How cells Make ATP: Energy Releasing Pathways

Metabolism Metabolism has two complementary components: catabolism, which releases energy by splitting complex molecules into smaller components anabolism, the synthesis of complex molecules from simpler building blocks Most anabolic reactions are endergonic and require ATP or some other energy source to drive them

Cellular Respiration Every organism extracts energy from food molecules that it manufactures by photosynthesis or obtains from the environment Exergonic metabolic pathways (cellular respiration and fermentation) release free energy that is captured by the cell cellular respiration Catabolic processes that convert energy in the chemical bonds of nutrients to chemical energy stored in ATP May be either aerobic or anaerobic

Aerobic Respiration Cells use aerobic respiration to obtain energy from glucose C6H12O6 + 6 O2 + 6 H2O → 6 CO2 + 12 H2O + energy (chemical bonds of ATP) aerobic respiration Cellular respiration that requires molecular oxygen (O2) Nutrients are catabolized to carbon dioxide and water

Aerobic Respiration (cont.) Aerobic respiration is a redox reaction in which glucose becomes oxidized and oxygen becomes reduced Aerobic respiration transfers electrons (associated with hydrogen atoms in glucose) to oxygen in a series of steps that control the amount of energy released Free energy of the electrons is coupled to ATP synthesis Aerobic respiration is an exergonic redox process in which glucose becomes oxidized, oxygen becomes reduced, and energy is captured to make ATP

The Four Stages of Aerobic Respiration

Summary of Aerobic Respiration Table 8-1, p. 174

Reactions Involved in Aerobic Respiration dehydrogenations Reactions in which two hydrogen atoms are removed from the substrate and transferred to NAD+ or FAD decarboxylations Reactions in which part of a carboxyl group (COOH) is removed from the substrate as a molecule of CO2 Other reactions Reactions in which molecules are rearranged so they can undergo further dehydrogenations or decarboxylations

Introduction to Glycolysis Takes place in the cytosol Metabolizes the 6-carbon sugar glucose into two 3-carbon molecules of pyruvate Does not require oxygen; proceeds under aerobic or anaerobic conditions Net yield: 2 ATP molecules and 2 NADH molecules Two major phases: Endergonic reactions that require ATP (investment phase) Exergonic reactions that yield ATP and NADH (payoff phase)

Energy capture phase Four ATPs and two NADH produced per glucose GLYCOLYSIS Glucose Energy investment phase and splitting of glucose Two ATPs invested per glucose 3 steps Fructose-1,6-bisphosphate Glyceraldehyde phosphate (G3P) Glyceraldehyde phosphate (G3P) Energy capture phase Four ATPs and two NADH produced per glucose (G3P) (G3P) Figure 8.3: An overview of glycolysis. The black spheres represent carbon atoms. The energy investment phase of glycolysis leads to the splitting of sugar; ATP and NADH are produced during the energy capture phase. During glycolysis, each glucose molecule is converted to two pyruvates, with a net yield of two ATP molecules and two NADH molecules. 5 steps Pyruvate Pyruvate Net yield per glucose: Two ATPs and two NADH Fig. 8-3, p. 176

First Phase of Glycolysis Phosphate groups are transferred from ATP to glucose In two separate phosphorylation reactions The phosphorylated sugar (fructose-1,6-bisphosphate) is broken enzymatically into two three-carbon molecules, yielding 2 glyceraldehyde-3-phosphate (G3P) glucose + 2 ATP → 2 G3P + 2 ADP

Second Phase of Glycolysis G3P is converted to pyruvate G3P is oxidized by removal of 2 electrons (as hydrogen atoms), which combine with NAD+ NAD+ + 2 H → NADH + H+ ATP is formed by substrate-level phosphorylation 2 G3P + 2 NAD+ + 4 ADP → 2 pyruvate + 2 NADH + 4 ATP

