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Home , Coenzyme Q10, Q cycle

Cell Mitochondria: The and Oxidative Phosphorylation

Dr. Fawwaz Al-Joudi PHM142 Oct 22 What are Mitochondria?

• These are cytoplasmic organelles that carry the task of oxidation of the final products of using the oxygen breathed in. What do Mitochondria do?

Mitochondria perform the task of oxidation of the final products of metabolism using the oxygen breathed in. Two major metabolic tasks are carried out in mitochondria, namely the tricarboxylic acid cycle and oxidative phosphorylation General Structure of Mitochondria

• Two bilayer membranes: • Outer membrane – smooth, contains large pores (porin channels) • Inner membrane – impermeable (even to H+), folded into cristae to increase surface area. of the respiratory chain are embedded in the inner mitochondrial membrane • Matrix: this is the innermost space of the mitochondria which is enclosed by inner membrane. It contains mitochondrial DNA and ribosomes. It is the site of Kreb’s cycle and acetyl-CoA production. • Intermembrane space: this is the area between the two membranes where the oxidative phosphorylation apparatus is situated. Mitochondrial structure Origin of mitochondria: the endosymbiont hypothesis • Evidence suggests that mitochondria have evolved from which were phagocytosed by an anaerobic, nucleus-containing eukaryotic cells. • These “bacterial ancestors” that live in eukaryotic cells are called “endosymbionts”.

6 Mitochondria: the power houses

• Mitochondria are organelles that act like a cellular digestive system that takes in nutrients, breaks them down, and creates energy for the cell. • The process of creating cell energy is known as and most of the chemical reactions involved in cellular respiration take place in the mitochondria. • The mitochondria are very small organelles yet they are shaped perfectly to maximize their hard work The citric acid cycle (CAC): An OVERVIEW

• The Citric Acid Cycle (CAC: also called the Kreb’s cycle or the ticarboxylic acid cycle) plays several roles in metabolism since it is the final pathway where the oxidative metabolism of carbohydrates, amino acids, and fatty acids converge, their carbon skeletons being converted to CO2. • This oxidation provides energy for the production of the majority of ATP in most , including humans. • The CAC occurs totally in the mitochondria and is, therefore, in close proximity to the reactions of electron transport, which oxidize the reduced coenzymes produced by the cycle. The CAC: an overview • The citric acid cycle is a sequence of reactions in mitochondria that oxidizes the acetal moiety of acetyl-CoA to CO2 and reduces coenzymes that are re- oxidized through the linked to the formation of AT P. • The CAC is the final common pathway for the oxidation of carbohydrate, lipid, and protein because glucose, fatty acids, and most amino acids are metabolised to acetyl-CoA or intermediates of this cycle. • It also has a central role in gluconeogenesis, lipogenesis and conversion of amino acids. • This cycle is not be viewed as a closed circle, but instead as a traffic circle with compounds entering and leaving as required, making it a central pathway of energy metabolism. The CAC is a central part in metabolism The major metabolic pathways The intracellular location and overview of the major metabolic pathways in a parenchymal cell. (Harper’s Illustrated Biochemistry, 30th Ed) The pyruvate produced in a eukaryotic cell

• In eukaryotic cells, the pyruvate molecules produced at the end of , are transported into mitochondria, which are sites of the citric acid cycle (mitochondrial matrix) and cellular respiration (inner membrane). • If oxygen is available, aerobic respiration will go forward. The TCA in brief

The cycle starts was reaction between the acetyl moiety of acetyl-CoA and the four carbon dicarboxylic acid, oxaloacetate, forming a 6- carbon tricarboxylic acid, citrate. In the subsequent reactions, two molecules of CO2 are released and oxaloacetate is regenerated. Also, two NADH molecules and one FADH2 molecule are liberated: these will then be utilized by the ETC in the oxidation process and ATP generation. TCA: the cycle

• The citric acid cycle is a closed loop in the sense that the last part of the pathway regenerates the compound used in the first step. • The eight steps of the cycle are a series of chemical reactions that produces: - two carbon dioxide molecules, - one ATP molecule (or an equivalent), and + + - reduced forms (NADH and FADH2) of NAD and FAD , important coenzymes in the cell. Pyruvate in mitochondria

• Pyruvate is transported into the mitochondria using a specific pyruvate transporter that helps pyruvate cross the inner mitochondrial membrane. • Once in the matrix, pyruvate is converted to acetyl CoA by the enzymes Complex. • Deficiency of this complex is a major cause of congenital lactic acidosis which is an X-linked trait. Reactions of the CAC: starting from Acetyl CoA and ending up with oxaloacetate • A. Synthesis of citrate, a six-carbon molecule, from the condensation of acetyl CoA and oxaloacetate is catalyzed by . • B. lsomerization of citrate to isocitrate by , an Fe-S protein. One NADH+ molecule is formed.

