Lecture 15 - GHOSE .Terminology Chapter 14 - Glycolysis & Gluconeogenesis 8 492 Bioenergetics and

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Text of Lecture 15 - GHOSE .Terminology Chapter 14 - Glycolysis & Gluconeogenesis 8 492 Bioenergetics and

  • Based on Profs. Kevin Gardner & Reza Khayat 1

    Biochemistry - I

    Mondays and Wednesdays 9:30-10:45 AM (MR-1307)

    SPRING 2017

    Lecture 15

  • Outline

    Bioenergetics Fates of glucose Glycolysis Feeder pathways for glycolysis

    Why learn about Glycolysis and Gluconeogenesis? Capable of supplying a huge array of metabolites used by many cellular processes,

    biofuels, high rate of glycolysis in tumors, lactose intolerance etc.

    2Chapter 14 - Glycolysis & Gluconeogenesis

  • Bioenergetics In aerobic organisms, the ultimate electron acceptor in the oxidation of carbon is O2 The oxidation product is CO2 and the reduction product is H2O The more reduced a carbon is, the more exergonic (energy releasing) its oxidation will be

    3

    Rapid but reduced amount of energy

    Slow but tremendous amount of energy

    Chapter 14 - Glycolysis & Gluconeogenesis

  • Bioenergetics - Two Helpful Pointers

    4Chapter 14 - Glycolysis & Gluconeogenesis

    Text uses SI units of energy: joules & kilojoules

    4.184 J = 1 cal 4.184 kJ = 1 kcal = 1 dietary Cal

    note 1 Cal (dietary) = 1000 cal (chemical)

  • Adenosine Triphosphate (ATP)

    5

    Reactions written with substrate as ATP, but true substrate is Mg/ATP-2 Mg+2 shields the negative charge of terminal phosphates and allows for nucleophilic attack by enzyme or other substrates Enzymes that bind ATP-4 need Mg+2 present

    ATP -> ADP + Pi has G of -30.5 kJ/mol

    Chapter 14 - Glycolysis & Gluconeogenesis

    Bioenergetics and Biochemical Reaction Types502

    phosphoric acid anhydride (phosphoanhydride) bondin ATP separates one of the three negatively chargedphosphates and thus relieves some of the electrostaticrepulsion in ATP; the Pi released is stabilized by the formation of several resonance forms not possible inATP; and ADP2!, the other direct product of hydrolysis,

    immediately ionizes, releasing H" into a medium ofvery low [H"] (!10!7 M). Because the concentrationsof the direct products of ATP hydrolysis are, in the cell,far below the concentrations at equilibrium (Table135), mass action favors the hydrolysis reaction in the cell.

    The free-energy change for ATP hydrolysis is!30.5 kJ/mol under standard conditions, but the ac-tual free energy of hydrolysis (#G) of ATP in livingcells is very different: the cellular concentrations ofATP, ADP, and Pi are not identical and are much lowerthan the 1.0 M of standard conditions (Table 135). Fur-thermore, Mg2" in the cytosol binds to ATP and ADP(Fig. 1312), and for most enzymatic reactions that in-volve ATP as phosphoryl group donor, the true sub-strate is MgATP2!. The relevant #G$% is therefore thatfor MgATP2! hydrolysis. We can calculate #G for ATPhydrolysis using data such as those in Table 135. Theactual free energy of hydrolysis of ATP under intracel-lular conditions is often called its phosphorylationpotential, !Gp.

    ADP3! " P i2! " H"

    #G$% & !30.5 kJ/mol ATP4! " H2O

    A

    BPO P

    !

    O

    O

    B

    A!O

    O OO

    O

    O Rib AdenineO

    O

    OHO

    ADP2"

    A

    B

    !O

    O

    O

    O OO Rib Adenine

    ADP3"POPO!O O

    B

    A!

    O

    O

    O

    H" "

    OPB

    AO!O

    O

    PO!O OB

    A!O

    O

    OA

    OBP

    !O

    O O

    O

    O OO Rib Adenine

    ATP4"

    H

    OH

    Pi

    !

    POO OA

    O

    O

    O

    POO OB

    A!O

    O

    3!

    OH

    ' !' !

    ' !

    ' !

    resonancestabilization

    AH"

    2

    ionization3

    hydrolysis,with reliefof chargerepulsion

    1

    FIGURE 1311 Chemical basis for the large free-energy change as-sociated with ATP hydrolysis. 1 The charge separation that resultsfrom hydrolysis relieves electrostatic repulsion among the four nega-tive charges on ATP. 2 The product inorganic phosphate (Pi) is sta-bilized by formation of a resonance hybrid, in which each of the fourphosphorusoxygen bonds has the same degree of double-bondcharacter and the hydrogen ion is not permanently associated withany one of the oxygens. (Some degree of resonance stabilization alsooccurs in phosphates involved in ester or anhydride linkages, butfewer resonance forms are possible than for Pi.) 3 The productADP2! immediately ionizes, releasing a proton into a medium ofvery low [H"] (pH 7). A fourth factor (not shown) that favors ATP hy-drolysis is the greater degree of solvation (hydration) of the productsPi and ADP relative to ATP, which further stabilizes the products rel-ative to the reactants.