Energy investment phase and splitting of glucose Two ATPs invested per glucose Hexokinase 1 Figure 8.4: A detailed look at glycolysis. A specific enzyme catalyzes each of the reactions in glycolysis. Note the net yield of two ATP molecules and two NADH molecules. (The black wavy lines indicate bonds that permit the phosphates to be readily transferred to other molecules; in this case, ADP.) 1 Glycolysis begins with preparation reaction in which glucose receives phosphate group from ATP molecule. ATP serves as source of both phosphate and energy needed to attach phosphate to glucose molecule. (Once ATP is spent, it becomes ADP and joins ADP pool of cell until turned into ATP again.) Phosphorylated glucose is known as glucose-6-phosphate. (Note phosphate attached to its carbon atom 6.) Phosphorylation of glucose makes it more chemically reactive. Glucose-6-phosphate Phosphoglucoisomerase Fig. 8-4a (1), p. 178

Dihydroxyacetone phosphate 2 Fructose-6-phosphate Phosphofructokinase 3 Fructose-1,6-bisphosphate Aldolase Figure 8.4: A detailed look at glycolysis. A specific enzyme catalyzes each of the reactions in glycolysis. Note the net yield of two ATP molecules and two NADH molecules. (The black wavy lines indicate bonds that permit the phosphates to be readily transferred to other molecules; in this case, ADP.) 2 Glucose-6-phosphate undergoes another preparation reaction, rearrangement of its hydrogen and oxygen atoms. In this reaction glucose-6-phosphate is converted to its isomer, fructose-6-phosphate. 3 Next, another ATP donates phosphate to molecule, forming fructose-1,6-bisphosphate. So far, two ATP molecules have been invested in process without any being produced. Phosphate groups are now bound at carbons 1 and 6, and molecule is ready to be split. 4 Fructose-1,6-bisphosphate is then split into two 3-carbon sugars, glyceraldehyde-3- phosphate (G3P) and dihydroxyacetone phosphate. 5 Dihydroxyacetone phosphate is enzymatically converted to its isomer, glyceraldehyde-3- phosphate, for further metabolism in glycolysis. 4 Isomerase 5 Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate (G3P) Fig. 8-4a (2), p. 178

Two glyceraldehyde-3-phosphate (G3P) from bottom of previous page Energy capture phase Four ATPs and two NADH produced per glucose Glyceraldehyde-3-phosphate dehydrogenase 6 Two 1,3-bisphosphoglycerate Phosphoglycerokinase Figure 8.4: A detailed look at glycolysis. A specific enzyme catalyzes each of the reactions in glycolysis. Note the net yield of two ATP molecules and two NADH molecules. (The black wavy lines indicate bonds that permit the phosphates to be readily transferred to other molecules; in this case, ADP.) 6 Each glyceraldehyde-3-phosphate undergoes dehydro- genation with NAD + as hydrogen acceptor. Product of this very exergonic reaction is phosphoglycerate, which reacts with inorganic phosphate present in cytosol to yield 1,3-bisphosphoglycerate. 7 One of phosphates of 1,3-bisphosphoglycerate reacts with ADP to form ATP. This transfer of phosphate from phosphorylated intermediate to ATP is referred to as substrate-level phosphorylation. 7 Two 3-phosphoglycerate Phosphoglyceromutase Fig. 8-4b (1), p. 179

8 Two 2-phosphoglycerate Enolase 9 Two phosphoenolpyruvate Figure 8.4: A detailed look at glycolysis. A specific enzyme catalyzes each of the reactions in glycolysis. Note the net yield of two ATP molecules and two NADH molecules. (The black wavy lines indicate bonds that permit the phosphates to be readily transferred to other molecules; in this case, ADP.) 8 3-phosphoglycerate is rearranged to 2-phosphoglycerate by enzymatic shift of position of phosphate group. This is a preparation reaction. 9 Next, molecule of water is removed, which results in formation of double bond. The product, phosphoenol- pyruvate (PEP), has phosphate group attached by an unstable bond ( wavy line ). 10 Each of two PEP molecules transfers its phosphate group to ADP to yield ATP and pyruvate. This is substrate-level phosphorylation reaction. Pyruvate kinase 10 Two pyruvate Fig. 8-4b (2), p. 179

Pyruvate is Converted to Acetyl CoA Pyruvate undergoes oxidative decarboxylation A carboxyl group is removed as CO2, which diffuses out of the cell Occurs in mitochondria of eukaryotes The two-carbon fragment is oxidized (NAD+ accepts the electrons), and is attached to coenzyme A, yielding acetyl coenzyme A (acetyl CoA) 2 pyruvate + 2 NAD+ + 2 CoA → 2 acetyl CoA + 2 NADH + 2 CO2