• N.b. The poison fluoracetate is found in some plants, and their consumption can be fatal to grazing animals. Some fluorinated compounds used as anti cancer agents and industrial chemicals (such as pesticides), are metabolised to fluoroacetate. Fluoro acetyl-CoA condenses with oxaloacetate to form fluorocitrate which inhibits aconitase stopping the cycle and causing citrate to accumulate. The Citric Acid Cycle (Harper’s Illustrated Biochemistry, 30th Ed) Followed by:

• C. Oxidation and decarboxylation of isocitrate by to give α-ketoglutarate. • D. Oxidative decarboxylation of α-ketoglutarate, catalyzed by the α- ketoglutarate dehydrogenase complex to give succinyl CoA. Mg+ or Mn+ ions are required for this reaction. • The reaction releases the second CO2 and produces the second NADH of the cycle. • This step is arsenite-sensitive. • By all means, hyperammonaemia inhibits the α-ketoglutarate dehydrogenase complex. Then

• E. Conversion of succinyl CoA to succinate by Succinate thiokinase which cleaves the high-energy thioester bond of succinyl CoA. • This reaction is coupled to phosphorylation of guanosine diphosphate (GDP) to guanosine triphosphate (GTP). GTP and ATP are energetically interconvertible by the nucleoside diphosphate kinase reaction: GTP + ADP GDP + ATP The Citric Acid Cycle (Harper’s Illustrated Biochemistry, 30th Ed) The last steps

• F. Oxidation of succinate to fumarate by , producing the reduced coenzyme FADH2. • G. Hydration of fumarate to malate by . • H. Oxidation of malate to oxaloacetate by . essential for the CAC

• Four of the B vitamins are essential in the citric acid cycle: • 1- Riboflavin in the form of flavin adenine dinucleotide (FAD),a of succinate dehydrogenase . • 2- Niacin, in the form of nicotine amide adenine dinucleotide (NAD+), the electron acceptors for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase. • 3- Thiamin ( B1), as thiamine diphosphate, the coenzyme for decarboxylation in the α-ketoglutarate dehydrogenase reaction . • 4- Pantothenic acid , as part of , the cofactor esterified to active carboxylic acid residues: acetyl-CoA and succinyl-CoA. REGULATION OF THE TCA CYCLE

• The TCA cycle is controlled by the regulation of several activities. • The most important of these regulated enzymes are those that catalyze reactions are isocitrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase complex. • Reducing equivalents needed for oxidative phosphorylation are generated by the pyruvate dehydrogenase complex and the TCA cycle, and both processes are upregulated in response to a rise in A DP. Inhibitors and activators of the CAC (Lippincott’s Illustrated Reviews: Biochemistry, 4th Ed.) The Respiratory Chain and Oxidative Phosphorylation Mitochondria. (a). A living fibroblast with a phase contrast microscope where mitochondria are seen as elongated dark bodies (b). TEM of a thin section through a revealing the internal structure of the organelle. (c) localization of mitochondria in the midpiece surrounding the proximal portion of the flagellum of a bat sperm. (Karp’s Cell and Molecular Biology, 9th Ed.) Structure of the Mitochondria (Harper’s Illustrated Biochemistry, 30th Ed) The mitochondrial matrix is enclosed by a double membrane. Inner membrane has cristae that vastly increase the surface area inside the organelle. The outer membrane is permeable to most metabolites and the inner membrane is selectively permeable. The phospholipid cardiolipin is concentrated in the inner membrane together with the enzymes of the respiratory chain and ATP synthase. The electron transport chain (ETC):

• It is the last component of aerobic respiration and • In ETC, energy is produced by transferring electrons down a chain of molecules along with various enzymes located on the inner membrane. • It is the only part of metabolism that uses atmospheric oxygen which enters through the respiratory system. • The mitochondria are the only places in the cell where oxygen can be combined with the food molecules (products) to provide the cell with usable energy molecules (ATP). Mitochondrial energy production-Oxidative Phosphorylation

