LEHNINGER PRINCIPLES OF BIOCHEMISTRY - VOL 2. BIOENERGETICS AND METABOLISM - 2014

PART II. BIOENERGETICS AND METABOLISM

19. OXIDATIVE PHOSPHORYLATION AND PHOTOPHOSPHORYLATION

19.5. Mitochondrial Genes: Origin and Mutations

Circular double-stranded molecules of Mitochondrial DNA harbor a set of mitochondrial genes. In a typical Cell, each of the hundreds or thousands of Mitochondria contains about five copies of its genome. Human mitochondrial Chromosomes (Fig. 19-38) contain 37 genes (16,569 bp), 13 of which encode subunits of the mitochondrial Respiratory Chain Enzymes (see Table 19-6). The remaining 24 genes encode mitochondrial rRNAs and tRNAs, which play a crucial role in mitochondrial Protein Synthesis. The vast majority of mitochondrial Proteins—approximately 900 different types—are encoded by nuclear DNA genes and synthesized by cytoplasmic Ribosomes before being imported into and assembled within the mitochondria. These topics are covered in detail in Chapter 27 (Vol. 3).

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Table 19-6. Respiratory chain enzymes of human mitochondria encoded by mitochondrial genes

Mitochondria arose through endosymbiosis

Mitochondria contain small amounts of DNA, as well as tRNAs and ribosomes (see Fig. 1-36, Vol. 1). Why do mitochondria contain DNA? To answer this question, the hypothesis of the endosymbiotic origin of mitochondria was proposed. According to this hypothesis, the first Cells capable of utilizing molecular oxygen to oxidize nutrients and synthesize ATP via electron-transfer-coupled energy were Prokaryotic Cells. Following the invasion of small aerobic prokaryotic cells (Bacteria) into the Cytoplasm of larger eukaryotic anaerobic (fermenting) cells and the subsequent evolutionary development of this symbiotic relationship between host and parasite cells, primitive eukaryotes acquired the capacity for Oxidative Phosphorylation, while the bacteria residing in the eukaryotic cytoplasm evolved into mitochondria.

This hypothesis of mitochondrial origin implies that ancient, free-living prokaryotic cells possessed the enzymatic machinery to carry out oxidative phosphorylation. Therefore, one would expect that modern bacteria, which are descendants of these aerobic prokaryotes, have a respiratory chain similar in Structure and properties to that of modern Eukaryotic cells. Indeed, in aerobic bacteria, the transfer to oxygen of electrons stripped from substrates by NAD-dependent dehydrogenases is coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial Cytosol, whereas the electron carriers of the respiratory chain reside in The Plasma Membrane. During electron transfer to oxygen in bacterial cells, H⁺ ions are pumped outward across the plasma membrane. The Plasma Membranes of the bacterium Escherichia coli contain F₀F₁ enzyme complexes, with the F₁ component protruding into the cytosol and catalyzing the synthesis of ATP from ADP and phosphate as protons flow back into The Cell through the proton channel of the F₀ component.

The transport of protons across the plasma membrane of bacterial cells, driven by The energy released during Electron transport along the respiratory chain, generates a proton-motive force utilized not only for ATP synthesis but also to sustain other biological processes. Transport systems in certain bacteria can drive the uptake of nutrients, such as lactose, against a concentration gradient via proton symport (see Fig. 11-42, Vol. 1). Bacterial flagella are driven by "proton turbines"—Molecular Motors that operate directly on the energy of the transmembrane Electrochemical Potential generated by proton extrusion from the matrix during Respiratory Electron Transport (Fig. 19-39). It is believed that the chemiosmotic mechanism coupling Electron transfer energy to ATP synthesis evolved well before eukaryotic cells appeared.