Formation of Acetyl CoA

Overview of the Citric Acid Cycle The citric acid cycle is also known as the Krebs cycle Takes place in the matrix of the mitochondria A specific enzyme catalyzes each of the eight steps Begins when acetyl CoA transfers its two-carbon acetyl group to the four-carbon acceptor compound oxaloacetate, forming citrate, a six-carbon compound: oxaloacetate + acetyl CoA → citrate + CoA

The Citric Acid Cycle (cont.) Citrate goes through a series of chemical transformations, losing two carboxyl group as CO2 One ATP is formed (per acetyl group) by substrate-level phosphorylation – most of the oxidative energy (in electrons) is transferred to NAD+, forming 3 NADH Electrons are also transferred to FAD, forming FADH2

Overview of the Citric Acid Cycle

Introduction to the Electron Transport Chain All electrons removed from a glucose during glycolysis, acetyl CoA formation, and the citric acid cycle are transferred as part of hydrogen atoms to NADH and FADH2 NADH and FADH2 enter the electron transport chain (ETC), where electrons move from one acceptor to another Some electron energy is used to drive synthesis of ATP by oxidative phosphorylation

Transfer of Electrons In eukaryotes, the ETC is a series of electron carriers embedded in the inner mitochondrial membrane Electrons pass down the ETC in a series of redox reactions, losing some of their energy at each step along the chain

Transfer of Electrons (cont.) Cytochrome c reduces O2, forming H2O Oxygen is the final electron acceptor in the ETC Lack of oxygen blocks the entire ETC – no additional ATP is produced by oxidative phosphorylation Some poisons also inhibit normal activity of cytochromes Example: Cyanide binds to iron in cytochrome, blocking ATP production

Overview of the Electron Transport Chain

The Chemiosmotic Model of ATP Synthesis 1961: Peter Mitchell proposed that electron transport and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane in eukaryotes (chemiosmosis) Mitchell’s experiments used a bacterial model: Bacterial cells placed in an environment with a high hydrogen ion (proton) concentration synthesized ATP even if electron transport was not taking place

KEY EXPERIMENT: Evidence for Chemiosmosis

Bacterial cytoplasm (low acid) Synthesized Figure 8.9: Evidence for chemiosmosis. Plasma membrane Acidic environment Fig. 8-9, p. 183

The Proton Gradient As electrons pass down the ETC, energy is used to move protons (H+) across the inner mitochondrial membrane into the intermembrane space The intermembrane space has a higher concentration of protons; the mitochondrial matrix has a lower concentration The resulting proton gradient is a form of potential energy that provides energy for ATP synthesis

The Proton Gradient Outer mitochondrial membrane Cytosol Inner mitochondrial membrane Figure 8.10: The accumulation of protons (H+) within the intermembrane space. As electrons move down the electron transport chain, the electron transport complexes move protons (H+) from the matrix to the intermembrane space, creating a proton gradient. The high concentration of H+ in the intermembrane space lowers the pH. Intermembrane space—low pH Matrix—higher pH Fig. 8-10, p. 184

Synthesis of ATP Protons diffuse from the intermembrane space (high concentration) to the matrix (low concentration) through the enzyme complex ATP synthase A central structure of ATP synthase rotates, catalyzing the phosphorylation of ADP to form ATP Chemiosmosis allows exergonic redox reactions to drive the endergonic reaction in which ATP is produced by oxidative phosphorylation

Overview of the ETC Cytosol Outer mitochondrial membrane Intermembrane space Complex V: ATP synthase Complex III Complex IV Inner mitochondrial membrane Complex I Complex II Matrix of mitochondrion Figure 8.11: A detailed look at electron transport and chemiosmosis. (a) The electron transport chain in the inner mitochondrial membrane includes three proton pumps that are located in three of the four elec- tron transport complexes. (The orange arrows indicate the pathway of electrons; and the black arrows, the pathway of protons.) The energy released during electron transport is used to transport protons (H + ) from the mitochondrial matrix to the intermembrane space, where a high concentration of protons accumulates. The protons cannot diffuse back into the matrix except through special channels in ATP synthase in the inner membrane. The flow of the protons through ATP synthase provides the energy for generating ATP from ADP and inorganic phos- phate (P i ). In the process, the inner part of ATP synthase rotates (thick red arrows) like a motor. Fig. 8-11a, p. 185

