Chapter 14
Glycolysis and the Catabolism of Hexoses
Having examined the organizing principles of
cell metabolism and bioenergetics, we are
ready to see how the chemical energy stored
in glucose and other fuel molecules is
released to perform biological work. DGlucose is the major fuel of most organisms
and occupies a central position in
metabolism. It is relatively rich in potential
energy; its complete oxidation to carbon
dioxide and water proceeds with a standard
free-energy change of -2,840 kJ/mol. By
storing glucose as a high molecular weight
polymer, a cell can stockpile large quantities Figure 14-1 Major pathways of glucose
in cells of higher plants and animals.
of hexose units while maintaining a relatively utilization
Although
low cytosolic osmolarity. When the cell's
not the only possible fates for glucose, these three
energy demands suddenly increase, glucose pathways are the most significant in terms of the
can be released quickly from these
amount of glucose that flows through them in most
intracellular storage polymers.
cells.
Glucose is not only an excellent fuel, it is
also a remarkably versatile precursor, capable
of supplying a huge array of metabolic
intermediates, the necessary starting
materials for biosynthetic reactions. E. coli
can obtain from glucose the carbon skeletons
for every one of the amino acids, nucleotides,
coenzymes, fatty acids, and other metabolic
intermediates needed for growth. A study of
the numerous metabolic fates of glucose
would encompass hundreds or thousands of
transformations. In the higher plants and
animals glucose has three major fates: it may
be stored (as a polysaccharide or as sucrose),
oxidized to a three-carbon compound
(pyruvate) via glycolysis, or oxidized to
pentoses via the pentose phosphate
(phosphogluconate) pathway (Fig. 14-1).
This chapter begins with a description of the
individual reactions that constitute the
glycolytic pathway and of the enzymes that
catalyze them. We then consider
fermentation, the operation of the glycolytic
pathway under anaerobic conditions. The
sources of glucose units for glycolysis are
diverse, and we next describe pathways that
bring carbon into glycolysis from hexoses
other than glucose and from disaccharides
and polysaccharides. Like all metabolic
pathways, glycolysis is under tight
regulation. We discuss the general principles
of metabolic regulation, then illustrate these
principles with the glycolytic pathway. The
chapter concludes with a brief description of
two other catabolic pathways that begin with
glucose: one leading to pentoses, the other to
glucuronate and ascorbic acid (vitamin C).
Glycolysis
In glycolysis (from the Greek glykys,
meaning "sweet," and lysis, meaning
"splitting") a molecule of glucose is
degraded in a series of enzymecatalyzed reactions to yield two
molecules of pyruvate. During the
sequential reactions of glycolysis some
of the free energy released from glucose
is conserved in the form of ATP.
Glycolysis was the first metabolic
pathway to be elucidated and is
probably the best understood. From the
discovery by Eduard Buchner (in 1897)
of fermentation in broken extracts of
yeast cells until the clear recognition by
Fritz Lipmann and Herman Kalckar (in
1941) of the metabolic role of
highenergy compounds such as ATP in
metabolism, the reactions of glycolysis
in extracts of yeast and muscle were
central to biochemical research. The
development of methods of enzyme
purification, the discovery and
recognition of the importance of
cofactors such as NAD, and the
discovery of the pivotal role in
metabolism of phosphorylated
compounds all came out of studies of
glycolysis. By now, all of the enzymes
of glycolysis have been purified from
many organisms and thoroughly studied,
and the three-dimensional structures of
all of the glycolytic enzymes are known
from x-ray crystallographic studies.
Glycolysis is an almost universal central
pathway of glucose catabolism. It is the
pathway through which the largest flux
of carbon occurs in most cells. In certain
mammalian tissues and cell types
(erythrocytes, renal medulla, brain, and
sperm, for example), glucose is the sole
or major source of metabolic energy
through glycolysis. Some plant tissues
that are modified for the storage of
starch (such as potato tubers) and some
plants adapted to growth in areas
regularly inundated by water
(watercress, for example) derive most of
their energy from glycolysis; many
types of anaerobic microorganisms are
entirely dependent on glycolysis.
Fermentation is a general term denoting
the anaerobic degradation of glucose or
other organic nutrients into various
products (characteristic for difl'erent
organisms) to obtain energy in the form
of ATP. Because living organisms first
arose in an atmosphere lacking oxygen,
anaerobic breakdown of glucose is
probably the most ancient biological
mechanism for obtaining energy from
organic fuel molecules. In the course of
evolution this reaction sequence has
been completely conserved; the
glycolytic enzymes of vertebrate
animals are closely similar, in amino
acid sequence and three-dimensional
structure, to their homologs in yeast and
spinach. The process of glycolysis
differs from one species to another only
in the details of its regulation and in the
subsequent metabolic fate of the
pyruvate formed. The thermodynamic
principles and the types of regulatory
mechanisms in glycolysis are found in
all pathways of cell metabolism. A
study of glycolysis can serve as a model
of many aspects of the pathways
discussed later in this book.
