Cellular Respiration - Essay Example

Cellular respiration After three steps during respiration, a molecule of glucose and 6 molecules of oxygen are converted to 6 molecules of carbon dioxide, water and energy (Rich, PR, 2003) as shown below. C6H12O6 + 6O2 ----- 6CO2 + 6H2O + 36 ATP However, the process of generating energy during cellular respiration is not as simple as the equation looks. Cellular respiration can be defined as a process of oxidizing fuel molecules such as glucose to carbon dioxide and water. The energy released during this process is held in the ATP molecules for all the energy consuming activities of the cell. The fuel molecules used by the cells include glucose, amino acids and fatty acids and the common electron acceptor or the oxidizing agent is the oxygen molecule. Organisms or cells that use molecular oxygen as the final electron acceptor are termed as aerobic, and those that do not are called as anaerobic. ATP is called as a "Universal energy currency" since all the energy transactions of the cell use this molecule. In terms of efficiency, aerobic respiration is more efficient than the anaerobic respiration which yields only two molecules of ATP. Both these modes of respiration share the initial Glycolysis step, which occurs in cytoplasm. After, the glycolysis, other reactions of the aerobic respiration, Krebs (Citric acid cycle) and oxidative phosphorylation takes place in mitochondria. While the Krebs cycle reactions take place in the mitochondrial matrix, the oxidative phosphorylation is staged at the inner membrane of the mitochondria that fold to form cristae (Figure 1) . Table 1. All the reactions of Glycolysis. Step Reaction Enzyme 1. -D-Glucose + ATP Glucose-6-phosphate + ADP Hexokinase 2. Glucose-6-phosphate Fructose-6-phosphate Phosphoglucose isomerase 3. Fructose-6-phosphate + ATP Fructose-1,6-bisphosphate + ADP Phosphofructokinase 4. Fructose-1,6-bisphosphate dihydroxyacetone phosphate + Glyceraldehyde-3-phosphate Aldolase 5 Dihydroxyacetone phosphate Glyceraldehyde-3-phosphate Triosephosphate isomerase 6. Glyceraldehyde-3-phosphate + NAD+ + Pi 1,3-Bisphosphoglycerate + NADH + H+ Glycerladehyde-3-phosphate dehydrogenase 7. 1,3-Bisphosphoglycerate + ADP 3-Phosphoglycerate + ATP Phosphoglycerate kinase 8. 3-Phosphoglycerate (3PG) 2-Phosphoglycerate (2PG) Phosphoglycerate mutase 9. 2-Phosphoglycerate Phosphoenolpyruvate + H2O Enolase 10. Phosphoenolpyruvate + ADP Pyruvate + ATP Pyruvate kinase Of the four different isozymes, type IV enzyme is not amenable to feedback inhibitory mechanism and is specific to liver and pancreas. During elevated levels of blood glucose, excess amount of Glucose-6-phosphate is made for conversion to glycogen for future use in liver (Table 1). 2. The second reaction of this cascade is catalyzed by phosphoglucose isomerase leading to interconversion of glucose-6-phosphate to fructose-6-phosphate during glycolysis and gluconeogenesis (Table 1). 3. Third step in the glycolysis in another priming reaction leading to the addition of another phosphate group by an enzyme called phosphofructokinase 1. This is a unidirectional reaction committing the cells to glycolysis. A phosphatase called fructose bisphosphatase is required for the reverse reaction. A balance of enzymatic activities of phosphofructokinase and fructose bisphosphatase determines whether the reaction proceeds towards glycolysis or gluconeogenesis. This reaction is activated by AMP and also fructose 2,6-bisphosphate which is a product of the reaction catalyzed by phosphofructokinase 2. These enzymes are downregulated by glucagon at the transcriptional level. Fructose bisphosphatase is inhibited by fructose 2,6 bisphosphate (Table 1). 4. The fourth step in the process of glycolysis is the reversible cleavage of fructose 1,6 bisphosphate to form two different triose phosphates, both of which continue through glycolysis, by an enzyme termed aldolase. The different isozymes of aldolase have different catalytic functions: aldolases A and C are mainly involved in glycolysis, while aldolase B is involved in both glycolysis and gluconeogenesis (Table 1). 5. The next step during the process of glycolysis is the conversion of dihydroxyacetone phosphate to glyceraldehydes-3-phosphate, since only glyceraldehydes-3-phosphate can be used in the subsequent reactions of glycolysis. This enzymatic reaction is catalyzed by triosephosphate isomerase (Table 1). 6. This reversible reaction is a crucial step in regulating glycolysis or gluconeogenesis through the oxidation and phosphorylation of glyceraldehydes-3-phosphate to energy rich intermediate 1,3-bisphosphoglycerate. NAD+ is a cosubstrate for this reaction. This reaction is catalyzed by glyceraldehydes-3-phosphate dehydrogenase (Table 1). 