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.