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

On how electrons meet oxygen, how ATR is generated in the process, and some related phenomena
Electron transport chain and oxidative phosphorylation
Transport across mitochondrial membranes

Like the outer Cell/33.html">Plasma Membrane, The inner mitochondrial membrane is highly selective. Some non-ionized substances pass through it easily, whereas The transport of ionic species, including dicarboxylic and tricarboxylic acid anions, is tightly regulated. In some cases, anions are transported via energy-dependent "Active Transport". In other cases, an anion can enter only in exchange for another anion exiting. All of these cases require the participation of specific translocating carrier Proteins (Ch. 5, Sec. B,2).

One of the translocation systems exchanges ADP for ATP. This adenine nucleotide carrier delivers ADP into the matrix, where it undergoes phosphorylation, and exports ATP into the Cytoplasm in a 1:1 ratio [60, 100—102]. A separate carrier is responsible for delivering P_i, probably in the form of Н₂РО^-₄. It is commonly assumed that the phosphorylation state R_p = [ATP]/[ADP] ∙ [P_i] has the same value outside and inside the mitochondrion. However, Klingenberg established that R_p is 10 times higher outside than inside [102]. This must mean that newly synthesized ATP is released predominantly on the outside of the inner mitochondrial membrane. A smaller portion of ATP must be released inside the mitochondrion, where it is consumed for fatty acid activation, Protein Synthesis, etc. Pyruvate also appears to enter the mitochondrion via its own carrier, most likely along with a proton. On the other hand, dicarboxylic acid anions, such as malate or a-ketoglutarate, are exchanged in a 1:1 ratio, as are aspartate and glutamate.

Mitochondrial membranes are impermeable to NADH. Thus, The transfer of reducing equivalents into Mitochondria from NADH generated in the cytoplasm is an important problem. In Fungi and green plants, this is resolved by the presence of two NADH dehydrogenases (Flavoproteins) embedded in the inner mitochondrial membrane [61, 103]. One faces the matrix and oxidizes endogenous NADH generated in the matrix, while the other faces outward into the intermembrane space and oxidizes exogenous NADH generated in the cytoplasm. Both Enzymes transfer electrons to the carrier chain via ubiquinone, but the exogenous NADH dehydrogenase is not inhibited by rotenone (Fig. 10-11).

In animals, the reducing equivalents of NADH enter mitochondria indirectly. Many different mechanisms have been postulated, and it is highly possible that several mechanisms actually operate simultaneously. In insect flight Muscle, NADH reduces dihydroxyacetone phosphate. The resulting a-glycerophosphate (sn-3-glycerophosphate) passes through the permeable outer mitochondrial membrane, after which it is reoxidized to dihydroxyacetone phosphate by an FAD-containing glycerophosphate dehydrogenase embedded in the outer surface of the inner membrane. Dihydroxyacetone can then return to the cytoplasm. This glycerophosphate shuttle mechanism (Fig. 10-13) ultimately provides mitochondrial oxidation of NADH generated in the cytoplasm. In mammals, a similar function is apparently performed by a more complex malate-aspartate shuttle mechanism (Fig. 10-13). In this case, the transfer of reducing equivalents into the mitochondrion involves the reduction of oxaloacetate to malate by NADH, the transport of malate into the mitochondrion, and its reoxidation by NAD⁺. However, mitochondrial membranes are poorly permeable to oxaloacetate. Therefore, the return of the latter to the cytoplasm occurs via its Transamination to aspartate, which leaves the mitochondrion along with a-ketoglutarate. Simultaneously, glutamate enters the mitochondrion in exchange for aspartate. The a-ketoglutarate presumably exits in exchange for the entering malate, as shown in Fig. 10-13.

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FIG. 10-13. Glycerophosphate (A) and malate-aspartate (B) shuttle mechanisms that mediate Electron transfer from cytoplasmic NADH into mitochondria. Bold arrows indicate The pathway of the transferred electrons.

It has been suggested that the export of aspartate from the mitochondrion is energy-dependent; in this case, an analogy can be drawn with the operation of the Na⁺ pump in The Plasma Membrane [104]. The Mechanism of such transport may be similar in nature to the mechanism of amino acid uptake by bacterial cytoplasmic membrane vesicles [105—107]. The accumulation of Amino Acids by such vesicles does not seem to depend on ATP but is coupled to electron transfer mediated by specific, membrane-bound flavin-containing dehydrogenases. Amino acid uptake by E. coli vesicles is particularly effectively promoted by the dehydrogenation of D-lactate. The removal of hydrogens from L-lactate or glycerophosphate promotes amino acid uptake by Staphylococcus aureus vesicles. The affinity of The amino acid for the membrane "pumps" of such vesicles likely depends on the redox state of the electron carriers in the membrane. These results suggest that the "pump" contains a protein in which the carrier is embedded; the redox state of the carrier regulates substrate affinity and simultaneously induces conformational changes. For example, changes could occur similar to those induced by phosphorylation and dephosphorylation in the hypothetical Na⁺ pump shown in Fig. 5-3. However, the interpretation of the experimental data underlying these assumptions remains controversial [107, 108].

The Role of carnitine in accelerating the transport of Fatty acids into mitochondria has already been discussed in Ch. 9, Sec. A,6.