• Oxidative phosphorylation involves oxidation of the high-energy electron carriers NADH and FADH2 (electron transport chain) coupled to the phosphorylation of ADP into AT P. • This process takes place at the inner mitochondrial membrane Correa, Jakob, Takala, 2015

29 Electron Transport

• Electron transport is a series of chemical reactions that resembles a human transport chain or a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced. The respiratory chain oxidizes reducing equivalents and acts as a • Most of the energy liberated during the oxidation of carbohydrates fatty acid and amino acids is made available within mitochondria as reducing equivalents (mainly NADH and FADH2). • The respiratory chain collects and transports reducing equivalents directing them to their final reaction with oxygen to form water, and oxidative phosphorylation is the process by which the liberated free energy is trapped as high energy phosphate. The Proton Gradient

• Protons will be generated creating a concentration difference i.e., the gradient is “potential energy” that is stored, hence, the high concentration of protons in the intermembrane space wants to flow downstream into the matrix and does so through specialized channels and then create the energy molecule (ATP). The protein complexes

• There are four complexes composed of proteins, labeled I through IV, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain.

• The electron transport chain is present in multiple copies in the inner mitochondrial membrane of and in the plasma membrane of prokaryotes. The electron transport chain and ATP

The TCA cycle produces NADH and FADH2, which enter the ETC at complex I and complex II, respectively. Both complexes pass the resulting electrons (e) to (Q) and the electrons continue until ultimately reacting with oxygen 02 to create water. Protons H+ are pumped at complexes I, III, and IV, creating the proton gradient. Protons flow back through ATP synthase to create ATP. (Mitochondria and the Future of Medicine, Lee Know, 2018) Complex I Also known as NADH dehydrogenase, Complex I is a large molecule made of 46 protein subunits. Complex I accepts electrons from NADH, passes them through iron sulfur (Fe-S) clusters to coenzyme Q10 (Q). This results in four protons (H+) being pumped from the matrix into the intermembrane space creating a proton gradient. This is the second most prevalent site in the ETC where electrons can fall out and react with oxygen to form superoxide free radicals. (Mitochondria and the Future of Medicine, Lee Know, 2018) Complex II This unique complex, also known as succinate dehydrogenase, consists of only four protein subunits. It is directly involved with TCA cycle where FADH2 produced. Electrons from FADH2 are passed through iron sulfur (Fe-S) clusters to coenzyme Q10 (Q). This is the only complex that doesn't pump electrons. (Mitochondria and the Future of Medicine, Lee Know, 2018) Complex III: also called cytochrome bc1 complex • This is a dimer made up of two identical complexes. Each part of the dimer comprises of 11 protein subunits giving a total of 22 proteins . • This is where the occurs, which is a multi-step process whereby (reduced Co Q10) is converted to ubiquinone (oxidized Co Q10). • In the process, four protons are pumped to contribute to the proton gradient. • This is the second most prevalent site in the ETC where electrons can fall out and react with oxygen to form superoxide free radicals. Complex III This complex accepts electrons from the reduced form of coenzyme Q10 (QH2) in a multistep process called the Q cycle. Electrons make their way to (Cyt C), and four protons (H+) are translocated to the intermembrane space. (Mitochondria and the Future of Medicine, Lee Know, 2018) Complex IV

This is also called , is made up of 13 protein subunits. Complex IV accepts electrons from cytochrome C (Cyt C), pumps four protons into the intermembrane space, and passes the electrons to the terminal/ ultimate receiver, oxygen (O2), to produce two molecules of water. (Mitochondria and the Future of Medicine, Lee Know, 2018) ATP Synthase: Complex V

• ATP synthase is an important enzyme, as it is the final step in the long chain of events that culminate in the synthesis of ATP. • This enzyme is what connects the proton gradient to phosphorylation: the process created by the ETC, and made possible by the presence of oxygen. • This process of adding a phosphate to adenosine diphosphate (ADP) which creates (ATP) all in all, is known as oxidative phosphorylation. ATP synthase: Complex V

• This large enzyme is the smallest known machine. • This Rotary machine constructed from many tiny moving protein parts has two main components, the drive shaft that is inserted straight through the membrane from one side to the other and a very large rotating head that is attached to the drive shaft . • The high concentration of protons on the outside of the membrane wants to flow downstream and does so by passing through the drive shaft to rotate the head . • In humans, a full rotation of the head of ATP synthase requires 10 protons and releases three molecules of ATP. 3 dimensional structure of the bacterial ATP synthase determined by cryo- electron microscopy

