chemotropic energy:glycolysis and fermentation

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CHEMOTROPIC ENERGY:GLYCOLYSIS AND FERMENTATION

Chapter 9


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 (G = –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


Go is an underestimate

• Because Go 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


Go is an underestimate (continued)

• .

• In most cells ATP/ADP is in the range of about 5:1

• The G 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


Gotransfer

• Gotransfer 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 donor in 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

• Go = –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 called fermentation


Two types of fermentation

• In some animals and many bacteria, the end product of fermentation is lactate, and so anaerobic glucose catabolism is called lactate fermentation

• In most plant cells and microorganisms such as yeast the process is termed alcoholic fermentation because the end product is ethanol


Based on Their Need for Oxygen, Organisms Are Aerobic, Anaerobic, or Facultative• Obligate aerobes have an absolute requirement

for oxygen

• Obligate anaerobes cannot use oxygen as an electron acceptor; oxygen is toxic to these organisms

• Facultative organisms can 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


The first reaction adds a phosphate to the sixth carbon atom

• The bond formed is a phosphodiester bond, a lower-energy bond than the phosphoanhydride bonds in ATP

• The enzyme that catalyzes the reaction is hexokinase, and is specific for phosphorylation of other six-carbon sugars as well

• Liver cells also have glucokinase, which is specific for just glucose


The second phosphate is added to carbon one

• The first carbon of glucose is not as easily phosphorylated as the sixth

• The Gly-2 reaction first converts glucose-6-phosphate to fructose-6-phosphate, allowing the Gly-3 reaction to add a phosphate to carbon one

• This reaction is catalyzed by the enzyme phosphofructokinase-1 (PFK-1)


Summary of Gly-1 to Gly-5

• The first phase of the glycolytic pathway can be summarized as


Phase 2: Oxidation and ATP Generation

• The net energy yield of phase one is negative

• Two molecules of ATP have been consumed per molecule of glucose

• In phase 2, ATP production is linked to an oxidative event, followed by the generation of ATP in phase 3


Gly-6 and Gly-7

• The oxidation of glyceraldehyde-3-phosphate to 3-phosphoglycerate is highly exergonic, and drives

– the reduction of NAD+ to NADH (Gly-6)

– the phosphorylation of ADP with inorganic phosphate, Pi (Gly-7)


Important features of Gly-6 and Gly-7

• NAD+ is an electron acceptor

• The oxidation is coupled to the formation of a high-energy, doubly phosphorylated intermediate, 1,3-bisphosphoglycerate

• ATP generation by transferring a phosphate group to ADP from a phosphorylated substrate such as 1,3-bisphosphoglycerate is called substrate-level phosphorylation


Summary of Gly-6 and Gly-7

• Gly-6 and Gly-7 can be summarized as

• Each reaction involving glyceraldehyde-3-phosphate occurs twice per starting molecule of glucose

• The two ATPs invested in the first phase are recovered in the second phase, for no net ATP


Phase 3: Pyruvate Formation and ATP Generation

• The phosphoester bond of 3-phosphoglycerate is converted to a phosphoenol bond

• The phosphate group is moved to the adjacent carbon, forming 2-phosphoglycerate (Gly-8)

• Water is removed from the 2-phosphoglycerate by the enzyme enolase (Gly-9) generating the high-energy compound phosphoenolpyruvate (PEP)


Phosphoenolpyruvate

• Hydrolysis of the phosphoenol bond of PEP is one of the most exergonic known in biological systems

• PEP hydrolysis drives ATP synthesis by transferring a phosphate to ADP, catalyzed by the enzyme pyruvate kinase (Gly-10)

• The transfer is irreversible in the direction of pyruvate and ATP formation


Summary of Gly-8 to Gly-1

• The third phase of glycolysis can be summarized as


Summary of Glycolysis

• The two molecules of ATP formed in the second phosphorylation event (Gly-10) represent the net yield of ATP for the glycolytic pathway

• .

