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Pentose phosphate pathway - Biochemistry
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BIOCHEMISTRY – LECTURE 10 PART 2pentose phosphate pathway
The pentose phosphate pathway is the pathway that catabolises glucose 6-
phosphate to form a pentose phosphate (and CO 2). Pentose phosphates
(including ribose 5-phosphate and many derived from it), are required for the
synthesis of nucleic acids, ATP, co-factors and precursors to amino acids. In the
pathway, nicotinamide adenine dinucleotide phosphate is reduced from the
oxidised form NADP+ to form NADPH. NADPH is an important electron donor in
the reductive biosynthesis of many biomolecules, such as fatty acids and steroid
hormones. It is also a vital reducing agent in cells to protect them from oxidative
damage.
The importance of the pathway is due to the essential nature of these products.
For some cells, it is the only mechanism to generate these molecules. Ribose 5-
phosphate is vital, but is also an intermediate for the other important molecules.
NADPH is very similar is very similar in structure to NADH, but has an additional
phosphate group.
The oxidative stage has 3 steps:
Oxidation
Ring opening
Oxidative decarboxylation
The non-oxidative stage involves the conversion of ribulose 5-phosphate to the
more useful ribose 5-phosphate or xyulose 5-phosphate. Subsequent steps form
products dependent on the needs of the cell. Only the oxidative stage generates
NADPH.
There are 3 enzyme catalysed steps in the first stage.
o In the first step glucose 6-phosphate is dehydrogenated, losing two
hydrogens to form 6-phosphoglucono--lactone and NADPH. The reaction
is catalysed by glucose 6-phosphate dehydrogenase. It is the slowest step
in the pathway and is the control point (by controlling the activity of this
enzyme cells control the pathway).
o The second step, is a hydrolysis reaction, where the enzyme lactonase
catalyses the formation of 6-phosphogluconate from 6-phosphoglucono--
lactone. The ring structure is split open.
o In the third and final step of the oxidative stage, 6-phosphogluconate is
decarboxylated (oxidatively), giving up a carbon dioxide molecule to form
the pentose phosphate, ribulose 5-phosphate. Like in the first step, NADP+
acts as the electron acceptor to form NADPH. This reaction is catalysed by
6-phosphogluconate dehydrogenase. A hydrogen is also lost from 6-
phosphogluconate in the reaction.
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