Fig. 19-39. Rotation of bacterial flagella driven by the proton-motive force. The rotational motion of bacterial flagella is driven by a specialized structure located in The cell membrane, consisting of a rod and rings, known as a "proton turbine." H⁺ ions pumped outward during electron transport re-enter the cell through this "turbine," causing the flagellar rod to rotate. The movement of bacterial flagella differs from that of eukaryotic cellular appendages, such as cilia and Muscle movement, which rely on energy released by ATP Hydrolysis.

Mutations accumulate in mitochondrial DNA over the lifespan of an Organism

The respiratory chain is the primary source of reactive oxygen species within the cell; consequently, mitochondria, including the Mitochondrial Genome, are more vulnerable to the damaging effects of oxygen free radicals than other cellular compartments. Furthermore, the mitochondrial METABOLISM/36.html">DNA Replication system lacks the high-fidelity error-correction mechanisms of nuclear DNA replication. As a result, errors accumulate in mitochondrial DNA over time. According to one theory, this gradual accumulation of errors is a major contributor to the "symptoms" of Aging, such as the progressive weakening of skeletal and cardiac muscle.

The distinctive mode of inheritance of mitochondrial genetic material means that individual cells and different organisms carry varying mutational loads. A typical cell contains hundreds or thousands of mitochondria, each bearing its own copy of The Genome (Fig. 19-40). Suppose that in a female organism, a germ-line cell destined to form an egg cell suffers damage to its mitochondrial genome, leaving it predominantly with wild-type mitochondria alongside a single defective mitochondrion. During oogenesis, as the egg cell undergoes multiple divisions, the number of defective mitochondria increases and is distributed randomly among the daughter cells. Consequently, mature oocytes end up with varying proportions of defective mitochondria. Following Fertilization and the numerous cell divisions of embryonic development, somatic cells are generated that harbor different proportions of mutated mitochondria (Fig. 19-41a). (Recall that all embryonic mitochondria are derived from the egg cell, not the sperm.) Such mitochondrial DNA heteroplasmy (as opposed to homoplasmy, in which the genomes of all mitochondria in all cells are identical) gives rise to mutant phenotypes of varying severity. Cells (and Tissues) containing predominantly wild-type mitochondria exhibit a wild-type phenotype and are normal. Other cells exhibiting mitochondrial DNA heteroplasmy display an intermediate phenotype: some are nearly normal, whereas others (with a higher proportion of mutant mitochondria) are abnormal (Fig. 19-41b). When such an abnormal phenotype is associated with a disease (see below), individuals harboring the same mitochondrial DNA mutation may present with Clinical symptoms of differing severity depending on the number and distribution of damaged mitochondria.

Fig. 19-40. Each cell contains numerous mitochondria. A typical animal cell contains several hundred or a thousand mitochondria, and the genomes of some of these mitochondria may harbor mutations that impair mitochondrial function. This sheep Kidney epithelial cell was cultured in vitro, fixed, and stained with fluorescent markers. When imaged by Fluorescence Microscopy, mitochondria appear gold, Actin microfilaments red, and nuclei green.

Fig. 19-41. Mitochondrial DNA heteroplasmy. (a) Following the fertilization of a mature egg cell, all mitochondria in the resulting diploid cell (zygote) are maternal—none originate from the sperm. If some mitochondria in the maternal cell contained a mutated Gene, random partitioning of mitochondria during subsequent cell divisions will yield daughter cells with high, intermediate, and low proportions of defective mitochondria. Thus, daughter cells differ in their degree of heteroplasmy. (b) Varying degrees of heteroplasmy lead to distinct phenotypes. Shown here is a section of human Muscle tissue with a cytochrome c oxidase deficiency. The sample is stained such that wild-type cells appear blue and cells with the mutant enzyme appear brown. As evident, the mitochondrial DNA mutation affects individual cells within the same tissue to varying extents.