• The pathway is highly exergonic in the direction of pyruvate formation; G in a cell is typically –20 kcal/mol


Conservation of Glycolysis

• The glycolytic pathway is one of the most common and highly conserved metabolic pathways known

• Virtually all cells have the ability to convert glucose to pyruvate, extracting energy in the process

• The next steps depend on the availability of oxygen


The Fate of Pyruvate Depends on Whether Oxygen Is Available

• Pyruvate occupies a key position as a branch point in chemotrophic energy metabolism

• In the presence of oxygen, pyruvate undergoes further oxidation to acetyl coenzyme A

• Acetyl CoA can be completely oxidized to CO2, generating more than 30 ATP per glucose


Figure 9-8A


The fate of pyruvate in the absence of oxygen

• Under anaerobic conditions no further oxidation of pyruvate occurs

• Pyruvate is reduced by accepting the electrons (and protons) that must be removed from NADH

• The most common products of pyruvate reduction are lactate or ethanol and CO2


Figure 9-8B,C


In the Absence of Oxygen, Pyruvate Undergoes Fermentation to Regenerate NAD+

• Fermentation must regenerate NAD+ from NADH so that glycolysis can continue

• Cells monitor and stabilize the NAD+/NADH ratio, an indicator of the cell’s redox state (general level of oxidation of cellular components)

• Electrons are transferred to pyruvate with two possible outcomes


Lactate Fermentation

• The anaerobic process that culminates with lactate is called lactate fermentation

• Lactate is generated by direct transfer of electrons from NADH to pyruvate by lactate dehydrogenase

• .


Overall metabolism of glucose to lactate

• The overall metabolism of glucose to lactate in the absence of oxygen can be summarized as

• Lactate fermentation is commercially important and also occurs in our muscles during strenuous exertion


Gluconeogenesis

• Lactate produced in muscles under hypoxic conditions is transferred to the liver

• In the liver, it is converted into glucose by the process of gluconeogenesis

• It is the reverse of lactate fermentation but with several differences


Alcoholic Fermentation

• Under anaerobic conditions plant cells carry out alcoholic fermentation, as do yeasts and other microorganisms

• Pyruvate loses a carbon (as CO2) and forms the two-carbon compound acetaldehyde (enzyme: pyruvate decarboxylase)

• Acetaldehyde reduction by NADH gives rise to ethanol (enzyme: alcohol dehydrogenase)


Summary of alcoholic fermentation

• .

• Adding the overall equation for glycolysis

• .


Other Fermentation Pathways

• In proprionate fermentation, bacteria reduce pyruvate to proprionate

• Bacteria that cause food spoilage do so by butylene glycol fermentation

• Other processes yield acetone, isopropyl alcohol, or butyrate, all variations on the common theme of reoxidizing NADH by the transfer of electrons to an organic acceptor


Fermentation Taps Only a Fraction of the Substrate’s Free Energy but Conserves That Energy Efficiently as ATP

• An essential feature of every fermentation process is

– no external electron acceptor is involved and no net oxidation occurs

• Fermentation gives a modest ATP yield of two ATP per glucose; most of the free energy of the glucose molecule is still present in the lactate or ethanol


Free energy of lactate

• The two lactate molecules produced from one glucose contain most of the energy present per mole of glucose

• Go = –319.5 kcal/mol, or 93% of the original free energy of glucose

• Although the energy yield is low, the free energy is conserved as ATP with an efficiency that probably exceeds 40%


Alternative Substrates for Glycolysis

• Glucose is a major substrate for both fermentations and respiration in a variety of organisms and some tissues

• But for some organisms and tissues, glucose is not significant at all

• There are a variety of alternatives to glucose, which are often converted into an intermediate in the glucose catabolism pathway


Other Sugars and Glycerol Are Also Catabolized by the Glycolytic Pathway

• Many sugars are available to cells, either monosaccharides or disaccharides that can be readily hydrolyzed into monosaccharides

• The monosaccharides are then converted into a glycolytic intermediate

• Glucose and fructose enter most directly after phosphorylation on carbon atom 6; mannose and fructose require more steps


Pentoses and glycerol can be channeled into the glycolytic pathway too

• Phosphorylated pentoses can enter the glycolytic pathway but must first be converted to hexose phosphates

• The conversion takes place via the phosphogluconate pathway, also called the pentose phosphate pathway

• Glycerol, a three-carbon molecule resulting from lipid breakdown, enters after conversion to dihydroxyacetone phosphate


