1 BCHEM 254: METABOLISM IN HEALTH AND DISEASES II Lecture 1: The Energetics of the Electron Transport Chain Lecturer: Dr. Christopher Larbie
2 Introduction The citric acid cycle oxidizes acetate into two molecules of CO 2 while capturing the electrons in the form of 3 NADH molecules and one molecule of FADH 2. These reduced molecules contain a pair of electrons with a high transfer potential. These electrons are ultimately going to be transferred by a system of electron carriers to O 2 to form H 2 O. This process occurs in the mitochondria and is the major energy source used to produce ATP by oxidative phosphorylation.
3 Let s look at the standard free energy change for the overall reaction shown below: NADH + H + + ½O 2 NAD + + H 2 O NAD + + 2e - + H + NADH E o = V ½O 2 + 2e - + 2H + H 2 O E o = V ΔE o = V ( V) = V ΔG o = nfδe o = 219 kj/mol ADP + Pi ATP ΔG o = kj/mol If we could convert all of this free energy generated by reducing oxygen with NADH to synthesize ATP from ADP and Pi: #ATP = 219/30.5 = 7.2 ATP
5 Mitochondria The processes involved in electron transport and oxidative phosphorylation occur within membranes. In prokaryote systems, electron transport and oxidative phosphorylation are carried out across the plasma membrane. Mitochondria are remarkably mobile organelles. In some cells they are anchored by attachment to the cell s cytoskeleton so that they remain fixed at one cellular location to target a site of high ATP utilization. In heart muscle for example the mitochondria are anchored close to the contractile muscle, In sperm they are wrapped tightly around the motile flagellum.
6 The outer membrane contains porins which are transmembrane proteins which allow molecules of low molecular weight (<10,000) freely diffuse in and out. The inner membrane is impermeable to molecules and ions. The inner membrane is folded into numerous cristae, which greatly increase the surface area of the inner membrane. Within this membrane are the proteins involved in the electron transport chain, ATP synthase and transport proteins.
7 The large internal space enclosed by inner mitochondrial membrane is called the matrix. It is densely packed with hundreds of enzymes, including pyruvate dehydrogenase, pyruvate carboxylase, and the soluble enzymes of the citric acid cycle. In addition there are the enzymes involved in the oxidation of fatty and amino acids. The matrix also contains the mitochondrial genome, the mitochondrial ribosomes, trna s and the enzymes required for the expression of mitochondrial genes.
8 The Components of the Electron Transport Chain 1. NADH NADH is generated in the matrix by the reactions of pyruvate dehydrogenase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase and malate dehyrogenase. The electron transport chain begins with reoxidizing NADH to form NAD + and channelling the electrons into the formation of reduced coenzymes. Important to note that NADH transfers 2 electrons at a time in the form of a hydride. NAD + + 2e- + H + NADH E o = V
9 Flavoproteins Flavoproteins have either a or a FMN prosthetic group. Flavoproteins can accept or donate electrons one at time or two at a time. Thus they are often intermediaries between two electron acceptors/donors and one electron acceptors/donors. For flavoproteins the typical standard reduction potentials are around 0 V. FAD + 2e - + 2H + FADH 2 ; FMN + 2e - + 2H + FMNH 2 E o 0 V
10 Coenzyme Q (CoQ) (or ubiquinone (UQ)) Coenzyme Q is a versatile cofactor because it is a soluble electron carrier in the hydrophobic bilipid layer of the inner mitochondrial membrane Q + 2e - + 2H + QH 2 E o =0.060 V Q + e - + H +.QH E o =0.030 V.QH + e - + H + QH 2 E o =0.190 V
11 Cytochromes Cytochromes are proteins that contain haem prosthetic groups which function as one electron carriers. The haem iron is involved in one electron transfers involving the Fe 2+ and Fe 3+ oxidation states. Cytochrome b contains the same iron porphyrin found in haemoglobin and myoglobin. Other cytochromes we will encounter in the electron transport complexes are cytochromes b, c, c1, a and a3.