Mitochondrial Gene Mutations cause human diseases

An increasing number of human disorders are linked to mutations in mitochondrial genes that impair the cellular capacity for ATP production. Certain cell and tissue types—such as Neurons, skeletal and cardiac muscle myocytes, and pancreatic cells—have a lower tolerance for ATP depletion and are therefore more severely damaged by mutations in mitochondrial protein genes. THE SPECTRUM OF recognized human diseases caused by mitochondrial gene mutations continues to expand alongside advancements in diagnostic Methods for detecting mutation-induced pathology.

Many of these conditions, known collectively as mitochondrial encephalopathies, primarily affect the Brain and skeletal Muscles. These are inherited disorders transmitted maternally, as the embryo inherits its mitochondrial genes, including mutants, from the maternal egg cell. A rare disorder, Leber's hereditary optic neuropathy (LHON), affects the Central Nervous system, including the optic nerves, and causes early-onset bilateral Vision loss. A single base-pair substitution in the mitochondrial ND4 gene results in an amino acid substitution in a Complex I polypeptide chain—replacing an Arginine residue with a Histidine—which compromises the efficiency of Electron transfer from NADH to ubiquinone (Fig. 19-38a). Mitochondria bearing this mutant gene retain the capacity to synthesize some ATP via electron transfer from succinate, but this proves insufficient to support the high metabolic demands of neurons, leading to Optic nerve degeneration and blindness. A single base-pair substitution in the mitochondrial gene encoding cytochrome b in Complex III also gives rise to LHON syndrome. These Examples demonstrate that human hereditary pathologies can stem from an overall decline in mitochondrial functional capacity rather than isolated defects in the electron-transfer machinery of Complex I.

A mutation in ATP6 that affects the proton channel of ATP synthase leads to a decreased rate of ATP synthesis without disrupting the respiratory chain sequence. Oxidative stress, associated with a continuous influx of electrons from NADH, is accompanied by elevated production of reactive oxygen species, and the resulting mitochondrial damage creates a vicious cycle. Half of all newborns harboring this gene mutation die within days or months of birth.

Myoclonic Epilepsy with ragged-red fibers (MERRF syndrome) results from mutations in a mitochondrial gene encoding a Transfer RNA that binds Lysine (tRNA^Lys). This disorder is characterized by uncontrolled muscle twitching, presumably resulting from impaired Biosynthesis of several proteins involving mitochondrial tRNAs. Mitochondria from the Skeletal Muscle fibers of individuals with MERRF syndrome have distinct outlines and sometimes contain paracrystalline inclusions (Fig. 19-38b). Other mutations in mitochondrial genes likely cause the progressive muscle weakness characteristic of mitochondrial myopathy, as well as the cardiac enlargement and dysfunction associated with hypertrophic cardiomyopathy. According to one hypothesis, progressive age-related changes in The Human Body result from the lifetime accumulation of mitochondrial DNA mutations induced by ROS such as ^•O₂^-. Mitochondria with damaged DNA fail to supply cells with sufficient amounts of ATP. Diseases caused by mitochondrial defects can also arise from mutations in any of the roughly 900 nuclear DNA genes that encode mitochondrial polypeptide chains. ■

Diabetes can result from mitochondrial defects in pancreatic β-cells

The mechanism regulating Insulin secretion by pancreatic β-cells is closely linked to the intracellular ATP concentration. When Blood glucose levels are high, glucose enters β-cells, where it is oxidized via Glycolysis and The Citric Acid Cycle, thereby raising the ATP concentration above a threshold level (Fig. 19-42). As soon as the ATP concentration exceeds this value, ATP-sensitive potassium channels in the plasma membrane close, leading to membrane depolarization and insulin release (see Fig. 23-28). Pancreatic β-cells with impaired oxidative phosphorylation cannot maintain an ATP concentration above the threshold, which disrupts insulin secretion and leads to diabetes. For example, defects in the glucokinase gene (an isoform of hexokinase IV present in β-cells) result in a rare form of diabetes with the MODY2 phenotype (see Box 15-3). Low glucokinase activity prevents the threshold ATP concentration from being reached, thereby blocking insulin secretion. Mutations in the mitochondrial tRNA^Lys or tRNA^Leu genes also impair mitochondrial ATP production, which is why individuals carrying such mutations frequently develop type 2 diabetes (although these cases account for only a small fraction of all diabetes cases).

