Читать книгу An Introduction to Molecular Biotechnology - Группа авторов - Страница 27

Mitochondria are very striking organelles that are found in nearly all eukaryotic cells (Figure 3.14). They look like worms or sausages and are between 1 μm and several micrometers long and 0.5 μm thick. Mitochondria have two separate membrane systems. The inner membrane forms a series of infoldings (cristae) that extend the surface considerably. The large surface area is needed because proteins and enzymes of the respiratory chain need to find space in or on the inner mitochondrial membrane (Figure 3.15). In a liver cell, mitochondria occupy about 22% of the cell volume.

Figure 3.14 Composition of a mitochondrion. (a) Electron microscope photograph.

Source: Courtesy of K.R. Porter/Photo Researchers, Inc.

(b) Schematic representation.

Source: Voet et al. (2016). Adapted with permission of John Wiley and Sons.

Figure 3.15 Function of mitochondrion: metabolism and respiratory chain. (a) Metabolism and respiration in mitochondrion. (b) Schematic representation of the respiratory chain with the complexes I–IV; the proton gradient is used by ATP synthase to produce ATP. Rotenone, malonate, antimycin A, and KCN are the inhibitors of complexes I–IV. FeS, iron–sulfur cluster; Cyt, cytochrome; CoQ, ubiquinone; FMN, flavin mononucleotide.

The respiratory chain produces ATP from the reduction equivalents NADH and FADH₂. During this process, electrons are transported through several intermediate stages, and a proton gradient is built up to provide energy for ATP synthase. This is also referred to as cellular respiration because the respiratory chain uses up oxygen. Without mitochondria, aerobic organisms such as animals, fungi, and plants would not be able to use oxygen from the air for the oxidation of organic matter – in other words, to produce energy. There are, however, some bacteria and a few eukaryotes that are anaerobic (i.e. they do not need oxygen). These organisms do not contain mitochondria.

During the citric acid cycle(Krebs cycle), which takes place in the mitochondria, acetyl CoA is introduced, and in each run of the cycle, CO₂ and reduction equivalents are generated. The acetyl CoA is derived from pyruvate, a product of glycolysis, which has been taken up by the mitochondria through a pyruvate transporter. It is then converted into acetyl CoA by a pyruvate decarboxylase complex. Another way of generating acetyl CoA is by β‐oxidation of fatty acids – a process that also takes place in mitochondria (Figure 3.15).

Mitochondria contain their own ring‐shaped DNA (Figure 3.16). In animals, the mitochondrial genome (mtDNA) is significantly smaller (16–19 kb) than in plants. It contains 13 genes coding for enzymes or other proteins involved in electron transport, 22 genes for tRNAs, and two genes for rRNAs. As every animal cell contains several hundred or even thousand mitochondria, each of which contains 5–10 mtDNA copies, the total of mtDNA copies amounts to several thousand per cell. mtDNA makes up about 1% of the total amount of DNA contained in a cell. The analysis of nucleotide sequences from mitochondrial genes has become an important tool in systematics to establish phylogenies and to define species.

Figure 3.16 Schematic overview of the arrangement of genes in the mtDNA of mammals.

Plant mitochondria, by contrast, have large genomes (150–2500 kb). Some of their genes even have an intron/exon structure.

Mitochondria contain functional ribosomes equivalent to the prokaryotic 70S type, and the nucleotide sequences in mitochondrial genes and the amino acid sequences of the respective proteins are more closely related to the corresponding prokaryotic genes than to equivalents coded in the nucleus. The genetic code of mitochondria shows a few differences to the universal code: UGA (stop codon) codes in animals and fungi for tryptophan, AUA (for isoleucine) codes in animals and fungi for methionine, and AGG (arginine) codes in mammals for stop and in invertebrates for serine.

These findings as well as other mitochondrial characteristics led to the endosymbiont hypothesis, which states that mitochondria are derived from α‐purple bacteria that were ingested by an ancestral eucyte 1.2 billion years ago and lived on as endosymbionts. The cell provides nutrients for the endosymbionts and receives ATP in return. Figure 3.17 shows a likely ingestion path for the α‐purple bacteria into the ancestral eucyte. It is assumed that the ancestral eucyte came into being by the infolding of a bacterial cytoplasmic membrane to form an ER. The membrane then began to surround the chromosome, thus forming a nucleus.

Figure 3.17 Development of an early eucyte and origin of mitochondria. α‐Purple bacteria were ingested by the early eucyte in a kind of phagocytosis. Hence, the outer mitochondrial membrane is derived from the host cell, whereas the inner mitochondrial membrane is the original bacterial cytoplasmic membrane.