12 Iron-Sulphur Proteins Iron-sulphur proteins participate in one electron transfers involving the Fe 2+ and Fe 3+ oxidation states. These are non-haem iron-sulphur proteins. 1. The simplest iron-sulphur protein is Fe-S in which iron is tetrahedrally coordinated by four cysteine residues. 2. The second form is Fe 2 S 2 which contains two irons complexed to 2 cysteine residues and two inorganic sulphides. 3. The third form is Fe 3 S 4 which contains 3 iron atoms coordinated to three cysteine residues and 4 inorganic sulphides. 4. The last form is the most complicated Fe 4 S 4 which contains 4 iron atoms coordinated to 4 cysteine residues and 4 inorganic sulphides.
13 Iron-Sulphur Protein structures Copper Proteins Copper bound proteins participate in one electron transfers involving the Cu + and Cu 2+ oxidation states.
14 Overview of the Electron Transport Chain The components of the electron transport chain are organized into 4 complexes. Each complex contains several different electron carriers. 1. Complex I also known as the NADH-coenzyme Q reductase or NADH dehydrogenase. 2. Complex II also known as succinate-coenzyme Q reductase or succinate dehydrogenase. 3. Complex III also known as coenzyme Q-cytochrome c reductase or coenzyme Q reductase. 4. Complex IV also known as cytochrome c reductase.
18 Complex I Complex I is also called NADH-Coenzyme Q reductase because this large protein complex transfers 2 electrons from NADH to coenzyme Q. Complex I was formerly known as NADH dehydrogenase. Complex I is huge, 850,000 kd and is composed of more than thirty subunits. It contains a FMN prosthetic group and seven or more Fe-S clusters. This complex has between iron atoms bound. The prosthetic group FMN is absolutely required for activity. Therefore this complex is a flavoprotein. This complex binds NADH, transfers two electrons in the form of a H - to FMN to produce NAD + and FMNH 2. The subsequent steps involve the transfer of electrons one at a time to a series of iron-sulphur complexes that includes both 2Fe-2S and 4Fe-4S clusters.
20 The final step of this complex is the transfer of 2 electrons one at a time to coenzyme Q. CoQ like FMN and FAD can function as a 2 electron donor/acceptor and as a 1 electron donor/acceptor. CoQ is a mobile electron carrier because its isoprenoid tail makes it highly hydrophobic and lipophillic. It diffuses freely in the bilipid layer of the inner mitochondrial membrane. The process of transferring electrons from NADH to CoQ by complex I results in the net transport of protons from the matrix side of the inner mitochondrial membrane to the inter membrane space where the H + ions accumulate generating a proton motive force. The intermembrane space side of the inner membrane is referred to as the P face (P standing for positive). The matrix side of the inner membrane is referred to as the N face. The transport of electrons from NADH to CoQ is coupled to the transport of protons across the membrane. This is an example of active transport. The stiochiometry is 4 H + transported per 2 electrons.
21 Complex II Complex II is succinate dehydrogenase. It is an integral membrane protein of the CAC. This complex is composed of four subunits; 2 of which are iron-sulphur proteins and the other two subunits together bind FAD through a covalent link to a histidine residue. These two subunits are called flavoprotein 2 or FP2. Complex II contains 3 Fe-S centres, 1 4Fe-4S cluster, 1 3Fe- 4S cluster and 1 2Fe-2S cluster. In the first step of this complex, succinate is bound and a hydride is transferred to FAD to generate FADH 2 and fumarate. FADH 2 then transfers its electrons one at a time to the Fe-S centres. The final step of this complex is the transfer of 2 electrons one at a time to coenzyme Q to produce CoQH 2.
23 The overall reaction for this complex is: Succinate Fumarate + 2H + + 2e - CoQ + 2 H + + 2e - CoQH 2 Net Succinate + CoQ Fumarate + CoQH 2 ΔE o = V (+0.031V) = V ΔG o = nfδe o = 5.6 kj/mol Compare this with complex I NADH + H + + CoQ NAD + + CoQH 2 ΔE o = V ( 0.315V) = V ΔG o = nfδe o = 72.4 kj/mol
24 Complex III This complex is also known as coenzyme Q-cytochrome c reductase because it passes the electrons from CoQH 2 to cyt c through a very unique electron transport pathway called the Q-cycle. Cytochrome b contains the same iron protoporphyrin as haemoglobin and myoglobin. The c cytochromes contain haem c through covalent attachment by cysteine residues. Cytochrome a is found in two forms in complex IV. In complex III we find two b-type cytochromes and one c-type cytochrome.