When genetic mutations occur in NADP⁺ transhydrogenase—an essential component of the ROS defense system (Fig. 19-18)—the accumulation of free radicals leads to mitochondrial damage, diminishing ATP production and blocking insulin secretion by β-cells (Fig. 19-42). Free radical-mediated damage, including damage to mitochondrial DNA, may also underlie other human pathologies and has been implicated in Alzheimer's, Parkinson's, and Huntington's diseases, Heart disorders, and the aging process. ■

Figure 19-42. Impaired oxidative phosphorylation in pancreatic β-cells blocks insulin secretion. Under normal conditions, an increase in glucose concentration enhances cellular ATP production. ATP blocks K⁺ channels, leading to membrane depolarization and the opening of voltage-gated Ca²⁺ channels. The resulting influx of Ca²⁺ triggers the exocytosis of insulin-containing secretory granules and the release of insulin. When Oxidative phosphorylation is impaired, the intracellular ATP concentration never reaches the level required to trigger this process, and consequently, insulin is not released.

Summary of Section 19.5 Mitochondrial Genes: Origin and Mutations

■ Mitochondrial DNA genes in human cells encode only a small subset of mitochondrial proteins (about 13 proteins) that are synthesized within mitochondria and participate in oxidative phosphorylation.

■ Mitochondria evolved from aerobic bacteria that were engulfed by eukaryotic cells and became permanently established within them during evolution.

■ Mutations in the mitochondrial genome accumulate over the lifespan of an organism. Mutations in genes encoding respiratory chain components, ATP synthase, the ROS defense system, as well as mutations in tRNA genes, cause a variety of human diseases that most commonly affect The Muscular System, heart, pancreatic β-cells, and brain.

Photosynthesis: HARVESTING SOLAR ENERGY

We now turn to the molecular processes by which light energy, absorbed by photosynthetic organisms, drives electron flow and the coupled synthesis of ATP. Ultimately, solar energy converted into the chemical energy of reduced Organic compounds serves as the source of nearly all biological energy. Photosynthetic and heterotrophic organisms coexist in the biosphere in a balanced steady state (Fig. 19-43). Photosynthetic organisms absorb solar energy and store it in the form of ATP and NADPH, which provide the energy required to synthesize CARBOHYDRATES and other organic cellular components from carbon dioxide and Water, releasing oxygen into the atmosphere in the process. Aerobic heterotrophs—such as humans and plants in the dark—use this oxygen to break down energy-rich photosynthetic products into CO₂ and H₂O, harnessing that energy to synthesize ATP for their own metabolic needs. The carbon dioxide produced by heterotrophic Respiration is returned to the atmosphere, where it is reused by photosynthetic organisms. Solar energy acts as the driving force for a cycle in which atmospheric carbon dioxide and oxygen continuously circulate through the biosphere, generating reduced substrates such as glucose, which serves as an energy source for non-photosynthetic organisms.

Figure 19-43. Solar energy as the primary source of all biological energy. Photosynthetic cells use sunlight energy to produce glucose and other organic molecules. These organic products serve as sources of energy and carbon for heterotrophic cells.

Photosynthesis occurs not only in familiar vascular plants but also in Unicellular Eukaryotes (Algae) and a diverse array of bacteria. In principle, the photosynthetic process is fundamentally similar across different organisms, although certain variations exist depending on the specific type of photosynthetic cell. Insights gained from studying photosynthesis in primitive organisms can be applied to elucidate photosynthetic mechanisms in higher plants. For plant photosynthesis, where water serves as the electron (hydrogen) donor to reduce carbon dioxide to carbohydrates, the overall equation can be written as follows: