What is the Pentose Phosphate Pathway?

The pentose phosphate pathway is a sequence of events a cell uses to convert a type of glucose into other molecules. The pathway uses several steps and different enzymes to achieve this. The products of the pentose phosphate pathway include molecules commonly known as NADPH, which donate electrons to other molecules in reducing reactions, and pentose molecules, which are used as building blocks for nucleic material.

The pentose phosphate pathway can be divided into two separate phases. The first phase is irreversible and involves taking electrons and a carbon atom from glucose-6-phosphate. This phase has two reaction steps. The second phase is reversible and converts the product of the first step into alternate sugar molecules.

The pathway begins with a glucose-6-phosphate molecule. This molecule is converted to 6-phosphogluconolactone by the enzyme glucose-6-phosphate dehydrogenase, losing two electrons in the process. The electrons are used to change a molecule of nicotinamide adenine dinucleotide phosphate (NADP) to its reduced form, NADPH, through a process called oxidization. The second step uses a water (H₂O) molecule to help remove a carbon from the 6-phosphogluconolactone product of the first step. During this reaction, catalyzed by the enzyme 6-phosphogluconolactone-dehydrogenase, a carbon dioxide (CO₂) molecule is released and another NADP gains electrons and is converted to NADPH.

The product of the first phase of the pentose phosphate pathway is ribulose-5-phosphate. This molecule can be changed into several useful substances in the second phase of the pathway. The ribulose-5-phosphate can be structurally changed, without altering the molecular weight, to ribose-5-phosphate, which is used in making nucleotides and deoxynucleotides, which are the building blocks of genetic material. Ribulose-5-phosphate can also be converted into xylulose-5-phosphate, which is also used for making nucleic material.

These five-carbon sugar molecules can further be used to produce sugars with six or three carbon atoms called fructose-6-phosphate and glyceraldehyde-3-phosphate. This occurs if the cell needs NADPH more than it needs ribose-5-phosphate. These six- and three-carbon sugars can also be used to produce glucose again if the cell requires it. The pathway can also operate in reverse, with the six- and three-carbon sugars converting into ribose-5-phosphate, if necessary.

The pentose phosphate pathway is quite active in fatty tissue and mammalian red blood cells. It is active in fatty tissue because, to break down energy sources into glucose, NADPH electron donation is necessary, so NADPH levels must be maintained. Mammalian blood cells use the pathway for a slightly different reason. The NADPH produced keeps a molecule called glutathione in a form that helps prevent hemoglobin iron from being oxidized. The reduced form of hemoglobin is more effective at binding oxygen than the oxidized form.