Pentoosifosfaattirata

Pentoosifosfaattirata, also known as the pentose phosphate pathway (PPP) or the hexose monophosphate shunt, is a metabolic pathway parallel to glycolysis that generates NADPH and precursor molecules for nucleotide biosynthesis. It is a fundamental process in all living organisms, playing crucial roles in cellular redox balance and biosynthesis. The pathway occurs primarily in the cytoplasm of cells.

The pentose phosphate pathway has two distinct phases: the oxidative phase and the non-oxidative phase. The oxidative phase is irreversible and produces NADPH, a vital reducing agent used in various anabolic processes like fatty acid synthesis and nucleotide synthesis, as well as for protecting cells against oxidative stress. During this phase, glucose-6-phosphate is converted into ribulose-5-phosphate, with the generation of two molecules of NADPH.

The non-oxidative phase is a reversible series of reactions that interconverts various phosphorylated sugars, including fructose-6-phosphate and glyceraldehyde-3-phosphate, which can then re-enter glycolysis. This phase is responsible for the production of pentose phosphates, particularly ribose-5-phosphate, which is essential for the synthesis of nucleotides and nucleic acids (DNA and RNA). It also produces erythrose-4-phosphate, a precursor for the synthesis of aromatic amino acids.

The pentose phosphate pathway is tightly regulated to meet the cell's specific needs for NADPH and pentose phosphates. Its importance is highlighted in red blood cells, where it is the sole source of NADPH, crucial for maintaining the reduced state of glutathione, which protects against oxidative damage. In actively growing cells or tissues involved in lipid synthesis, the pathway is particularly active.