Before examining each step of the
pathway in some detail, we will take a
look at glycolysis as a whole.
An Overview: Glycolysis Has Two Phases
The breakdown of the six-carbon glucose into two molecules of the three-carbon pyruvate
occurs in ten steps, the first five of which constitute the preparatory phase (Fig. 14-2a). In
these reactions glucose is first phosphorylated at the hydroxyl group on C-6 (step 1). The
nglucose-6-phosphate thus formed is converted to D-fructose-6phosphate (step 2), which
is again phosphorylated, this time at C-1, to yield n-fructose-1,6-bisphosphate (step 3 ).
For both phosphorylations, ATP is the phosphate donor. As all of the sugar derivatives that
occur in the glycolytic pathway are the D isomers, we will omit the D designation except
when emphasizing stereochemistry.
Figure 14-2 The two phases of glycolysis. For each molecule of glucose that passes through the preparatory
phase (a), two molecules of glyceraldehyde-3-phosphate are formed; both pass through the payoff phase (b).
Pyruvate is the end product of the second phase under aerobic conditions, but under anaerobic conditions
pyruvate is reduced to lactate to regenerate NAD+. For each glucose molecule, two ATP are consumed in the
preparatory phase and four ATP are produced in the payoff phase, giving a net yield of two molecules of ATP
per one of glucose converted to pyruvate. The number beside each reaction step corresponds to its numbered
heading in the text discussion. Keep in mind that each phosphate group, represented here as (P) has two
negative charges (-PO32-).
Fructose-1,6-bisphosphate is next split to yield two three-carbon molecules,
dihydroxyacetone phosphate and glyceraldehyde-3-phosphate (step 4); this is the "lysis"
step that gives the process its name. The dihydroxyacetone phosphate is isomerized to a
second molecule of glyceraldehyde-3-phosphate (step 5); this ends the first phase of
glycolysis. Note that two molecules of ATP must be invested to activate, or prime, the
glucose molecule for its cleavage into two three-carbon pieces; later there will be a good
return on this investment. To sum up: in the preparatory phase of glycolysis the energy of
ATP is invested, raising the free-energy content of the intermediates, and the carbon
chains of all the metabolized hexoses are converted into a common product,
glyceraldehyde-3-phosphate.
The energetic gain comes in the payoff phase of glycolysis (Fig. 14-2b). Each molecule of
glyceraldehyde-3-phosphate is oxidized and phosphorylated by inorganic phosphate (not
by ATP) to form 1,3bisphosphoglycerate (step 6)?. Energy is released as the two
molecules of 1,3-bisphosphoglycerate are converted into two molecules of pyruvate (steps
7 through 10)?. Much of this energy is conserved by the coupled phosphorylation of four
molecules of ADP to ATP. The net yield is two molecules of ATP per molecule of glucose
used, because two molecules of ATP were invested in the preparatory phase of glycolysis.
Energy is also conserved in the payoff phase in the formation of two molecules of NADH
per molecule of glucose.
In the sequential reactions of glycolysis, three types of chemical transformation are
particularly noteworthy: (1) the degradation of the carbon skeleton of glucose to yield
pyruvate, (2) the phosphorylation of ADP to ATP by high-energy phosphate compounds
formed during glycolysis, and (3) the transfer of hydrogen atoms or electrons to NAD+,
forming NADH. The fate of the product, pyruvate, depends on the cell type and the
metabolic circumstances.
Fate of Pyruvate Three
alternative catabolic
routes are taken by the
pyruvate formed by
glycolysis. In aerobic
organisms or tissues,
under aerobic conditions,
glycolysis constitutes
only the first stage in the
complete degradation of
glucose (Fig. 14-3).
Pyruvate is oxidized,
with loss of its carboxyl
group as CO2, to yield
the acetyl group of
acetylcoenzyme A, which
is then oxidized
completely to CO2 by the
citric acid cycle (Chapter
15). The electrons from
these oxidations are
passed to O2 through a
Figure 14-3 Three possible catabolic fates of the pyruvate formed in the
chain of carriers in the
payoff phase of glycolysis. Pyruvate also serves as a precursor in many
mitochondrion, forming anabolic reactions, not shown here.
H2O. The energy from
the electron transfer
reactions drives the
synthesis of ATP in the
mitochondrion (Chapter
18).
The second route for
pyruvate metabolism is
its reduction to lactate via
lactic acid fermentation.
