Mitochondria, oxidative phosphorylation

Mitochondria Oxidative phosphorylation Citric acid cycle Fe, (Cu), (Ca), (Mg)... 

Keywords Membrane protein folding phosphatidylethanolamine cardiolipin mitochondria oxidative phosphorylation... 

When they further observed that the normal nucleus contains a high proportion of mono-, di-, and trinucleotides of adenine, they claimed to have provided direct proof of their theory by demonstrating that the mono-or dinucleotides in the nucleus may be converted to ATP when oxygen is present. (The nucleotides can be extracted from the nucleus with acetate buffer at pH 5.1.) This conversion certainly suggested the existence of an intranuclear process of oxidative phosphorylation. As in mitochondria, oxidative phosphorylation in the nucleus is inhibited by uncouplers or agents blocking the electron transport chain. Nuclear oxidative phosphorylation is blocked by cyanide, azide, and antimycin A, or by dinitrophenol but, in contrast to mitochondria, it is resistant to Janus green, methylene blue, carbon monoxide, Dicumarol, and calcium. 

Mechanistic P/0 ratio Oxidation of reducing equivalents produced during glycolysis and in the TCA cycle takes place in the mitochondria (oxidative phosphorylation). The maximal ATP production rate, coupled with electrons exchanged, is determined by the mechanism of sites III and IV in the electron transport chain. The mechanistic P/0 ratio is equal to the number of moles ATP synthesized by the oxidation of one mole of NAD(P)H+Fr by 0.5 mole O2. The electrochemical potential change in site III of the respiratory chain is 41.4 kJ per mole NAD(P)FI+H oxidized and 99.6 kJ moP for the site IV, whereas the standard energy of ADP phosphorylation to ATP is 30.5 kJ moP. Thus, a maximum of 1 mole of ATP can be produced in site III per mol NAD(P)H+H oxidized, and a maximum of 2 moles ATP in site IV, leading to a maximal mechanistic P/O ratio of 3. However, it is not demonstrated that the mechanistic P/0 ratio of oxidative phosphorylation is effectively 3 due to the inefficiency of sites III and IV. 

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

Mitochondrion A subcellular organelle in which oxidative phosphorylation occurs, leading to the generation of ATP. 

Atractyloside inhibits oxidative phosphorylation by inhibiting the transporter of ADP into and ATP out of the mitochondrion (Figure 12-10). 

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

It has been known for many years that the mitochondrion shows a respiration-linked transport of a number of ions. Of these, calcium has attracted the most attention since it depends on a specific transport system with high-affinity binding sites. The uptake of calcium usually also involves a permeant anion, but in the absence of this, protons are ejected as the electron transfer system operates. The result is either the accumulation of calcium salts in the mitochondrial matrix or an alkalinization of the interior of the mitochondrion. The transfer of calcium inwards stimulates oxygen utilization but provides an alternative to the oxidative phosphorylation of ADP618 ... 

How Many Protons in a Mitochondrion Electron transfer translocates protons from the mitochondrial matrix to the external medium, establishing a pH gradient across the inner membrane (outside more acidic than inside). The tendency of protons to diffuse back into the matrix is the driving force for ATP synthesis by ATP synthase. During oxidative phosphorylation by a suspension of mitochondria in a medium of pH 7.4, the pH of the matrix has been measured as 7.7. 

The combined effect of exchanging extramitochon-drial ADP-3 and H2P04 for mitochondrial ATP-4 and OH is to move one proton into the mitochondrial matrix for every molecule of ATP that the mitochondrion releases into the cytosol. This proton translocation must be considered, along with the movement of protons through the ATP synthase, to account for the P-to-O ratio of oxidative phosphorylation. If three protons pass through the ATP synthase, and the adenine nucleotide and Pj transport systems move one additional proton, then four protons in total move into the matrix for each ATP molecule provided to the cytosol. 

Mitochondrion. An organelle, found in eukaryotic cells, in which oxidative phosphorylation takes place. It contains its own genome and unique ribosomes to carry out protein synthesis of only a fraction of the proteins located in this organelle. 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

To put it differently, more than one proton should be transferred through the membrane per molecule of synthesized ATP, i.e. the minimal ratio equals 2. At the same time, only one proton can be transferred through the membrane to mitochondrion. If this transfer is related to dissociation of the water molecule (3.51), produced during oxidative phosphorylation, the number of transferred ions must equal 1. 

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

MITOCHONDRION A membrane-bound organelle of eukaryotic organisms that replicates independently of the cell nucleus and contains its own ENA and its own protein-synthesizing apparatus its function is to provide energy to the cell in the form of adenosine triphosphate by oxidative phosphorylation. 

The inconsistency between experiment and prediction must lead to the rejection of the model used to describe the system. In the case of oxidative phosphorylation this has led to a refined model, in which the chemiosmotic coupling is visualized as taking place within units of one (or a few) respiratory chain(s) plus ATP synthase, while the pumped protons have only limited access to the bulk phase inside and/or outside the mitochondrion [42]. This more refined model can again be tested by deriving from it flux-force relations according to the MNET approach. A discussion of the refined model can be found in Ref. 43. 

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