26 Q-Cycle The Q-cycle is initiated when CoQH 2 diffuses through the bilipid layer to the CoQH 2 binding site which is near the intermembrane face. This CoQH 2 binding site is called the QP site. The electron transfer occurs in two steps. First one electron from CoQH 2 is transferred to the Rieske protein (a Fe-S protein) which transfers the electron to cytochrome c1. This process releases 2 protons to the intermembrane space. Coenzyme Q is now in a semiquinone anionic state, CoQH. - still bound to the Q P site. The second electron is transferred to the cyt b L haem which converts CoQH. - to CoQ. This reoxidized CoQ can now diffuse away from the QP binding site. The b L haem is near the P-face. The b L haem transfers its electron to the b H haem which is near the N-face. This electron is then transferred to second molecule of CoQ bound at a second CoQ binding site which is near the N-face and is called the Q n binding site. This electron transfer generates a CoQ. - radical which remains firmly bound to the Q n binding site. This completes the first half of the Q cycle.
28 The second half of the Q-cycle is similar to the first half. A second molecule of CoQH 2 binds to the Q P site. In the next step, one electron from CoQH 2 (bound at Q P ) is transferred to the Rieske protein which transfers it to cytochrome c 1. This process releases another 2 protons to the intermembrane space. The second electron is transferred to the b L haem to generate a second molecule of reoxidized CoQ. The b L haem transfers its electron to the b H haem. This electron is then transferred to the CoQ. - radical still firmly bound to the Q n binding site. The take up of two protons from the N-face produces CoQH 2 which diffuses from the Q n binding site. This completes Q cycle.
29 The net result of the Q-cycle is 2e - transported to cytochrome c 1. Two protons were picked up from the N-face in the second half of the Q-cycle and 4 protons total were released into the intermembrane space.
30 Cytochrome c The electrons that end up on cytochrome c 1 are transferred to cytochrome c. Cytochrome c is the only water soluble cytochrome. Cytochrome c is coordinated to ligands, histidine nitrogen and to the sulphur of methionine residues that protect the iron contained in the haem from oxygen and other oxidizing agents. Cytochrome c is a mobile electron carrier that diffuses through the intermembrane space shuttling electrons from the c 1 haem of Complex III to CuA site of complex IV.
31 Complex IV Cytochrome c oxidase Cytochrome c oxidase accepts the electrons from cytochrome c and directs them towards the fourelectron reduction of O 2 to form 2 molecules of H 2 O. 4 cyt c (Fe 2+ ) + 4 H + + O 2 4 cyt c (Fe 3+ ) + 2H 2 O
32 Cytochrome c oxidase contains 2 haem centres, cytochrome a and cytochrome a 3 and two copper proteins. Each of the protein bound coppers are associated with one of the cytochromes. The copper sites are called Cu A and Cu B. Cu A is associated with cytochrome a; Cu B is associated with cytochrome a3. The copper sites function as 1 electron carriers cycling between the cuprous state Cu + and the cupric state Cu 2+ one at a time.
34 The reduction of oxygen by complex IV involves the transfer of four electrons. Four protons are abstracted from the matrix and two protons are released into the intermembrane space.
35 Chemiosmotic Coupling There is no experimental evidence that the four complexes associate with one another in the membrane. However, evidence supports the existence of multimeric supercomplexes or respirasomes. Mutation studies suggest that Complex III is required to maintain Complex I in the mitochondria of human and mouse cells, and Complex IV is necessary for the proper assembly of Complex I in mouse fibroblasts. Further evidence suggests the requirement of cardiolipin in the assembly and stability of supercomplexes. The process of electron transfer is coupled to transporting protons from the matrix to the intermembrane space of the mitochondria. This generates a chemical potential and an electrostatic potential. This potential energy is used to drive the synthesis of ATP. This is known as the Mitchell s Chemiosmotic hypothesis.