(Karp/s Cell and Molecular Biology, 9th Ed.) Video https://www.bing.com/videos/search?q=atp+synthas e+video&docid=608000505647203026&mid=C5EA 80C4CEDF1A76F7F5C5EA80C4CEDF1A76F7F5 &view=detail&FORM=VIRE Structure of ATP synthase

The enzyme complex consists of an Fo sub complex which is a disk made up of “C” protein subunits. Attached is a gamma subunit in the form of a bent axle. The gamma subunit fits inside the F1 sub complex of 3 alfa and 3 beta subunits which are fixed to the membrane and do not rotate. (Harper’s Illustrated Biochemistry, 30th Ed) Mechanism of ATP production by ATP synthase

Protons passing through the disk of C units cause both the gamma and the C subunit to rotate. ADP and Pi are taken up sequentially by the beta subunits to form ATP which is expelled as the rotating gamma subunit squeezes each beta subunit int turn and changes its confirmation. Thus three ATP molecules are generated per revolution. Not all the subunits that have been identified are shown like the axle also contains an epsilon sub unit. The Analogy to a Hydro-electric power Dam

• This is very much like The pumping of water (protons) into a reservoir (intermembrane space) contained by a dam (inner membrane). • As water flows out through a channel in the dam it is used to drive turbines to create hydroelectric electricity. A hydroelectric dam

The water (protons) filling the reservoir (intermediate space) that is contained by a dam (inner membrane) pressure build up and is used to drive turbines (ATP synthase) to generate the electricity (ATP). (Mitochondria and the Future of Medicine, Lee Know, 2018) Requirement for oxygen

• This requirement for oxygen makes the electron transport process the respiratory chain, which accounts for the greatest portion of the body’s use of oxygen. • When oxygen is absent in the mitochondrion, the electrons could not be removed from the system, and the entire electron transport chain would back up and stop. • The mitochondria would then be unable to generate new ATP, and the cell would ultimately die from lack of energy. • This is the reason living creatures must breathe to draw in new oxygen. Photosynthesis in plants and the bacterial system • A similar event takes place in plants where the sun's energy is used to pump protons across the membrane in chloroplasts which represent the mitochondria in plants. • However, in contrast to the human system and the mammalian system, the electrons in plants pass down the ETC to a terminal electron acceptor that can be one of many different molecules, not just oxygen. • • Bacteria, being the ancestors to mitochondria, also function in a similar way- by generating a proton gradient across their cell membranes. The Universal Concept

• In every case , the energy extracted from the ETC is used to move protons across a membrane. • This concept is so universal, it seems that pumping protons across a membrane is a central signature of life on earth. The ATP Yield • The number of ATP molecules generated from the catabolism of glucose varies. • For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. • Another source of variance stems from the shuttle of electrons across the mitochondrial membrane. • Fewer ATP molecules are generated when FAD+ acts as a carrier. Distribution of energy produced

• Some molecules that would otherwise be used to harvest energy in glycolysis or the citric acid cycle may be removed to form nucleic acids, amino acids, , or other compounds. • Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose. How many mitochondria are there in a cell?

• The number depends on what the cell needs to do: they are found in large numbers in and skeletal muscle. They are also found in large numbers in most active organs such as the pancreas with its biosynthetic ability for insulin, the liver where detoxification takes place, and in the brain where tremendous amounts of energy are required by the nerve cells. • The egg cell and oocyte has some 100,000 mitochondria in contrast to sperms which usually have fewer than 100 . • Red blood cells and skin cells have very few as well. • Up to 10% of the weight of the is mitochondria and in numbers there are about 10,000,000 billion mitochondria per human body. Uncoupling of Mitochondria

• Uncoupling is a loss of output efficiency for any given input to a process. • Uncoupling of mitochondria is the loss of ATP productivity from the input of protons into the inter-membrane space. • This can be protein driven as part of a natural feedback loop for the regulation of energy and temperature, and • It can be chemical/ toxin driven Mitochondrial Toxicity

Ways by which chemicals can affect mitochondrial bioenergetics: 1. By interfering with electrochemical proton gradient generation by using inhibitors of ETC components. 2. They cause uncoupling of ETC with ATP synthase by causing dissipation of the proton gradient. 3. By inhibiting ATP synthase

54 Slide adapted from Latif 2018 Uncouplers • Decreased ATP/Increased ADP + enhanced oxygen consumption/NADH oxidation  energy from ETC’s proton gradient dissipates  released as heat  fever • Uncoupling can occur naturally in some organisms to generate heat and maintain body temperature (occurring e.g. in brown adipose tissues in hibernating animals, newborns, and mammals adapted to the cold).