7. Phosphoglycerate kinase catalyzes the next reversible reaction leading to the formation of ATP or ADP using one of the high energy phosphate group on 1,3-BPG. One molecule of ATP per molecule of 1,3-BPG or two ATP molecules per molecule of glucose is generated in this reaction (Table 1). 8. Transfer of phosphor group from C3 to C2 position is catalyzed by phosphoglycerate mutase. Human phosphoglycerate mutase uses 2,3-bisphophoglycerate as a cofactor to phosphorylate a serine residue to prime the reaction (Table 1). 9. The reversible dehydration of 2-phosphoglycerate to yield a high energy intermediate phosphoenolpyruvate is catalyzed by enolase. Fluoride inhibits the activity of enolase by forming a phosphofluoride complex with magnesium at the active site (Table 1). 10. The final energy yielding step in glycolysis is catalyzed by pyruvate kinase leading to the formation of pyruvate from phosphoenolpyruvate with a simultaneous transfer of phosphate group to ADP generating ATP (Table 1). Thus the net amount of ATP generated during the process of glycolysis is two. During aerobic conditions pyruvate generated in this reaction can be transported to mitochondria where it is utilized to generate more ATP through the process of oxidative phosphorylation. In an anaerobic scenario, pyruvate is reduced to lactate in cells lacking mitochondria or in hypoxic conditions such as that of muscle tissue or core of tumors. Oxidation and decarboxylation of pyruvate is catalyzed by an enzyme called pyruvate dehydrogenase complex, which is a complex of three enzymes that transform pyruvate into acetyl-CoA, which is then be used in the TCA cycle to carry out cellular respiration (Holtzer, 1959). Pyruvate + CoA + NAD+ Acetyl CoA + CO2 + NADH + H+ Krebs or Citric acid cycle. In the citric acid, or tricarboxylic acid (TCA) cycle, a two-carbon unit, the acetyl group of acetyl CoA is completely oxidized to CO2. All reactions of the citric acid cycle take place in the mitochondrial matrix (Figure 1). This cyclical nature of the reactions was first suggested by Hans Krebs, from biochemical studies of pigeon breast muscle, therefore these reactions are also known as Krebs cycle (reviewed in Holtzer, 1959). Step Reaction Enzyme Cofactors Type 1. Acetyl CoA + oxaloacetate + H2O Citrate + CoA + H+ Citrate synthase ondensation 2a. Citrate cis-aconitate + H2O Aconitase Fe-S dehydration 2b. Cis-aconitate + H2O isocitrate Aconitase Fe-S hydration 3. Isocitrate + NAD+ -ketoglutarate + CO2 +NADH Isocitrate dehydrogenase oxidation + decarboxylation 4. -ketoglutarate + NAD+ + CoA succinyl CoA + CO2 +NADH -ketoglutarate dehydrogenase complex Lipoic acid, FAD, TPP oxidattion + decarboxylation 5. Succinyl CoA + Pi +GDP Succinate + GTP + CoA Succinyl CoA synthetase substrate level phosphorylation 6. Succinate + FAD (enzyme bound) fumarate + FADH2 (enzyme bound) Succinate dehydrogenase FAD, Fe-S oxidation 7. Fumarate + H2O L-malate Fumarase addition 8. L-malate + NAD+ oxaloacetate + NADH + H+ Malate dehydrogenase oxidation 1. Citrate synthase catalyzes the condensation reaction of the two-carbon acetate residue from acetyl coenzyme A and four-carbon oxaloacetate to form the six-carbon citrate. Oxaloacetate is regenerated after the completion of one round of the Krebs Cycle. Though this enzyme is localized in the mitochondrial matrix it is encoded by nuclear DNA. High ratio of ATP/ADP, acetyl CoA/CoA, NADH/NAD inhibit the activity of this enzyme. Citrate and succinyl CoA inhibit this enzyme is a feedback regulatory mechanism (Table 2). 2. Aconitase catalyzes the stereo specific reversible isomerization of citrate to isocitrate through cis-aconitate. An active form of aconitase contains a 4Fe-4S iron-sulfur cluster; three cysteine residues have been shown to be ligands of the 4Fe-4S cluster (Table 2). 3. The third step of the TCA cycle is catalyzed by isocitrate dehdrogenase producing -ketoglutarate and CO2 in an oxidative decarboxylation reaction, while converting NAD+ to NADH. It is composed to three subunits and is allosterically regulated and requires Mn2+ or Mg2+ ions (Table 2). 4. The next step of the reaction proceeds in three steps including decarboxylation of -ketoglutarate, oxidation and transfer to CoA to yield succinyl CoA. The enzyme catalyzing this reaction is oxoglutarate dehydrogenase or -ketoglutarate dehydrogenase, is inhibited by its products succunyl CoA and NADH (Table 2). 5. Succinyl synthetase (succinate thiokinase) catalyzes the next reversible reaction involving substrates level phosphorylation of GDP to yield succinate and GTP (Table 2). 