37 Complex I: NADH + 5H + N + Q NAD + + QH 2 + 4H + P Complex II: FADH 2 + Q FAD + QH 2 Complex III: QH 2 + 2H + N + 2 Cyt c (Fe 3+ ) 2 Cyt c (Fe 2+ ) + Q + 4H + P Complex IV: 4Cyt c (Fe 2+ ) + 8 H + N + O 2 4 Cyt c (Fe 3+ ) + 4H + P + 2H2O Overall reaction beginning with NADH: NADH + 11H + N + ½O 2 NAD H + P + H 2 O (10H + P/2e - ) NAD + + 2e - + H + NADH E o = V ½O 2 + 2e - + 2H + H 2 O E o = V ΔE o = V ( V) = V ΔG o = nfδe o = 219 kj/mol FADH 2 + ½O 2 FAD + H 2 O FAD + 2e - + H + FADH 2 E o V ½O 2 + 2e - + 2H + H 2 O E o = V ΔE o = V (0 V) = V ΔG o = nfδe o = kj/mol Overall reaction beginning with FADH 2 : FADH 2 + 6H + N + ½O 2 FAD + 6H + P + H 2 O (6H + P/2e - ) NADH + H + + ½O 2 NAD + + H 2 O Most of the free energy has been used to pump electrons out of the matrix across the inner mitochondrial membrane into the intermembrane space.
38 ATP Synthase or F 1 F 0 -ATPase
39 The F 0 unit forms a transmembrane pore through which protons are channelled to drive ATP synthesis. The F 1 unit is composed of five polypeptide chains α,β,γ,δ, and ε which a stoichiometry of α3β3γδε. The α and β subunits are homologous to each other, each subunit contains an ATP binding site. The catalytic sites are located in the 3 β subunits The γ subunit forms a shaft. The single γ subunit associates primarily with one of the αβ pairs forcing each of the β subunits into a different conformation. One β subunit has an ADP bound, the next β-subunit contains ATP the next is empty. This difference in nucleotide binding among the three β subunits is critical to the mechanism of the complex.
40 Structure of F 0 The F 0 subunit is composed of three subunits denoted a, b and c in stoichiometry of ab 2 c The c subunits are the alpha helices that span the membrane with a small extending out into the matrix side of the membrane. The c subunits are arranged into two concentric circles with a 55Å diameter. The ring of c subunits forms a rotor that turns with respect to the a-subunit. The 2 b-subunits associate firmly with the α and β subunits of F 1 holding them fixed relative to the membrane. In the membrane embedded cylinder (subunit c) of F 0 is attached the shaft composed of the γ and ε subunits of F 1. As protons flow through the membrane from the P side to the N side through the F 0 channel, the c subunits turn which in turn turns the embedded shaft which rotates causing the β-subunits to change conformation as the γ subunit turns.
42 ATP Synthesis The F 1 complex of ATP Synthase has three interacting conformations and three conformationally distinct active sites. The Open conformation is inactive and has a low affinity for ligands. The L form has a loose affinity for ADP and Pi and also inactive. The Tight conformation is active and has a high affinity for ADP and Pi. The synthesis of ATP is initiated by the binding of ADP and Pi to an open L site. In the next step, a proton driven conformational change converts the L conformation into the T conformation and simultaneously converts the O form to the L form and the T form to the O form. In the third step ATP is synthesized at the T site and ATP is released from the O site. This cycle continues over and over.
44 Inhibitors and Uncouplers of Oxidative Phosphorylation The electron transport chain was determined by studying the effects of particular inhibitors.
46 Amytal is a barbiturate that inhibits the electron transport of complex I. Demerol is painkiller that also inhibits complex I. They block the oxidation of the Fe-S clusters of complex I. 2-Thenoyltrifluoroacetone and carboxin specifically block electron transport in Complex II. Antimycin A1 is an antibiotic that inhibits electron transfer in complex III by blocking the transfer of electrons between Cyt bh and coenzyme Q bound at the Q N site. Cyanide, azide and carbon monoxide all inhibit electron transport in Complex IV. They all inhibit electron transfer by binding tightly with the iron coordinated in Cyt a 3.
