Chemotrophic Energy Metabolism: Glycolysis and F FIGURE 9-11 Pathways for Glycolysis and Gluconeogenesis

  • View
    0

  • Download
    0

Embed Size (px)

Transcript

  • Chemotrophic Energy

    Metabolism:

    Glycolysis and

    Fermentation

    Chapter 9

    Becker’s The World of Cell

  • Metabolic Pathways

     Metabolic pathways in cells are

    usually either anabolic (synthetic) or

    catabolic (degradative).

    Catabolic reactions provide the

    energy necessary to drive the

    anabolic reactions.

  • ATP: The Universal Energy Coupler

     ATP is useful for storing chemical energy in cells

    because its terminal anhydride bond has an

    intermediate free energy of hydrolysis. This allows ATP to

    serve as a donor of phosphate groups to a number of

    biologically important molecules such as glucose. It also

    allows ADP to serve as an acceptor of phosphate

    groups from molecules such as PEP.

  • ATP Hydrolysis Is Highly Exergonic Because of

    Charge Repulsion and Resonance Stabilization

  • Chemotrophic Energy Metabolism

     Most chemotrophs derive the energy needed to

    generate ATP from the catabolism of organic nutrients

    such as carbohydrates, fats, and proteins. They do so

    either by fermentative processes in the absence of

    oxygen or by aerobic respiratory metabolism in the

    presence of oxygen.

     Although glycolysis may seem overly complex, it

    represents a mechanism by which glucose can be

    degraded in dilute solution at temperatures compatible

    with life, with a large portion of the free energy yield

    conserved as ATP.

  • Biological Oxidations Usually Involve the Removal of Both Electrons

    and Protons and Are Highly Exergonic

    Coenzymes Such as NAD Serve as Electron Acceptors in Biological

    Oxidations

  • Glycolysis and Fermentation: ATP Generation

    Without the Involvement of Oxygen

     Using glucose as a prototype substrate, catabolism

    under both anaerobic and aerobic conditions begins

    with glycolysis, a ten-step pathway that converts

    glucose into pyruvate. In most cases, this leads to the

    production of two molecules of ATP per molecule of

    glucose.

     In the absence of oxygen, the reduced coenzyme

    NADH generated during glycolysis must be reoxidized at

    the expense of pyruvate, leading to fermentation end-

    products such as lactate or ethanol plus carbon

    dioxide.

  • Alternative Substrates for Glycolysis

     Although usually written with glucose as the starting

    substrate, the glycolytic sequence is also the

    mainstream pathway for catabolizing a variety of

    related sugars, such as fructose, galactose, and

    mannose.

     Glycolysis is also used to metabolize the glucose-1-

    phosphate derived by phosphorolytic cleavage of

    storage polysaccharides such as starch or glycogen.

  • FIGURE 9-11 Pathways for Glycolysis and Gluconeogenesis Compared. The pathways for glycolysis (left) and gluconeogenesis (right) have nine intermediates and seven enzyme-

    catalyzed reactions in common. The three essentially irreversible reactions of the glycolytic pathway (in green shading) are circumvented in gluconeogenesis by four bypass reactions (in yellow shading). Gluconeogenesis, on the other hand, is an anabolic pathway, requiring the coupled hydrolysis of six phosphoanhydride bonds (four from ATP, two from GTP) to drive it in the direction of glucose formation. The enzymes

    that catalyze the bypass reactions are shown in gold and are identified in the box. In animals, glycolysis occurs in muscle and various other tissues, whereas gluconeogenesis occurs mainly in the liver and to a lesser degree in

    the kidneys.

  • Gluconeogenesis

     Gluconeogenesis is, in a sense, the opposite of glycolysis because it is the pathway used by some cells to synthesize glucose from three- and four- carbon starting materials such as pyruvate. However, the gluconeogenic pathway is not just glycolysis in reverse.

     The two pathways have seven enzyme-catalyzed reactions in common, but the three most exergonic reactions of glycolysis are bypassed in gluconeogenesis by reactions that render the pathway exergonic in the gluconeogenic direction by the input of energy from ATP and GTP.

  • The Regulation of Glycolysis and Gluconeogenesis

     Glycolysis and gluconeogenesis are regulated by

    modifying the activity of enzymes that are unique to

    each pathway. These enzymes are regulated by one or

    more key intermediates in aerobic respiration, including

    ATP, ADP, AMP, acetyl CoA, and citrate.

     An important allosteric regulator of both glycolysis and

    gluconeogenesis is fructose-2,6-bisphosphate. Its

    concentration depends on the relative kinase and

    phosphatase activities of the bifunctional enzyme PFK-2.

     PFK-2 in turn is regulated by the hormones glucagon

    and epinephrine via their effects on the cyclic AMP

    concentration in cells.

  • Novel Roles for Glycolytic Enzymes

     In addition to their roles as catalysts, several well-known

    glycolytic enzymes have recently been shown to play

    regulatory roles in cells, affecting processes such as cell

    division, programmed cell death, and cancer cell

    migration.