6. Succinate dehydrogenase is the only enzyme complex bound to the inner mitochondrial membrane and is the only enzyme to participate in both the TCA cycle and mitochondrial electron transport chain. Succinate is oxidized to fumarate by succinate dehdrogenase subunit A using FAD cofactor which is reduced to FADH2. The electron transfer subunit B has several Fe-S clusters used in relaying the electrons from subunit A. The electrons are passed to the ubiquinone molecule bound to subunit C and D dimmer, reducing it to ubiquinol. This molecule freely diffuses through the inner mitochondrial membrane to interact with subsequent enzymes of electron transport chain (Table 2). 7. Fumarase (or fumarate hydratase) is the next enzyme involved in the Krebs Cycle that catalyzes the hydration (addition of H2O across a double bond) to convert Fumarate to L-malate (Table 2). 8. Malate dehydrogenase catalyzes the reversible conversion of malate to oxaloacetate using NAD+. This is a crucial enzyme in the process of gluconeogenesis. The sum of all the reactions in TCA is presented below Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O CoA-SH + 3 NADH + 3 H+ + FADH2 + GTP + 2 CO2 Thus, TCA cycle reactions yields one high-energy phosphate bond (as GTP) and four reducing equivalents (three NADH + H+, and one FADH2). Oxidative phosphorylation: Oxidative phosphorylation is the process of generating energy in the form of ATP through the oxidation of nutrients. This process is a highly efficient way of generating and storing energy compared to glycolysis or fermentation. During this process electron acceptor (such as oxygen) receives the electrons from an electron donor in a redox reaction occurring within mitochondria by a series of five main protein complexes. The redox reactions release energy, which is used to transport protons across the inner mitochondrial membrane in a process, called as chemiosmosis, leading to the generation of potential and electrical energy. This energy is captured when the protons are allowed to get back in the matrix by an enzyme called ATP synthase culminating in the generation of ATP (reviewed in Holtzer, 1959; Chance, 1977). 1.NADH-coenzyme Q oxidoreductase (Complex I) is the first protein complex in the electron transport system. It causes reduction of coenzyme Q by NADH as depicted below NADH + Q + 5H+ (Matrix) -- NAD + QH2 + 4H+ (intermembrane space) Electrons enter the complex 1 through the prsothetic group FMN which is reduced to FMNH2. The electrons are then transferred through a series of Fe-S clusters. During this process 4 protons are pumped into the intermembrane space. 2. Succinate Q oxidoreductase (Complex II) is the second entry to the electron transport system. As discussed earlier this 4 subunit enzyme is unique in that it participates in both the TCA cycle and electron transport. FAD serves as cofactor with Fe-S clusters and heme group bound to the complex. This enzyme catalyzes the conversion of succinate to fumarate. Since not much energy is released during this process no protons are transported across the membrane. Succinate + Q --- Fumarate + QH2 3. Electron transfer flavoprotein Q reductase is the third point of entry to the electron transport system. This enzyme accepts electrons from the electron transferring flavoprotein and reduces the ubiquinone in the matrix. ETFreduced + Q -- ETFoxidized + QH2 4. Q-cytochrome c oxidoreductase (complex III) is also known as cytochrome c reductase or complex III. The iron in the one heme group that is present fluctuates between a reduced and oxidized state as electrons are transferred through the protein. Four protons are trasnported to the intermembrane space QH2 + 2 Cyt Cox + 2H+(matrix) -- Q + 2Cyt Cred + 4H+ (intermembrane space) 5. Cytochrome c oxidase (complex IV) is the final protein complex of the electron transport system. This complex has two heme groups with multiple metal ion cofactors. This complex mediates the final reaction in the electron transport system and delivers electrons to the oxygen, while pumping four protons across the membrane. 4 Cyt cred + O2 + 8H+ (Matrix) - 4 Cyt coxid +2H2O + 4H+ (intermembrane space) All these complexes are arranged in a higher ordered structures called supercomplexes or respirosomes that permits chanelling of substrates to increase the effeciency and rate of electron transfer. 6. ATP synthase, also called as complex V is the final enzyme of the oxidative phosphorylation mechanism. In the absence of proton motive force the reaction will proceed from right to left. ADP + Pi + 4H+ (Intermembrane space) Read More