47 Azide and cyanide bind to the iron when the iron is in the ferric state. Carbon monoxide binds to the iron when it is in the ferrous state. Cyanide and azide are potent inhibitors at this site which accounts for the acute toxicity. Carbon monoxide is toxic due to its affinity for the haem iron of haemoglobin. Animals carry many molecules of haemoglobin, therefore it takes a large quantity of carbon monoxide to die from carbon monoxide poisoning. Animals have relatively few molecules of Cyt a3. Consequently an exposure to a small quantity of azide or cyanide can be lethal. The toxicity of cyanide is solely from its ability to arrest electron transport.
48 Inhibitors of ATP Synthase DCCD forms covalent bonds to a glutamate residue of the c subunit of F 0. When DCCD is covalently attached it blocks the proton channel, which causes the rotation and ATP synthesis to cease. Oligomycin binds directly to ATP synthase F0 subunit and blocks the flow of protons through the channel.
49 Endogenous Uncouplers Enable Organisms to Generate Heat The uncoupling of oxidative phosphorylation from electron transport generates heat. Hibernating animals and new-born animals (including human beings) contain brown adipose tissue. The adipose tissue is brown due to the high mitochondria content of the tissue. An endogenous protein called thermogenin uncouples ATP synthesis from electron transport by opening up a passive proton channel (UCP-1) through the inner mitochondrial membrane. The collapse of the ph gradient generates heat.
51 P/O Ratios The P/O ratio is the number of ATP molecules formed per 2 electrons transferred to oxygen. Beginning with NADH, two electrons are transferred via the electron transport chain to oxygen resulting in 10 protons being pumped across the mitochondrial inner membrane. 4 protons are channelled back the matrix per ATP synthesized. Beginning with FADH 2, 6 protons pumped per 2 electrons transferred. Still 4 protons/atp. P/O ratio = 6/4 = 1.5
52 Proton Motive Force Drives Transport The primary purpose of the proton gradient is to generate ATP by oxidative phosphorylation. The potential energy of the gradient can also be used for active transport. The inner mitochondrial membrane is impermeable to charged molecules. Examples The ATP-ADP Translocase Phosphate Carrier Protein Dicarboxylate Carrier Protein Tricarboxylate Carrier Protein Pyruvate Carrier Protein
53 ATP-ADP Translocase There are two specific systems to transport ADP and Pi into the mitochondrial matrix. A specific transport protein ATP-ADP translocase enables ATP and ADP to transverse the inner mitochondrial membrane. The transport of ADP in and ATP out are coupled. ADP only enters the matrix if ATP exits or vice versa.
54 The ATP-ADP translocase can be specifically inhibited by a low concentration of atractyloside which is a plant glycoside. Atractyloside binds to the translocase when its nucleotide binding site is facing the intermembrane space. This inhibitor completely blocks ATP-ADP translocation and stops oxidative phosphorylation.
55 Phosphate Carrier Protein The combined activity of ATP-ADP translocase and the phosphate carrier is the exchange of ADP + Pi for ATP at the cost of one proton transported from the P-phase to the N-phase.
56 Other Examples of Secondary Active Transport
57 Electron Shuttle Systems Most of the NADH used in electron transport is produced in the mitochondrial matrix. The binding site of Complex I for NADH is on the matrix side of the inner membrane. The inner mitochondrial membrane is impermeable to NAD + and NADH. NADH formed in the cytosol of cells needs to reoxidized into NAD + to keep glycolysis going. Under anaerobic conditions, NAD + is regenerated by lactate dehydrogenase. Under aerobic conditions, the electrons of NADH need to be shuttled into the matrix of the mitochondria. Hence the need for shuttles for transporting the reducing equivalence for the cytosol to the mitochondrial matrix.
