSPHORYLATION

C:  ATP AND OXIDATIVE PHOSPHORYLATION

BIOCHEMISTRY - DR. JAKUBOWSKI

 04/15/16

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 

 C11.  Proton Gradient Collapse and ATP synthesis - Thermodynamics

Experimental evidence shows that it can. The FoF1ATPase complex can be removed from membranes and placed in a liposome into which ADP and Pi have been encapsulated. The pH of the outside of the vesicles is then lowered several pH units. Under these circumstances, ATP is generated inside the vesicle proving that a gradient alone can drive its synthesis.

Mathematical analyses show that it can as well. Consider a typical pH gradient (-1.4 pH units) across the inner membrane of respiring mitochondria (with the outside having a lower pH than inside making the inside more depleted in protons). Clearly there is a chemical potential difference in protons across the membrane. However, another factor determines the thermodynamic driving force for proton translocation across the membrane. A transmembrane potential exists across the inner membrane of the mitochondria, as it does across most membranes. The source of the membrane potential will be discussed in signal transduction chapter. The inside is more negative than the outside, giving the membrane a transmembrane electrical potential. of about -0.14 V. Clearly, protons would be attracted to the other side of the membrane (into the matrix) by this potential difference, which then augments the chemical potential difference as well. A simple mathematical derivation shows that indeed, a proton gradient can supply enough energy for ATP synthesis, especially when coupled to a transmembrane electrical potential.

Figure:  A simple mathematical derivation

The sum of the electrical and chemical potentials are called the electrochemical potential, which when divided by nF gives the proton motive force.

Note: In the above discussion, we dealt with two different proton translocating methods: 

Figure:  two different proton translocating methods

  1. Complex I, III, and IV, which couple uphill proton movement (from the higher pH matrix to the lower pH intermembrane space)  to oxidation (NADH + O2 to NAD+ + H2O). 
  2. Downhill movement of protons through F0F1 ATPase which couples to ATP synthesis by the enzyme. 

 

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