2
Ν. G. PON
I. General Considerations
Without exception, all living things require energy to support life.
They derive this energy either from photosynthesis or from the oxidation
of inorganic or organic compounds. Those organisms which oxidize
organic compounds often utilize glucose as the source of energy, and
they generally oxidize glucose via glycolysis (J). [The term glycolysis is
used here to describe the Embden-Meyerhof-Parnas (EMP) pathway,
although the original meaning of glycolysis is simply the breakdown of
glucose (2).] There are, however, certain organisms which degrade
glucose by alternate pathways, or which, under certain conditions, have
to call upon alternate pathways for the metabolism of glucose. One such
alternate pathway is the pentose phosphate cycle. To complicate matters
further, there exist at least two types of pentose phosphate cycles. The
one referred to above is the oxidative pentose phosphate cycle and is a
catabolic pathway. The other cycle is a synthetic pathway and is sometimes called the reductive pentose phosphate cycle. Both cycles are still
being actively studied, as is evident from the enormous mass of literature accumulated to date. Fortunately, a number of excellent reviews
over the past decade have been made available by Horecker and Mehler
(3), Racker (4), Axelrod and Beevers (5), Gibbs (6), and many others.
II. Description of the Pentose Phosphate Cycle
The oxidative pentose phosphate cycle (7) comes under the guise of
many aliases. Notable among them are such names as the hexose monophosphate shunt, the phosphogluconate oxidative pathway, the WarburgLipmann-Dickens pathway, the direct oxidative pathway (δ). These
names, however, do not describe the operation of this cycle completely.
In brief, the operation is as follows: A hexose molecule enters the pathway in the form of its monophosphate. The latter is oxidized to an
aldonolactone, which is then decarboxylated oxidatively to a pentose
phosphate. The pentose phosphate rearranges and condenses with
another pentose phosphate to form a heptulose monophosphate and a
triose phosphate. These two compounds eventually are reconverted to a
hexose monophosphate and a tetrose phosphate. The pathway is shown
in detail in Fig. 1. Noteworthy is the production of reduced pyridine
nucleotides during the operation of the cycle.
The reductive pentose phosphate cycle (9) is also a cycle of many
names, two of them being the photosynthetic carbon reduction cycle
(10) and the Calvin cycle (11). [A slight difference between the reductive pentose phosphate cycle and the carbon reduction cycle of photosynthesis has been noted by Krebs and Kornberg (12). This difference
Ν. G. PON
I. General Considerations
Without exception, all living things require energy to support life.
They derive this energy either from photosynthesis or from the oxidation
of inorganic or organic compounds. Those organisms which oxidize
organic compounds often utilize glucose as the source of energy, and
they generally oxidize glucose via glycolysis (J). [The term glycolysis is
used here to describe the Embden-Meyerhof-Parnas (EMP) pathway,
although the original meaning of glycolysis is simply the breakdown of
glucose (2).] There are, however, certain organisms which degrade
glucose by alternate pathways, or which, under certain conditions, have
to call upon alternate pathways for the metabolism of glucose. One such
alternate pathway is the pentose phosphate cycle. To complicate matters
further, there exist at least two types of pentose phosphate cycles. The
one referred to above is the oxidative pentose phosphate cycle and is a
catabolic pathway. The other cycle is a synthetic pathway and is sometimes called the reductive pentose phosphate cycle. Both cycles are still
being actively studied, as is evident from the enormous mass of literature accumulated to date. Fortunately, a number of excellent reviews
over the past decade have been made available by Horecker and Mehler
(3), Racker (4), Axelrod and Beevers (5), Gibbs (6), and many others.
II. Description of the Pentose Phosphate Cycle
The oxidative pentose phosphate cycle (7) comes under the guise of
many aliases. Notable among them are such names as the hexose monophosphate shunt, the phosphogluconate oxidative pathway, the WarburgLipmann-Dickens pathway, the direct oxidative pathway (δ). These
names, however, do not describe the operation of this cycle completely.
In brief, the operation is as follows: A hexose molecule enters the pathway in the form of its monophosphate. The latter is oxidized to an
aldonolactone, which is then decarboxylated oxidatively to a pentose
phosphate. The pentose phosphate rearranges and condenses with
another pentose phosphate to form a heptulose monophosphate and a
triose phosphate. These two compounds eventually are reconverted to a
hexose monophosphate and a tetrose phosphate. The pathway is shown
in detail in Fig. 1. Noteworthy is the production of reduced pyridine
nucleotides during the operation of the cycle.
The reductive pentose phosphate cycle (9) is also a cycle of many
names, two of them being the photosynthetic carbon reduction cycle
(10) and the Calvin cycle (11). [A slight difference between the reductive pentose phosphate cycle and the carbon reduction cycle of photosynthesis has been noted by Krebs and Kornberg (12). This difference