Chemotropic Energy:glycolysis and fermentation

Chapter 9 Chemotropic Energy:glycolysis and fermentation

Chemotrophic Energy Metabolism: Glycolysis and Fermentation Cells cannot survive without a source of energy or a source of chemical building blocks In many organisms these requirements are related Chemotrophs obtain energy from the food they engulf or ingest

Metabolic Pathways To accomplish any task, a cell requires a series of reactions occurring in an ordered sequence This requires many different enzymes to catalyze each individual reaction All the chemical reactions in a cell are referred to as its metabolism, which consists of many specific metabolic pathways

General types of metabolic pathways Anabolic pathways synthesize cellular components, often polymers such as starch and glycogen They usually involve an increase in order and a decrease in entropy So, they are endergonic (energy-requiring)

General types of metabolic pathways (continued) Catabolic pathways are involved in the breakdown of cellular constituents, such as the hydrolysis of glucose These degradative pathways typically involve a decrease in order and increase in entropy So, they are exergonic, energy-liberating reactions

Catabolic pathways Catabolic pathways involve the production of metabolites, small organic building blocks However, the reactions are not just the reversal of an anabolic pathway; enzymes and intermediates may be different Catabolism can be carried out in the presence (aerobic) or absence (anaerobic) of oxygen

ATP: The Universal Energy Coupler The efficient linking (coupling) of energy-yielding and energy-requiring processes is crucial to cell function The most common energy intermediate is adenosine triphosphate (ATP) It is the primary (but not the only) energy currency of the biological world

Other high-energy molecules High-energy molecules such as GTP and creatine phosphate store chemical energy that can be converted to ATP Chemical energy is also stored as reduced coenzymes such as NADH

ATP Contains Two Energy-Rich Phosphoanhydride Bonds ATP contains the aromatic base, adenine, the five-carbon sugar, ribose, and a chain of three phosphate groups The phosphate groups are linked by phosphoanhydride bonds Adenine linked to ribose is adenosine

Forms of adenosine Adenosine occurs in cells in the unphosphorylated form It can also be phosphorylated up to three times, called adenosine monophosphate (AMP), adenosine diphosphate (ADP), or adenosine triphosphate (ATP) Hydrolysis of ATP releases energy (DG = –7.3kcal/mol)

Figure 9-1

Phosphoanhydride bonds Phosphoanhydride bonds are referred to as energy-rich bonds This term is a shorthand way of saying that free energy is released when the bond is hydrolyzed The energy is a feature of the reaction the molecule is involved in, and not of a particular bond in the molecule

ATP Hydrolysis Is Highly Exergonic Because of Charge Repulsion and Resonance Stabilization • Hydrolysis of ATP to ADP and Pi is exergonic because of • Charge repulsion between the adjacent negatively charged phosphate groups • Resonance stabilization of both products of hydrolysis • Increased entropy and solubility of the products of hydrolysis

ATP and ADP are higher-energy than AMP • . • . • . • So, ATP and ADP are both higher-energy compounds than is AMP

DGo is an underestimate Because DGo from equation 9-1 is based on equal concentrations of ADP and ATP (1M each), it is an underestimate This is because under most biological conditions, the concentration of ATP is much larger

DGo is an underestimate (continued) . In most cells ATP/ADP is in the range of about 5:1 The DG is thus in the range of –10 to –14 kcal/mol in cells

ATP Is an Important Intermediate in Cellular Energy Metabolism ATP occupies an intermediate position in the overall spectrum of energy-rich phosphorylated compounds in the cell Under standard conditions, a compound can phosphorylate a less energy-rich compound, but not a more energy-rich compound

Table 9-1

ATP is intermediate among the energy-rich phosphorylated compounds in the cell ATP can be formed from ADP by the transfer of a phosphate group from PEP, but not from glucose-6-phosphate The reverse is also true; ATP can phosphorylate glucose but not pyruvate

Figure 9-3

DGotransfer DGotransfer refers to the standard free energy change that accompanies the transfer of a phosphate from a donor to an acceptor . .

Group transfer reactions Reactions that involve the movement of a chemical group from one molecule to another are called group transfer reactions The phosphate group is one of the most frequently transferred, especially in energy metabolism It is important that ATP/ADP occupy an intermediate position in terms of bond energy

ATP/ADP: intermediate in terms of bond energy ATP can serve as a phosphate donorin some reactions Its dephosphorylated form, ADP, can serve as an acceptor in other reactions That is because there are compounds both above and below the pair in energy

Figure 9-4A

Figure 9-4B

Chemotrophic Energy Metabolism Chemotrophic energy metabolism describes the reactions and pathways by which cells catabolize nutrients and conserve the released energy in the form of ATP Much of chemotrophic energy metabolism involves energy-yielding oxidative reactions (oxidation)

Biological Oxidations Usually Involve the Removal of Both Electrons and Protons and Are Highly Exergonic Substances that are energy sources for cells are oxidizable compounds, the oxidation of which is highly exergonic Oxidation is the removal of electrons .

