MICROBIAL METABOLISM CONTD...
The Embden-Meyerhof or glycolytic pathway is undoubtedly
the most common pathway for glucose degradation to pyruvate in
stage two of catabolism. It is found in all major groups of microorganisms
and functions in the presence or absence of O2.
Glycolysis [Greek glyco, sweet, and lysis, a loosening] is located
in the cytoplasmic matrix of procaryotes and eucaryotes.
The pathway as a whole may be divided into two parts.
In the initial six-carbon stage, glucose
is phosphorylated twice and eventually converted to fructose 1,6-
bisphosphate. Other sugars are often fed into the pathway by conversion
to glucose 6-phosphate or fructose 6-phosphate. This preliminary
stage does not yield energy; in fact, two ATP molecules
are expended for each glucose. These initial steps “prime the
pump” by adding phosphates to each end of the sugar. The phosphates
will soon be used to make ATP.
The three-carbon stage of glycolysis begins when the enzyme
fructose 1,6-bisphosphate aldolase catalyzes the cleavage
of fructose 1,6-bisphosphate into two halves, each with a phosphate
group. One of the products, glyceraldehyde 3-phosphate, is
converted directly to pyruvate in a five-step process. Because the
other product, dihydroxyacetone phosphate, can be easily
changed to glyceraldehyde 3-phosphate, both halves of fructose
1,6-bisphosphate are used in the three-carbon stage. Glyceraldehyde
3-phosphate is first oxidized with NAD as the electron acceptor,
and a phosphate is simultaneously incorporated to give a
high-energy molecule called 1,3-bisphosphoglycerate. The highenergy
phosphate on carbon one is subsequently donated to ADPfrom a single glucose (one by way of dihydroxyacetone phosphate),
the three-carbon stage generates four ATPs and two NADHs per
glucose. Subtraction of the ATP used in the six-carbon stage from
that produced in the three-carbon stage gives a net yield of two ATPs
per glucose. Thus the catabolism of glucose to pyruvate in glycolysis
can be represented by the following simple equation.
Glucose 2ADP 2Pi 2NAD+
2 pyruvate 2ATP 2NADH 2H+
The Pentose Phosphate Pathway
A second pathway, the pentose phosphate or hexose monophosphate
pathway may be used at the same time as the glycolytic
pathway or the Entner-Doudoroff sequence. It can operate either
aerobically or anaerobically and is important in biosynthesis as
well as in catabolism.
The pentose phosphate pathway begins with the oxidation of
glucose 6-phosphate to 6-phosphogluconate followed by the oxidation
of 6-phosphogluconate to the pentose ribulose 5-phosphate and
CO2 (figure 9.6 and appendix II). NADPH is produced during these
oxidations. Ribulose 5-phosphate is then converted to a mixture of
three- through seven-carbon sugar phosphates. Two enzymes
unique to this pathway play a central role in these transformations:
(1) transketolase catalyzes the transfer of two-carbon ketol groups,
and (2) transaldolase transfers a three-carbon group from sedoheptulose
7-phosphate to glyceraldehyde 3-phosphate (figure 9.7). The
overall result is that three glucose 6-phosphates are converted to two
fructose 6-phosphates, glyceraldehyde 3-phosphate, and three CO2
molecules, as shown in the following equation.
3 glucose 6-phosphate 6NADP+ 3H2O
2 fructose 6-phosphate glyceraldehyde 3-phosphate
3CO2 6NADPH 6H+
These intermediates are used in two ways. The fructose 6-
phosphate can be changed back to glucose 6-phosphate while
glyceraldehyde 3-phosphate is converted to pyruvate by glycolytic
enzymes. The glyceraldehyde 3-phosphate also may be returned
to the pentose phosphate pathway through glucose 6-phosphate
formation. This results in the complete degradation of glucose 6-phosphate to CO2 and the production of a great deal of NADPH.
Glucose 6-phosphate 12NADP+ + 7H2O
6CO2 12NADPH 12H+ Pi
The pentose phosphate pathway has several catabolic and anabolic
functions that are summarized as follows:
1. NADPH from the pentose phosphate pathway serves as a
source of electrons for the reduction of molecules during
2. The pathway synthesizes four- and five-carbon sugars for a
variety of purposes. The four-carbon sugar erythrose
4-phosphate is used to synthesize aromatic amino acids and
vitamin B6 (pyridoxal). The pentose ribose 5-phosphate
is a major component of nucleic acids, and ribulose
1,5-bisphosphate is the primary CO2 acceptor in
photosynthesis. Note that when a microorganism is growing on a pentose carbon source, the pathway also can supply
carbon for hexose production (e.g., glucose is needed for
3. Intermediates in the pentose phosphate pathway may be used
to produce ATP. Glyceraldehyde 3-phosphate from the
pathway can enter the three-carbon stage of the glycolytic
pathway and be converted to ATP and pyruvate. The latter
may be oxidized in the tricarboxylic acid cycle to provide
more energy. In addition, some NADPH can be converted to
NADH, which yields ATP when it is oxidized by the electron
transport chain. Because five-carbon sugars are intermediates
in the pathway, the pentose phosphate pathway can be used
to catabolize pentoses as well as hexoses.
Although the pentose phosphate pathway may be a source of
energy in many microorganisms, it is more often of greater importance
in biosynthesis. Several functions of the pentose phosphate
pathway are when biosynthesis is considered more directly.