Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

How electrons meet oxygen, how ATP is formed in the process, and other related phenomena.
Electron Transport Chain and Oxidative Phosphorylation
Chemical Activity of Mitochondria

Mentioning Cell/35.html">Mitochondria typically brings to mind The Tricarboxylic Acid Cycle, the fatty acid ß-Oxidation pathway, and Oxidative Phosphorylation. Alongside these core processes, a multitude of other chemical transformations take place within mitochondria. Perhaps The most significant of these is the accumulation of ions, such as Ca²⁺. Mitochondria also regulate the influx and efflux of numerous compounds, including ATP. Thus, they perform vital regulatory Functions in both catabolic and biosynthetic pathways. As they grow and replicate, mitochondria synthesize a portion of their own Proteins, while receiving various others from the Cytoplasm.

Where precisely within the mitochondrion are specific Enzymes localized? One approach to answering this question involves studying the release of enzymes from mitochondria. Some enzymes readily escape into a hypotonic medium. Others are liberated only upon Treatment with ultrasound, indicating that they reside within the mitochondrial matrix. Several enzymes, including Cytochromes and Flavoproteins acting on succinate and NADH, are bound so tightly to the mitochondrial membranes that they can be solubilized only after detergent treatment. Current evidence suggests that these firmly bound enzymes are embedded within the inner membrane.

If (as is generally believed) the Enzymes of the tricarboxylic acid cycle and ß-oxidation are localized in the matrix, then the reduced carriers must presumably approach the inner membrane from the matrix side (the M-side). Consequently, the membrane-embedded enzymes that catalyze The oxidation of NADH, succinate, and other reduced substrates should be accessible from the matrix side. However, the flavoprotein a-glycerophosphate dehydrogenase proves to be accessible from the "outer side" of the inner membrane (the C-side).

Specific fluorescent Antibodies directed against cytochrome c bind exclusively to the C-side of the inner membrane, whereas antibodies to cytochrome oxidase bind to both sides; this suggests that the latter protein spans the entire membrane [66, 66a]. However, the oxidation of cytochrome c (mediated by cytochrome a) occurs solely on the C-side, while the reduction of O₂ (mediated by cytochrome a3) takes place exclusively on the M-side [66]. Furthermore, antibodies against the "coupling factor," which forms the knob-like projections, bind only from the matrix side.

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FIG. 10-9. A. Cytology/cytology/92.html">SCHEMATIC Structure OF a mitochondrion. B. Model of particle Organization in mitochondrial membranes, based on freeze-fracture electron micrographs. Characteristic structural features are visible on four fracture faces exposing the interior of the outer and inner membranes (EF and PF). Four smooth membrane faces (ES and PS) were revealed by deep-etching (see Supplement 1-C). Adapted from Packer and Worthington [59a]. 1 — P-face of the outer membrane (PS), 2 — E-face of the inner membrane (ES), 3 — P-face of the inner membrane (PS), 4 — E-face of the outer membrane (ES), 5 — E-face of the outer membrane fracture (EF), 6 — P-face of the inner membrane (PF), 7 — matrix, 8 — E-face of the inner membrane (EF), 9 — P-face of the outer membrane fracture (PF). C. Thin-section electron micrograph of mitochondria in cultured chick embryo Kidney Cells. Courtesy of Judie Walton.

Based on these and other findings, the scheme shown in Fig. 10-10, B was constructed.

The outer mitochondrial membrane contains monoamine oxidase and cytochrome b₅, along with other proteins. In its composition, it likely resembles the membranes of The Endoplasmic reticulum. For the intermembrane space (situated between the inner and outer membranes), adenylate kinase (myokinase) [67] is considered one of the characteristic enzymes — a key catalyst involved in maintaining equilibrium between ATP, on the one hand, and AMP and ADP, on the other (Supplement 3-A; Chapter 7, Section D, 6).

At what concentrations are electron carriers present in mitochondrial membranes? One experiment demonstrated that the cytochrome concentration in Liver mitochondria is 0.28 µmol/g of protein. Assuming that the total protein concentration in a mitochondrion is approximately 22%, the average cytochrome concentration would be ~0.06 mM. Because cytochromes are localized within the inner membrane — which accounts for 10% or slightly less of the total mitochondrial volume — their concentration in this membrane must approach 1 mM. This is sufficient to support rapid reaction rates with their substrates. Analytical data regarding the relative Abundance of various Respiratory Chain components are widely available (Table 10-1).

FIG. 10-10. A. Putative distribution of enzymes in the mitochondrial inner membrane, based on analytical data from Heart Muscle mitochondria. Abbreviations: a, a₃, b, c, c₁ — cytochromes; NADH-DH, GluDH, GlyP-DH, MDH, SDH — NADH, glutamate, glycerophosphate, malate, and succinate dehydrogenases, respectively; (▲) — ubiquinone, (●) — NAD. B. Proposed transmembrane localization of certain respiratory chain carriers and dehydrogenases. From Kröger and Klingenberg [67a].

Kröger and Klingenberg [67a] mapped the distribution of enzymes within The inner mitochondrial membrane to better reflect what the membrane surface actually looks like (Fig. 10-10). Of course, it remains unknown whether Membrane Proteins are arranged in a regular lattice or diffuse randomly while colliding with one another.

¹⁾ Isoenzymes of adenylate kinase are present in somewhat lower amounts in The Nucleus and cytoplasm [68].

Table 10-1 Relative abundance of components in the Mitochondrial Electron Transport chain^a,b

^a According to Wainio [59] and cited literature.

^b Molecular ratios are given. Data for cytochromes are expressed per heme group.