ATP Production Aerobic respiration of one glucose molecule: Glycolysis: glucose + 2 ATP → 2 pyruvates + 2 NADH + 4 ATPs (net profit of 2 ATPs) Pyruvate conversion: 2 pyruvates → 2 acetyl CoA + 2 CO2 + 2 NADH Citric acid cycle: 2 acetyl CoA → 4 CO2 + 6 NADH + 2 FADH2 + 2 ATPs Total = 4 ATP + 10 NADH + 2 FADH2

ATP Production (cont.) Oxidation of NADH in the electron transport chain yields up to 3 ATPs per molecule (10 NADH X 3 = 30 ATPs) Oxidation of FADH2 yields 2 ATPs per molecule (2 FADH2 X 2 = 4 ATPs)

ATP Production (cont.) Summing all the ATPs: 2 from glycolysis 2 from the citric acid cycle 32 to 34 from electron transport and chemiosmosis Complete aerobic metabolism of one molecule of glucose yields a maximum of 36 to 38 ATPs

Energy Yield from Oxidation of Glucose by Aerobic Respiration

Cells Regulate Aerobic Respiration Glycolysis is partly controlled by feedback regulation of the enzyme phosphofructokinase Phosphofructokinase has two allosteric sites: An inhibitor site that binds ATP (at very high ATP levels) An activator site to which AMP binds (when ATP is low)

KEY CONCEPTS 8.2 Aerobic respiration consists of four stages: glycolysis, formation of acetyl coenzyme A, the citric acid cycle, and the electron transport chain and chemiosmosis

Animation: Recreating the reactions of glycolysis

Nutrients Other Than Glucose Nutrients other than glucose are transformed into metabolic intermediates that enter glycolysis or the citric acid cycle Amino acids: The amino group (NH2) is removed (deamination) The carbon chain is used in aerobic respiration Lipids: Glycerol is converted to a compound that enters glycolysis Fatty acids are converted by β-oxidation to acetyl CoA, which enters the citric acid cycle

Energy from Proteins, Carbohydrates, and Fats Amino acids Glycerol Fatty acids Glycolysis Glucose G3P Pyruvate Energy from Proteins, Carbohydrates, and Fats CO 2 Acetyl coenzyme A Citric acid cycle Figure 8.13: Energy from proteins, carbohydrates, and fats. Products of the catabolism of proteins, carbohydrates, and fats enter glycolysis or the citric acid cycle at various points. This diagram is greatly simplified and illustrates only a few of the principal catabolic pathways. Electron transport and chemiosmosis End products: NH3 H2O CO2 Fig. 8-13, p. 187

Anaerobic Respiration Does not use oxygen as the final electron acceptor Used by prokaryotes in anaerobic environments, such as waterlogged soil, stagnant ponds, and animal intestines Electrons from glucose pass from NADH down an ETC coupled to ATP synthesis by chemiosmosis End products of this of anaerobic respiration are CO2, one or more reduced inorganic substances, and ATP

Fermentation fermentation An anaerobic pathway that does not involve an ETC Only two ATPs are formed per glucose (by substrate-level phosphorylation during glycolysis) NADH molecules transfer H atoms to organic molecules, regenerating NAD+ needed for glycolysis Fermentation is highly inefficient, because fuel is only partially oxidized

Alcohol Fermentation Yeasts are facultative anaerobes that carry out aerobic respiration when oxygen is available but switch to alcohol fermentation when deprived of oxygen alcohol fermentation Enzymes decarboxylate pyruvate, forming acetaldehyde NADH produced during glycolysis transfers hydrogen atoms to acetaldehyde, reducing it to ethyl alcohol

Lactate Fermentation Certain fungi bacteria perform lactate fermentation – vertebrate muscle cells also produce lactate when oxygen is depleted during exercise lactate (lactic acid) fermentation NADH produced during glycolysis transfers hydrogen atoms to pyruvate, reducing it to lactate

Fermentation

Aerobic Respiration, Anaerobic Respiration, and Fermentation Table 8-2, p. 188