58 The Glycerophosphate Shuttle This shuttle system uses two distinct glycerol 3-phosphate dehydrogenases. The first is found in the cytoplasm, the other is found on the intermembrane side of the inner mitochondrial membrane. In the first step, NADH produced in the cytosol transfers its electrons to dihydroxyacetone phosphate to form glycerol-3- phosphate. Glycerol-3-phosphate enters the inter-mitochondrial space through a porin. Glycerol-3-phosphate is then reoxidized into dihydroxyacetone phosphate by an FAD-dependent mitochondrial membrane glycerol 3- phosphate dehydrogenase. In this shuttle the electrons of NADH are transferred to FAD to form FADH 2. The two electrons bound by the FADH 2 are transferred directly to coenzyme Q forming QH 2. QH 2 carries the electrons to complex III. The result of this shuttle is 1.5 ATP/NADH. Note that this shuttle is essentially irreversible
60 The Malate-Aspartate Shuttle
61 In summary. Glycolysis: Glucose 2 pyruvate + 2ATP + 2NADH Back to the very beginning. Glucose + 6O 2 6CO 2 + 6H 2 O ΔG = 2937 kj/mol Under cellular conditions, ΔG for ATP hydrolysis 50 kj/mol 32 ATP /glucose = 1,600 kj/mol Efficiency 1600/2937 X 100 = 54% 3.5 billion years of evolution has resulted in 54 % efficiency.
62 OXYGEN METABOLISM AND TOXICITY
63 Chemiosomotic Theory Chemiosomotic Theory state that the free energy of electron transport is coupled to the pumping of protons from the matrix to the intermembrane space to create a ph gradient. This potential energy stored in this ph gradient is used to drive the synthesis of ATP. This process requires: An intact inner mitochondrial membrane that is impermeable to protons, The inner mitochondrial membrane must also be impermeable to ions such as OH -, K +, Na + and Cl - because free diffusion of these ions would discharge the trans-membrane electrical potential which is a key component of the proton motive force. Compounds that increase the permeability of the membrane to protons dissipate the electrochemical gradient and uncouple electron transport from oxidative phosphorylation.
64 Ca 2+ transport in the mitochondria There are two separate transport proteins for calcium ions. One integral membrane protein transport calcium ions into the matrix using the transmembrane electrical potential. The rate of Ca 2+ influx depends on the cytosol concentration of calcium because the Km for Ca 2+ for this transport protein is greater than physiological cytosolic concentrations of Ca 2+. A separate protein transports Calcium ions out of the matrix antiporting in a sodium ion. This transporter protein always operates at its maximal velocity.
65 Regulation of ATP Producing Pathways The electron transport chain functions near equilibrium from NADH to cytochrome c. There is only one irreversible step in electron transportoxidative phosphorylation; that is the reduction of oxygen by cyctochrome c oxidase, Complex IV. Cytochrome c oxidase is regulated primarily by the concentration of reduced cytochrome c. The concentration of reduced cytochrome c is in equilibrium with the rest of the electron transport pathway. Thus a high NADH/NAD + ratio and a low ATP/ADP ratio increase the concentration of reduced Cyt c. Increased concentration of reduced Cyt c results in an increase in the rate of oxygen reduction.
66 Oxygen Metabolism Oxidation by molecular oxygen can only occur by the transfer of single electrons. Organic molecules that serve as substrates for oxidation do not contain unpaired electrons. For O 2 to accept a pair of electrons from an organic substrate, one of the electrons of oxygen or one of the electrons from the donating substrate has to invert its spin. As a result two electron oxidations by molecular oxygen have to occur stepwise by two single electron transfers. The large barrier to spin inversions keeps us from spontaneously combusting in our oxygen atmosphere. The two unpaired electrons of molecular oxygen have a biradical nature which has a tendency to form highly reactive oxygen species (ROS) which initiate free radical reactions.
67 Free radicals react indiscriminately with any molecule they come in contact with. Free radical reactions are a chain of single electron transfer reactions which damage cellular components. Reactive Oxygen Species (ROS). OH Hydroxyl radical. O 2 - Superoxide H 2 O 2 Hydrogen peroxide
69 Chronic effects of ROS
70 Coenzyme Q generates superoxide One of the major sites of superoxide generation is the electron transport chain which leaks free radicals in the form of semiquinone radicals of coenzyme Q. The one electron form of CoQ occasionally leaks into the inner mitochondrial membrane. The nonspecific interaction of a CoQH. with molecular oxygen results in the formation of a superoxide radical which abstracts an electron from some other molecule and initiates a free radical chain reaction
71 Cellular Defences Against ROS
72 Vitamins E and C act as antioxidants Vitamin E (tocopherol) is the most abundant antioxidant in nature. It is a lipophilic free radical scavenger to protect lipids from peroxidation in the membranes. It sole biological purpose is to quench free radical reactions in membranes.