Figure 9-9


Polysaccharides Are Cleaved to Form Sugar Phosphates That Also Enter the Glycolytic Pathway

• Glucose occurs primarily in the form of storage polysaccharides, most often starch in plants and glycogen in animals

• The polysaccharides are mobilized by phophorolysis, using inorganic phosphate to break the (→ 4) bond between glucose units

• Glucose is liberated as glucose-1-phosphate


Gluconeogenesis

• The process of glucose synthesis is called gluconeogenesis

• Glucose is synthesized from three- and four- carbon precursors

• Pyruvate and lactate are the most common starting materials


Gluconeogenesis and glycolysis

• Gluconeogenesis occurs by simple reversal of glycolysis using the same enzyme in both directions

• But not all the steps are simple reversals of glycolysis: Gly-1, Gly-3, and Gly-10 are accomplished by other means

• These are the most exergonic reactions of glycolysis


The Regulation of Glycolysis and Gluconeogenesis• Cells have enzymes for both glycolysis and

gluconeogenesis, so the processes must be regulated

• Spatial regulation keeps the two processes confined to separate places in the body

• There is also temporal regulation in which the two processes take place at different times in one cell


Key Enzymes in the Glycolytic and Gluconeogenic Pathways Are Subject to Alllosteric Regulation

• Allosteric regulation involves the interconversion of an enzyme between two forms, one catalytically active and the other inactive

• The enzyme will be active or not depending on whether an allosteric effector is bound to the allosteric site

• The effector might be an activator or inhibitor


Figure 9-12


Key regulatory enzymes of glycolysis and gluconeogenesis

• For glycolysis the enzymes are hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase

• For gluconeogenesis they are fructose-1,6-bisphosphatase, and pyruvate carboxylase

• Each of the enzymes is unique to its pathway so the pathways can be regulated independently


Glycolysis and gluconeogenesis are reciprocally regulated

• AMP and acetyl CoA, the two effectors to which both pathways are sensitive, have opposite effects

• AMP activates glycolysis and inhibits gluconeogenesis

• Acetyl CoA activate gluconeogenesis but inhibits glycolysis


Fructose-2,6-Bisphosphate Is an Important Regulator of Glycolysis and Gluconeogenesis• Fructose-2,6-Bisphosphate (F2,6BP) is the most

important regulator of both glycolysis and gluconeogenesis

• Synthesis of F2,6BP is catalyzed by phosphofructokinase-2 (PFK-2)

• F2,6BP activates the glycolytic enzyme (PFK-1) that phosphorylates fructose-6-phosphate and it inhibits FBPase that catalyzes the reverse reaction


Figure 9-13


Effect of cAMP on F2,6BP

• cAMP affects the F2,6BP concentration in two ways

• 1. It inactivates the PFK-2 kinase activity

• 2. It stimulates the F2,6BP phosphatase activity

• These two effects tend to decrease the concentration of F2,6BP in the cell


Effect of cAMP on hormone regulation

• cAMP level in cells is controlled primarily by the hormones glucagon and epinephrine (adrenaline)

• These cause an increase in cAMP concentration, stimulating gluconeogenesis when more glucose is needed


Novel Roles for Glycolytic Enzymes

• Glycolysis is connected to other cell processes• Hexokinase (Gly-1) is a transcriptional repressor

in yeast cells under high glucose levels• Mammals have four isoforms of hexokinase

- One is expressed in highly catabolically active tumor cells

- Another binds to mitochondria and helps coordinate glycolysis with mitochondrial functions

• Many other examples exist


Additional functions of other glycolytic enzymes

• Glyceraldehyde-3-phosphate dehydrogenase (Gly-6) and enolase (Gly-9) have DNA-binding abilities

• They can act as transcriptional regulators

• They connect the glycolytic pathway with processes such as cell division and programmed cell death


Cancer connections

• Phosphoglucoisomerase (PGI; Gly-2) is involved in cell motility and migration during cancer cell metastasis

• Metastasis: the release of cells from malignant tumors into the bloodstream; these can form secondary tumors throughout the body

• PGI stimulates cell proliferation and migration

chemotropic energy:glycolysis and fermentation

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