• Examples • 2,4 dinitrophenol • NSAIDs (salicylates) • antipsychotics/antidepressants, antitumor drugs, lipid lowering drugs.

55 Slide adapted from Latif 2018 ETC Inhibitors • They cause muscle weakness, fatigue, fever, hypotension, , , acidosis. • Complex I • Most vulnerable • >60 inhibitors known (e.g. pesticides and rodenticides; antipsychotics, antihistamines, antianginal drugs, etc.) • Specific examples: • (plant-derived pesticide) • Amytal (barbiturate) • Complex II • • Complex IV • Carbon Monoxide (CO), Cyanide (CN-), Azides (N3-)

56 Slide adapted from Latif 2018 Rotenone

Rotenone is a naturally occurring compound found in the roots of several plant species used as pesticide, insecticide, etc. It is a potent specific complex I inhibitor and is used as a molecular tool in mechanistic studies on complex I. Binding suggested to occur at Fe-S protein. It causes ROS production (superoxide was suggested as the primary product, with hydrogen peroxide as the secondary product) and subsequent apoptosis. Amytal (amobarbital): a barbiturate

• Amytal works in the brain by increasing the amounts of a neurotransmitter called GABA amytal (Gamma aminobutyric acid), which in turn calms the nerves, relaxes muscles, slows down the central nervous system, and induces sleep. • At the mitochondrial level, amytal has been localized on the ubiquinone side of the flavoprotein segment of NADH dehydrogenase (complex I)- between FMN flavoprotein and Fe-S proteins.

58 Antimycin A

• An antibiotic produced by Streptomyces species amytal QH2 that demonstrates antifungal, insecticidal,

nematocidal, and pesticidal properties • Antimycin A prevents the transfer of electrons Blocked by between the b-cytochromes and ubiquinone (co- antimycin enzyme Q) at the Q (inner) site of complex III. • Stimulate ROS superoxide and hydrogen peroxide formation, and decrease ATP production. Cytochrome c

59

• Cyanide is a familiar poison that kills by shutting down the ETC. • It inhibits, specifically, the activity of complex IV by binding to the iron (Fe) component, to stop the flow of electrons. • The newest approved antidote is hydroxocobalamin which is a form of vitamin B12. This reacts with the cyanide to form cyanocobalamin which can then be safely eliminated by the kidneys. Azides (sodium azide) • Sodium azide is best known as the chemical found in automobile airbags. An electrical charge triggered by automobile impact causes sodium azide to explode and convert to nitrogen gas inside the airbag. • Sodium azide is used in agriculture (farming) for pest control. • Sodium azide is also used in detonators and other explosives.

• They have an action on the respiratory chain very similar to cyanide: binding to Fe+++ and inhibiting the groups of cytochromes in Cytochrome Oxidase (Complex IV). • As a consequence, reactions in the respiratory chain will stop, energy will not be released, proton pumps will not function, so they will not be available for complex V (ATP synthase), and the production of ATP will cease.

61 Carbon monoxide poisoning

• Carbon monoxide is a toxin that displaces oxygen at the cytochrome oxidase C, the final destination of the ETC: this destination becomes no longer available, causing respiratory block, because the electrons no longer have their exit. • Unless the carbon monoxide is removed, the mitochondria will sto working causing the cells to die, and ultimately killing the exposed to carbon monoxide. • DNP (2,4-Dinitrophenol) has a 2,4-Dinitrophenol variety of industrial uses; it is used in photography, as a fertilizer, in the manufacturing of dyes and in explosives. • Also used for losing weight: causes weight loss by burning fat and carbohydrates and converting them into heat associated with increased metabolic rate, which can be fatal. • It is a lipid-soluble aromatic weak acid that acts as a protonophore, allowing protons to leak across the inner mitochondrial membrane and thus bypass ATP synthase.

chem.libretexts.org 63 ATP synthase inhibitors

• Oligomycin is an inhibitor of the Fo unit of the ATP synthase. The “o” of the ”Fo” stands for “oligomycin”, the toxin that works by binding to the Fo unit. • Oligomycin is an antibiotic derived from Streptomyces bacteria and it blocks proton translocation (by binding to F0 subunit). ATP synthase: a cause of some diseases and a treatment target in others • ATP synthase is associated directly or indirectly with various human diseases such as Alzheimer's disease or presenile dementia where a deficiency of ATP synthase has been observed in mitochondria. • However, ATP synthase been suggested as a good molecular target for drugs in the treatment of various diseases: • R207910/ Bedaquiline is a new drug developed for the treatment of tuberculosis, and was shown to be active against a number of drug- resistant strains of Mycobacterium tuberculosis: the drug can block the synthesis of ATP by targeting ATP synthase.