When a tissue such as
vigorously contracting
skeletal muscle must
function anaerobically,
the pyruvate cannot be
oxidized further for lack
of oxygen. Under these
conditions pyruvate is
reduced to lactate.
Certain tissues and cell
types (retina, brain,
erythrocytes) convert
glucose to lactate even
under aerobic conditions.
Lactate (the dissociated
form of lactic acid) is
also the product of
glycolysis under
anaerobic conditions in
microorganisms that
carry out the lactic acid
fermentation (Fig. 14-3).
The third major route for catabolism of pyruvate leads to ethanol. In some plant tissues
and in certain invertebrates, protists, and microorganisms such as brewer's yeast, pyruvate
is converted anaerobically into ethanol and CO2, a process called alcohol (or ethanol)
fermentation (Fig. 14-3).
The focus of this chapter is catabolism, but pyruvate has other, anabolic, fates. It can, for
example, provide the carbon skeleton for the synthesis of the amino acid alanine. We shall
return to these anabolic reactions of pyruvate in later chapters.
ATP Formation Coupled to Glycolysis During glycolysis some of the energy of the
glucose molecule is conserved in the form of ATP, while much remains in the product,
pyruvate. The overall equation for glycolysis is
Glucose + 2NAD+ + 2ADP + 2Pi
2 pyruvate + 2NADH + 2H+ +
2ATP + 2H2O (14-1)
For each molecule of glucose degraded to pyruvate, two molecules of ATP are generated
from ADP and Pi. We can now resolve the equation of glycolysis into two processes: (1)
the conversion of glucose into pyruvate, which is exergonic:
Glucose + 2NAD+
2 pyruvate + 2NADH + 2H+ (14-2) ,ΔG°'1 = -146 kJ/mol
and (2) the formation of ATP from ADP and Pi, which is endergonic:
2ADP + 2Pi
2ATP + 2H2O (14-3) ,ΔG°'2 = 2(30.5 kJ/mol) = 61 kJ/mol
If we now write the sum of Equations 14-2 and 14-3, we can also determine the overall
standard free-energy change of glycolysis (Eqn 14-1), including ATP formation, as the
algebraic sum, ΔG°'S, of ΔG°'l and ΔG°'2
ΔG°'s = ΔG°'1' + ΔG°'2 = -146 kJ/mol + 61 kJ/mol = -85 kJ/mol
Under either standard or intracellular conditions, glycolysis is an essentially irreversible
process, driven to completion by this large net decrease in free energy. At the actual
intracellular concentrations of ATP, ADP, and Pi (see Table 13-5) and of glucose and
pyruvate, the efficiency of recovery of the energy of glycolysis in the form of ATP is over
60%.
Energy Remaining in the Pyruuate Produced by Glycolysis Glycolysis releases only a
small fraction of the total available energy of the glucose molecule. When glucose is
oxidized completely to CO2 and H2O, the total standard free-energy change is -2,840
kJ/mol. The glycolytic degradation of glucose to two molecules of pyruvate (ΔG°'= -146
kJ/mol) therefore yields only (146/2,840)100 = 5.2% of the total energy that can be
released by complete oxidation. The two molecules of pyruvate formed by glycolysis still
contain most of the biologically available energy of the glucose molecule, energy that can
be extracted by oxidative reactions in the citric acid cycle.
Importance of Phosphorylated Intermediates Each of the nine glycolytic intermediates
between glucose and pyruvate is phosphorylated (Fig. 14-2). The phosphate groups appear
to have three functions.
1. The phosphate groups are ionized at pH 7, thus giving each of the intermediates of
glycolysis a net negative charge. Because the plasma membrane is impermeable to
molecules that are charged, the phosphorylated intermediates cannot diffuse out of the
cell. After the initial phosphorylation, the cell does not have to spend further energy in
retaining phosphorylated intermediates despite the large difference between the
intracellular and extracellular concentrations of these compounds.
2. Phosphate groups are essential components in the enzymatic conservation of metabolic
energy. Energy released in the breakage of phosphoric acid anhydride bonds (such as
those in ATP) is partially conserved in the formation of phosphate esters such as glucose6phosphate. High-energy phosphate compounds formed in glycolysis (1,3-
bisphosphoglycerate and phosphoenol pyruvate) donate phosphate groups to ADP to form
ATP.
3. Binding of phosphate groups to the active sites of enzymes provides binding energy
that contributes to lowering the activation energy and increasing the specificity of
enzyme-catalyzed reactions (Chapter 8). The phosphate groups of ADP, ATP, and the
glycolytic intermediates form complexes with Mg2+, and the substrate binding sites of
many of the glycolytic enzymes are specific for these Mg2+ complexes. Nearly all the
glycolytic enzymes require Mg2+ for activity.