    OP

    Mg2"

    A!

    OPO OB

    AO

    O

    O OO Rib Adenine

    MgADP"

    OPB

    Mg2"

    AO!O

    O

    PO!O OB

    A!O

    O

    O OBP

    !O

    O O

    O

    O OO Rib Adenine

    MgATP2"

    O!

    O!O

    OB

    A

    FIGURE 1312 Mg2" and ATP. Formation of Mg2"

    complexes partially shields the negative chargesand influences the conformation of the phosphategroups in nucleotides such as ATP and ADP.

    Adenine Nucleotide, Inorganic Phosphate, andPhosphocreatine Concentrations in Some CellsTABLE 135

    Concentration (mM)*

    ATP ADP AMP Pi PCr

    Rat hepatocyte 3.38 1.32 0.29 4.8 0

    Rat myocyte 8.05 0.93 0.04 8.05 28

    Rat neuron 2.59 0.73 0.06 2.72 4.7

    Human erythrocyte 2.25 0.25 0.02 1.65 0

    E. coli cell 7.90 1.04 0.82 7.9 0

    *For erythrocytes the concentrations are those of the cytosol (human erythrocytes lack a nucleus andmitochondria). In the other types of cells the data are for the entire cell contents, although the cytosol and themitochondria have very different concentrations of ADP. PCr is phosphocreatine, discussed on p. 510.This value reflects total concentration; the true value for free ADP may be much lower (p. 503).

    each of us has about 0.2 mol of ATP at any given time

    cannot be stored, usually recycled ~500 cycles per day

  • Fates of Glucose The complete oxidation of glucose

    to carbon dioxide and water proceeds with a standard free-energy change of -2,840 kJ/mol

    Glycolysis is first part of this, handling oxidation to pyruvate

    6Chapter 14 - Glycolysis & Gluconeogenesis

  • Overview of Glycolysis

    7Chapter 14 - Glycolysis & Gluconeogenesis

    Occurs in cytoplasm

  • Terminology

    8Chapter 14 - Glycolysis & Gluconeogenesis

    Bioenergetics and Biochemical Reaction Types492

    KEY CONVENTION: In another simplifying convention usedby biochemists, when H2O, H

    !, and/or Mg2! are reac-tants or products, their concentrations are not includedin equations such as Equation 132 but are insteadincorporated into the constants K"eq and #G"$.

    Just as K"eq is a physical constant characteristic foreach reaction, so too is #G"$ a constant. As we noted inChapter 6, there is a simple relationship between K"eqand #G"$:

    (133)

    The standard free-energy change of a chemical reac-tion is simply an alternative mathematical way ofexpressing its equilibrium constant. Table 132shows the relationship between #G"$ and K"eq. If theequilibrium constant for a given chemical reaction is 1.0,the standard free-energy change of that reaction is 0.0(the natural logarithm of 1.0 is zero). If K"eq of a reactionis greater than 1.0, its #G"$ is negative. If K"eq is less than1.0, #G"$ is positive. Because the relationship between#G"$ and K"eq is exponential, relatively small changes in#G"$ correspond to large changes in K"eq.

    It may be helpful to think of the standard free-energy change in another way. #G"$ is the difference be-tween the free-energy content of the products and thefree-energy content of the reactants, under standardconditions. When #G"$ is negative, the products containless free energy than the reactants and the reaction willproceed spontaneously under standard conditions; allchemical reactions tend to go in the direction that re-sults in a decrease in the free energy of the system. A

    G % &RT ln Keq

    positive value of #G"$ means that the products of the re-action contain more free energy than the reactants, andthis reaction will tend to go in the reverse direction if westart with 1.0 M concentrations of all components (stan-dard conditions). Table 133 summarizes these points.

    Table 134 gives the standard free-energy changesfor some representative chemical reactions. Note thathydrolysis of simple esters, amides, peptides, and glyco-sides, as well as rearrangements and eliminations, pro-ceed with relatively small standard free-energy changes,whereas hydrolysis of acid anhydrides is accompaniedby relatively large decreases in standard free energy.The complete oxidation of organic compounds such asglucose or palmitate to CO2 and H2O, which in cells re-quires many steps, results in very large decreases instandard free energy. However, standard free-energy

    WORKED EXAMPLE 131 Calculation of !G"#Calculate the standard free-energy change of the reac-tion catalyzed by the enzyme phosphoglucomutase

    Glucose 1-phosphate glucose 6-phosphate

    given that, starting with 20 mM glucose 1-phosphate andno glucose 6-phosphate, the final equilibrium mixture at25 $C and pH 7.0 contains 1.0 mM glucose 1-phosphateand 19 mM glucose 6-phosphate. Does the reaction inthe direction of glucose 6-phosphate formation proceedwith a loss or a gain of free energy?

    Solution: First we calculate the equilibrium constant:

    We can now calculate the standard free-energy change:

    Because the standard free-energy change is negative,the conversion of glucose 1-phosphate to glucose 6-phosphate proceeds with a loss (release) of freeenergy. (For the reverse reaction, #G"$ has the samemagnitude but the opposite sign.)

    % &7.3 kJ/mo