CHEM 440
Biochemistry I
J. D. Cronk   Syllabus [ Previous | Next ] Pick a lecture:
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Lecture 32. Citric acid cycle in context

Wednesday 1 December 2010

Amphibolic nature of the citric acid cycle. Anaplerotic reactions and the example of pyruvate carboxylase. The glyoxylate cycle. Mitochondria: structure and function. Mitochondrial transporters. Electron carriers and electron transfer processes. Exergonic processes drive the generation of a transmembrane hydrogen ion gradient.

Reading: Voet, Voet, and Pratt; Ch.17, pp.588-594; Ch.18, pp.596-601.

32. Summary

Lecture 32 Summary

We begin to turn our attention to the bioenergetic payoffs that are the end result of oxidative metabolism with a look at mitochondria and crucial role played by mitochondrial transporters in preparation for a survey of the electron transport chain, and an introduction to ATP synthase and oxidative phosphorylation.

 

Mitochondria and mitochondrial transport

Mitochondria are organelles of eukaryotic cells that are energy specialists. The mitochondria is bounded by a relatively porous outer membrane. Inside the mitochondria is the highly folded and reticulated inner membrane.

Schematic drawing of a mitochondrion   The intermembrane space is separated by the inner membrane from an inner compartment called the matrix. Specialized transporters - integral membrane proteins within the very protein-rich inner membrane - mediate the exchange of metabolites into and out of the matrix.
The inner membrane is otherwise highly impervious to the movement of ions or small molecules across it. Such compartmentalization has quite an important bearing on metabolism and its regulation in eukaryotic cells. For example, glycolysis takes place in the cytoplasm of cells. The product of glycolysis, pyruvate, is transported into the mitochondrial matrix before it is oxidized by pyruvate dehydrogenase complex. All of the citric acid cycle takes place in the matrix. If excess citrate accumulates in the matrix it can be exported to the cytosol to provide raw material for gluconeogenesis and fatty acid synthesis. Most of the ATP formed by oxidative phosphorylation, which takes place within the inner membrane and matrix, must be exported to the cytoplasm, while ADP and Pi must continually be imported to the matrix.
 

Oxidative phosphorylation

In oxidative phosphorylation, the free energy of the transmembrane difference in pH, created by the oxidation of metabolic fuel to water and carbon dioxide, as taken to completion by the electron transport chain, is in turn utilized to drive phosphorylation of ADP to produce ATP.

Schematic summary of oxidative phosphorylation, illustrating the transmembrane pH difference and its relation to the electron transport chain and ATP synthesis  

Left: Schematic summary of oxidative phosphorylation, illustrating the transmembrane pH difference and its relation to the electron transport chain and ATP synthesis. The equation is an expression for the Gibbs free energy of the electrochemical gradient. The term ΔV is defined as ΔV = Vout − Vin . The definition of pH can be used to write the first term of the equation as −2.3026RTΔpH.

 
 

Learning objectives

Page updated 07-21-10

References
 
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