Six Functions of Respiration: Isn't It Time to Take Control over ROS Production in Mitochondria, and Aging Along with It?

Abstract

Cellular respiration is associated with at least six distinct but intertwined biological functions. (1) biosynthesis of ATP from ADP and inorganic phosphate, (2) consumption of respiratory substrates, (3) support of membrane transport, (4) conversion of respiratory energy to heat, (5) removal of oxygen to prevent oxidative damage, and (6) generation of reactive oxygen species (ROS) as signaling molecules. Here we focus on function #6, which helps the organism control its mitochondria. The ROS bursts typically occur when the mitochondrial membrane potential (MMP) becomes too high, e.g., due to mitochondrial malfunction, leading to cardiolipin (CL) oxidation. Depending on the intensity of CL damage, specific programs for the elimination of damaged mitochondria (mitophagy), whole cells (apoptosis), or organisms (phenoptosis) can be activated. In particular, we consider those mechanisms that suppress ROS generation by enabling ATP synthesis at low MMP levels. We discuss evidence that the mild depolarization mechanism of direct ATP/ADP exchange across mammalian inner and outer mitochondrial membranes weakens with age. We review recent data showing that by protecting CL from oxidation, mitochondria-targeted antioxidants decrease lethality in response to many potentially deadly shock insults. Thus, targeting ROS- and CL-dependent pathways may prevent acute mortality and, hopefully, slow aging.

Keywords: aging; cardiolipin oxidation; mitochondria; reactive oxygen species; respiration.

Figure 1

Mitochondrial respiratory chain and the complex between ATP synthase, ADP/ATP carrier, VDAC, and a mitochondrial kinase. Complexes I-IV are positioned along the scale of redox potentials on the right in accordance with their actual redox properties. The following crystal structures were used (from top to bottom): bovine Complex I (PDB ID 7R43 [4], human respiratory Complex II (PDB ID 8GS8 [5]), bovine respiratory Complex III (PDB ID 1BGY, [6]), bovine respiratory Complex IV (PDB ID 6J8M [7]), bovine respiratory cytochrome c (PDB ID 2B4Z [8]), bovine mitochondrial ATP synthase (PDB ID 6ZQM [9]), bovine mitochondrial ADP/ATP carrier (PDB ID 1OKC [10]), putative glycerol kinase-like proteins anchored on an array of voltage-dependent anion channels in the outer mitochondrial membrane of pig sperm mitochondria (PDB ID 7NIE [11]) as a model of a VDAC dimer interacting with a mitochondrial kinase. Green arrows, electron transfer steps; red arrows, proton transfer steps; orange arrows, ADP/ATP exchange. Abbreviations: SOD, superoxide-dismutase.

Figure 2

Influence of CL and its oxidation on the packing of integral membrane proteins. Top row: The scheme shows how an integral membrane protein interacts with the lipid bilayer in the presence of (a) cylindrical two-tail lipids, (b) cone-shaped four-tail CL molecules, and (c) after the oxidation of a CL molecule. Red arrows indicate proton leakage in cases (a,c). Bottom row: 3D structure of the bovine ADP/ATP carrier (PDB ID: 2C3E [75]) without (d) and with (e) bound CL molecules. (f), a scheme of a chain reaction of lipid peroxidation in the presence of an antioxidant. Abbreviations: L, lipid; LOO•, lipid peroxide; LOOH, a quenched lipid peroxide; QH₂, a quinol-type antioxidant; see the main text and [62,72] for details.

Figure 3

Structure of different mitochondria-targeted compounds. Adapted from [94].

Figure 4

Mitochondria-targeted antioxidant SkQ1 prevents the lethal toxicity of four different shocks. (A) SkQ1 prevents the lethal toxicity of bacterial lipopolysaccharide (LPS). LPS at doses of 30 mg/kg was injected intravenously. SkQ1 (1.5 mmol/kg/day) was administered intraperitoneally for five days. С57Bl6 (3 months) mice were used; (B) SkQ1 prevents the rapid death of mice after injection of mitochondria (10 mg protein/kg body weight) into the tail vein. SkQ1 (1.5 μmol/kg) was administered intraperitoneally daily for 5 days before and after mitochondrial injection. 5 mM succinate (Succ) was added to mitochondria prior to injection. (С) SkQ1 protects mice against toxic doses of C₁₂TPP. Survival of mice after a single intravenous injection of 34 μmol/kg C₁₂TPP. SkQ1 (1.5 μmol/kg/day) was injected as in (A); (D) SkQ1 prevents mice from rapid death caused by cooling. Mice were placed at −20 °C for 1 h and then returned to room temperature. SkQ1 (1.5 μmol/kg) was administered intraperitoneally for 5 days before cooling and for 3 days after cooling. Adapted from [130].

Figure 5

Mechanism of mild depolarization and its dependence on aging. (A), The interplay between the respiratory chain (Complexes I, II, III, and IV), H⁺-ATP-synthase (Complex V, also known as F_OF₁), the transporters of ATP and ADP (ADP/ATP carrier and VDAC), and hexokinase I or II bound to VDAC. Creatine kinase is is in the intermembrane space, contacting the outer surface of the ADP/ATP carrier. The creatine phosphate produced by this kinase is released into the intermembrane space of the mitochondrion. The hexokinase-mediated depolarization should be driven by the dissipation of some energy due to the transfer of the phosphoryl residue from ATP to glucose when G6P is formed. During aging, the level of mitochondria-associated hexokinase decreases, which prevents mild depolarization and consequently leads to increased mROS production. (B) Experimental data illustrating the age-dependent decline in mitochondrial-bound hexokinase activity in mouse embryos and 3-, 12-, 24-, or 30-month-old mice. * p < 0.01; ** p < 0.001 (mouse embryos, 12-, 24-, or 30-mo-old mice compared with 3-mo-old mice). Date adapted from [145].