73 Vitamin C, ascorbic acid is a water soluble free radical scavenger. It accepts electrons from superoxide, hydrogen peroxide, hypochlorite, hydroxyl radicals and peroxyl radicals. It also quenches ozone and nitrous oxide.
74 Hypoxic Injury Hypoxia- deficiency of oxygen. Hypoxic injury-injury caused by a deficiency of oxygen. Acute hypoxic tissue injury has been extensively studied. The occlusion of a major coronary artery produces an array of biochemical and physiological complications. When a tissue is deprived of oxygen, the mitochondrial electron transport-oxidative phosphorylation is inhibited, resulting in a severe decline in cellular ATP levels. Anaerobic glycolysis is activated in an attempt to restore cellular ATP levels. Glycogen stores are rapidly depleted, and lactic acid concentrations increase, lowering the cellular ph (lactic acidosis). Hypoxic cells begin to swell as they can no longer maintain their normal electrolyte concentrations. The mitochondria begin to swell and accumulate calcium which precipitates as calcium phosphate. As the membranes swell they become permeable leading to the leakage of enzymes, coenzymes and other cellular constituents. As the ph continues to fall, the lysosomal membranes release hydrolytic proteases, lipases, glucosides and phosphatases that digest the cell. Amazingly cells that have been exposed to short periods of hypoxia can recover without irreversible damage upon reperfusion with oxygen containing medium.
75 Enzymes that use oxygen as a substrate Oxidases An oxidase is an enzyme that reduces molecular oxygen to water or hydrogen peroxide. These oxidases play specific roles in metabolism. Oxidases reduce oxygen to either water or hydrogen peroxide. Cytochrome c oxidase (Complex IV) is an example of an oxidase. This enzyme complex catalyses the four electron reduction of O 2 to 2H 2 O. The electrons are transferred to oxygen one at a time to overcome the spin restrictions. The mechanism of this enzyme allows the peroxy intermediates to formed in a controlled fashion without interaction between oxygen free radicals and other mitochondrial components. Other oxidases reduce oxygen to peroxide instead of water. These enzymes are compartmentalized into peroxisomes and lysosomes where catalase and glutathione peroxidase remove the hydrogen peroxides.
76 Oxygenases An oxygenase directly incorporates oxygen into the molecules being oxidized. Oxygenases incorporate oxygen into the substrate. Monooxygenases incorporate one of the atoms of oxygen into the substrate while reducing the other atom of oxygen into water. The common name for monooxygenases is hydroxylases (i.e. phenylalanine hydroxylase which adds a hydroxyl group to phenylalanine to form tyrosine.) Monooxygenases require an electron donor such as NADPH, a coenzyme capable of mediating between a two electron donor and a one electron acceptor (FAD) and a metal cofactor to form a stable reactive oxygen complex. Cytochrome P450 enzymes are a superfamily of structurally related monooxygenases which hydroxylate many compounds such as steroids, fatty acids, drugs, carcinogens, etc. There are over 100 isozymes of P450 enzymes in humans, each with a different but overlapping specificity. Dioxygenases incorporate both atoms of oxygen into the substrate.
77 Mitochondrial Myopathies Mitochondrial myopathies are mitochondrial diseases that affect the muscles. Clinically, patients with mitochondrial myopathies complain of weakness and severe cramping of the affected muscles. Infants have difficulty feeding and crawling. Continued severe fatigue results in minimal exertion and muscle wasting. Examining the mitochondria of these myopathies have discovered primary reasons for mitochondrial malfunction. Deficiencies in mitochondrial transport. Deficiencies in electron transport in either Complex I, Complex II, Complex III, Complex IV and ATP synthase have been described. Crystalline inclusions within the mitochondrial matrix. Under these conditions electron transport and oxidative phosphorylation are only loosely coupled. Example of paracrystalline inclusions in the mitochondria from muscles of ocular myopathic patients.
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