Electron Transport, Oxidative Phosphorylation, and Hydroxylation

The outer membranes of mitochondria can be removed from the inner membranes by osmotic rup-ture. Analyses on separated membrane fractions show that the outer membrane is less dense (density 1.1 g/ cm ) than the inner (density 1.2 g / cm ). It is highly permeable to most substances of molecular mass 10 kDa or less because of the presence of pores of 2 nm diameter. These are formed by mitochondrial porins, which are similar to the outer membrane porins of gram-negative bacteria (Fig. 8-20). The ratio of phospholipid to protein ( 0.82 on a weight basis) is much higher than in the inner membrane. Extraction of the phospholipids by acetone destroys the membrane. Of the lipids present, there is a low content of cardiolipin, a high content of phosphatidylinositol and cholesterol, and no ubiquinone. 

The inner membrane is impermeable to many substances. Neutral molecules of 150 Da can penetrate the membranes, but the permeability for all other materials including small ions such as H, K, Na+, and Cl is tightly controlled. The ratio of phospholipid to protein in the inner membrane is 0.27, and cardiolipin makes up 20% of the phospholipid present. Cholesterol is absent. Ubiquinone and other components of the respiratory chain are all found in the inner membrane. Proteins account for 75% of the mass of the membrane. 

Another characteristic of the inner mitochondrial membrane is the presence of projections on the inside surface, which faces the mitochondrial matrix. See Fig. 18-14. These spherical 85-kDa particles, discovered by Fernandez Moran in 1962 and attached to the membrane through a stalk , display ATP-hydrolyzing (ATPase) activity. The latter was a clue that the knobs might participate in the synthesis of ATP during oxidative phosphorylation. In fact, they are now recognized as a complex of proteins called coupling factor 1 (Fj) or ATP synthase. 

In addition to bacterialike mitochondrial ribosomes and small circular molecules of DNA, mitochondria may contain variable numbers of dense granules of calcium phosphate, either Ca3(P04)2 or hydroxylapa-tite (Fig. 8-34), 48 g gf phospholipoprotein.  

Quantitatively the major constituents of the matrix space are a large number of profeins. These accormt for about 56% by weight of the matrix material and exist in a state closer to that in a protein crystal than in a true solution. Mitochondrial membranes also contain proteins, both tightly bormd relatively nonpolar intrinsic proteins and extrinsic proteins bormd 

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Chapter 18. Electron Transport, Oxidative Phosphorylation, and Hydroxylation... 

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