Oxidation in biological chemistry In biological systems oxidation involves removal of hydrogen ions (protons) in addition to electrons . This process is also a dehydrogenation .

Transfer of electrons Because oxidation reactions involve the removal (in effect) of two hydrogen atoms, many of the enzymes involved are called dehydrogenases The electrons must be transferred to another molecule, which is reduced Reduction, the addition of electrons, is an endergonic process

Hydrogenation In reduction, the electrons that are transferred are frequently accompanied by protons Therefore, the overall reaction is a hydrogenation .

Oxidation and reduction Equations describing reductions or oxidations are half reactions In real situations, reduction and oxidation always take place simultaneously Any time an oxidation occurs, the electrons (and protons) must be added to another molecule in a reduction

Coenzymes Such as NAD+ Serve as Electron Acceptors in Biological Oxidations Usually electrons and hydrogens removed during biological oxidation are transferred to one of several coenzymes Coenzymes are small molecules that function along with enzymes by serving as carriers of electrons or small functional groups They are in low concentrations in the cell as they are recycled

NAD+ The most common coenzyme involved in energy metabolism is nicotinamide adenine dinucleotide, NAD+ It serves as an electron acceptor, adding two electrons and a proton to its aromatic ring, generating NADH plus a proton .

Figure 9-5

Most Chemotrophs Meet Their Energy Needs by Oxidizing Organic Food Molecules Most chemotrophs depend on organic food molecules as oxidizable substrates Oxidation of these organic compounds—carbohydrates, fats, and proteins—produces energy for the cell in the form of ATP and reduced coenzymes

Glucose Is One of the Most Important Oxidizable Substrates in Energy Metabolism The six-carbon sugar glucose is the main energy source for most of the cells in the body Blood glucose comes mainly from dietary carbohydrates, starch, or sucrose, or from the breakdown of stored glycogen In plants, glucose is the monosaccharide released upon starch breakdown

The Oxidation of Glucose Is Highly Exergonic Glucose is a good source of energy because its oxidation is a highly exergonic process DGo = –686 kcal/mol for complete conversion of glucose to carbon dioxide and water, with oxygen as the final electron acceptor .

Glucose Catabolism Yields Much More Energy in the Presence of Oxygen than in Its Absence It is not possible to obtain the full 686 kcal/mol for complete oxidation of glucose; energy conversion is not 100% efficient Complete oxidation of glucose in the presence of oxygen is called aerobic respiration Many organisms, such as bacteria, carry out anaerobic respiration, using electron acceptors such as S, H+, and Fe3+

Anaerobic respiration Even in the absence of oxygen, most organisms can extract limited energy from glucose They do so via glycolysis Electrons removed during glucose oxidation are returned to an organic molecule later in the same pathway This is calledfermentation

Two types of fermentation In some animals and many bacteria, the end product of fermentation is lactate, and so anaerobic glucose catabolism is called lactatefermentation In most plant cells and microorganisms such as yeast the process is termed alcoholicfermentation because the end product is ethanol

Based on Their Need for Oxygen, Organisms Are Aerobic, Anaerobic, or Facultative Obligate aerobeshave an absolute requirement for oxygen Obligateanaerobescannot use oxygen as an electron acceptor; oxygen is toxic to these organisms Facultative organismscan function under aerobic or anaerobic conditions

Glycolysis and Fermentation: ATP Generation Without the Involvement of Oxygen Anaerobes carry out oxidative reactions without using oxygen as an electron acceptor Most organisms generate two molecules of ATP for every glucose molecule that is oxidized However, some organisms are able to produce more ATP molecules per glucose

Glycolysis Generates ATP by Catabolizing Glucose to Pyruvate Glycolysis (or the glycolytic pathway) is a ten-step reaction sequence that converts one glucose molecule into two molecules of pyruvate Pyruvate is a three-carbon compound Both ATP and NADH are produced

Figure 9-6

Figure 9-7

Glycolysis is present in all organisms Glycolysis is common to both aerobic and anaerobic organisms In most cells the enzymes for glycolysis are found in the cytosol But in some parasitic protozoans called trypanosomes, the first seven enzymes are found in membrane-bounded organelles called glycosomes

Glycolysis in Overview In the absence of oxygen glycolysis leads to fermentation In the presence of oxygen glycolysis leads to aerobic respiration

Important features of the glycolytic pathway are The initial input of ATP (Gly-1) The sugar splitting reaction in which glucose is split into two three-carbon molecules The oxidative event that generates NADH (Gly-6) The two steps at which the reaction sequence is coupled to ATP generation (Gly-7 and Gly-10)

The glycolytic pathway can be divided into three phases Phase I: the preparatory and cleavage steps Phase II: the oxidative sequence, which is the first ATP-generating event Phase III: the second ATP-generating event

Phase I: Preparation and Cleavage The net result of the first three reactions is to convert glucose into a doubly phosphorylated molecule (fructose-1,6-bisphosphate) The phosphates are transferred to glucose from ATP ATP hydrolysis is also the driving force that makes the phosphorylation exergonic and thus irreversible

Chemotropic Energy:glycolysis and fermentation