65 of Mitochondrial DNA (mtDNA)

• The human mitochondrial DNA contains 37 genes, of which 22 are transcribed into transfer RNAs and two into ribosomal RNAs . • The remaining 13 genes encode subunits of the enzymes of the respiratory chain. • Because most mtDNA encodes enzymes involved in oxidative phosphorylation, mutations affecting these genes exert their deleterious effects primarily on the organs most dependent on oxidative phosphorylation such as the central nervous system, skeletal muscle, cardiac muscle, liver and kidneys. Expression of mitochondrial genes

• Human mitochondrial DNA is made up of two strands one of them is the heavy H strand and the other is the light L strand . • The promoters for the H and the L transcription are situated just upstream of the tRNA . • The transcripts that initiate at these points are extended in opposite directions around the entire circumference of the DNA molecule. Map of human mtDNA showing the pattern of transcription

Genes on the inner circle are transcribed from the L strand of DNA , whereas genes on the outer circle are transcribed from the H strand of the DNA. Arrows show the direction of transcription. ND 1-6 are genes encoding subunits of the enzyme NADH reductase. The tRNA genes in the mtDNA are indicated by abbreviations of the amino acids. (Principles of Genetics, Snustard, et al., 1997) Transcripts of the H and L strands

• A transcript from the H strand encodes the two ribosomal RNAs, 14 tRNAs, and 12 polypeptides . • The transcript from the L strand encodes 8 tRNAs and one polypeptide. • Each transcript is cleaved to separate the tRNAs from the ribosomal and mRNAs , and the mRNA's are polyadenylated. • Each mRNA is then translated into polypeptides using the mitochondrial ribosomes and a combination of nuclear and ribosomal tRNAs. Inheritance of mtDNA

• The classical mendelian inheritance does not exist with mtDNA and their genes are inherited through the unique “maternal inheritance”. • This peculiarity exists because ova contain numerous mitochondria within their abundant cytoplasm, whereas spermatozoa contain few, if any. Hence the mitochondrial DNA compliment of the zygote is derived entirely from the ovum. • So mothers transmit mtDNA to all their offspring, males and females, and daughters in turn, but not sons, further transmit mtDNA to their progeny. Mitochondria. (a). A living fibroblast with a phase contrast microscope where mitochondria are seen as elongated dark bodies (b). TEM of a thin section through a mitochondrion revealing the internal structure of the organelle . (c)localization of mitochondria in the midpiece surrounding the proximal portion of the flagellum of a bat sperm. DNA in Mitochondria

• Each mitochondrion contains thousands of copies of mitochondrial DNA, and, typically, deleterious mutations of this DNA affect some but not all of these copies. • Thus, tissues and, indeed, whole individuals may harbor both wild type and mutant mtDNA , a situation called heteroplasmy. • It should be evident that a minimum number of mutant mtDNA must be present in a cell or tissue before oxidative dysfunction gives rise to disease a situation that is called the “threshold effect”. Mitochondrial mutations

• Mutations to mtDNA accumulate with the passage of time. • If the mutations affect any one of the many protein subunits in the mitochondrial ETC, the rate of free leakage increases and the situation can spiral out of control quite rapidly and the odds are in favor of free radicals damaging the genes of the ETC proteins, simply because Mitochondrial DNA is stored next to the cell’s primary site of free radical generation. The house of power or the house weakness

• Mitochondrial DNA also doesn't have the protective histone proteins that nuclear DNA does. • The repair mechanisms are also severely deficient and a mutation anywhere is likely to result in a negative effect. • It is known that the primary cause of mitochondrial damage is the free radicals generated by the mitochondria themselves. • There is evidence to suggest that the majority all free radicals are generated by complexes I and III. Thank you Next, we will discuss Mitochondrial Disorders