Green plants and algae contain an additional organelle, the conspicuous chloroplasts, which are significantly larger and structurally more complex than mitochondria (Figure 3.18). Apart from the surrounding inner and outer biomembranes, the chloroplast contains an extensively folded membrane system, known as thylakoids. These contain chlorophyll, as well as the proteins and enzymes required for photosynthesis, to enable the plants to turn sunlight into energy in the form of ATP and NADPH (Figure 3.19). The electron transport between photosystem II and I and the production of NADPH are explained in Figure 3.19b. The light reaction leads to the buildup of a proton gradient, which is then used by ATP synthase to produce ATP. During the subsequent CO₂fixation process (Calvin cycle), CO₂ is first bound to ribulose‐1,5‐biphosphate, which is then cleaved into two C3 units (3‐phosphoglycerate). 3‐Phosphoglycerate is transformed into glycerol aldehyde‐3‐phosphate, which is used for the regeneration of ribulose‐1,5‐biphosphate and for building glucose, fatty acids, and amino acids. A plant cell can generate additional ATP from glucose for the energy supply of the cell. This makes plants autotrophic and a suitable basic nutrient for heterotrophic animals that live on organic matter.

Figure 3.18 Structure of a chloroplast. (a) Electron microscope photo of a chloroplast.

Source: Courtesy of T. Elliot Weier.

(b) Schematic representation.

Source: Voet et al. (2016). Adapted with permission of John Wiley and Sons.

Figure 3.19 Essential steps in photosynthesis. (a) Overview of photosynthetic reactions in chloroplasts. (b) Electron transport between photosystem II and I across the thylakoid membrane, resulting in NADPH production. ATP synthase uses the proton gradient for the production of ATP. Q, plastoquinone; FD, ferredoxin; PS I, photosystem I.

Like mitochondria, chloroplasts contain their own ring‐shaped DNA (cpDNA) as well as independent replication, transcription, and protein biosynthesis. The chloroplast genome has a size of 120–200 kb (Figure 3.20); it encodes 120 genes and is present at 20–300 copies in a single chloroplast. As a plant cell contains up to 40 chloroplasts, the total number of cpDNA copies is between 800 and 3200 per cell. The analysis of nucleotide sequences from chloroplast genes has become an important tool in systematics to establish phylogenies and to define species.

Figure 3.20 Overview of the arrangement of genes in chloroplast genomes.

For chloroplasts, too, it is assumed that there is an endosymbiotic origin (Table 3.6). The nucleotide sequences in chloroplast genes and the amino acid sequences of the corresponding proteins are more closely related to those of cyanobacteria than to the respective genes in the plant cell nucleus. Figure 3.21 gives a schematic overview of the presumptive origin of chloroplasts. Similar to the intake of mitochondria, an early eucyte seems to have taken up photosynthetic bacteria through phagocytosis and tamed them to develop an endosymbiosis. It is thought that the acquisition of chloroplasts happened several times in the phylogeny of photosynthetically active algae and plants.

Table 3.6 Prokaryotic properties of plastids and mitochondria.

Genome	Mostly circular DNA adhesive to biomembrane without histones and nucleosomes, several copies concentrated in nucleoids; gene arrangement more or less prokaryotic (operon structure); repetitive sequences rare or nonexistent
Ribosomes	70S‐type
Translation	No Cap structure at the 5′ end of mRNAs; prokaryotic complement of initiation factors
Tubulin, actin	Not found in organelles; FtsZ, a bacterial, tubulin‐homologous cell division protein is involved in the division of plastids
Plastid fatty acid synthesis	As in bacteria, using acyl carrier proteins
Cardiolipin	Membrane lipid found in many bacteria. Not present in eukaryotic membranes except the inner mitochondrial membrane

Figure 3.21 Development of chloroplasts through phagocytosis of cyanobacteria.

Mitochondria and chloroplasts never emerge de novo, but replicate through division. When the cell divides, mitochondria are distributed over the daughter cells. Mitochondria can also fuse with each other. Both mitochondria and mostly also chloroplasts are inherited maternally. Mitochondria in sperm cells are not incorporated into the fertilized egg. Although replication, transcription, and protein biosynthesis still happen in the same way in mitochondria and chloroplasts, they have become organelles and are no longer autonomous. They import most of their proteins from the cytoplasm. These proteins carry signaling sequences that bind to receptors on the organelles (see Chapter 5), and through complex transport mechanisms, they finally reach their working place inside the mitochondria and chloroplasts. The corresponding genes used to be part of the endosymbionts but have increasingly been moved into the nucleus. Only a relatively small set of genes has remained in mitochondria and chloroplasts. While this applies mostly to protein‐coding genes, tRNA and rRNA genes have remained in the organelles.