Pentosephosphatweg

The Pentose Phosphate Pathway, also known as the hexose monophosphate shunt or the phosphogluconate pathway, is a metabolic route parallel to glycolysis. It plays a crucial role in cellular redox homeostasis and biosynthesis. The pathway's primary function is to generate NADPH, a reducing agent essential for various biosynthetic processes and for protecting cells against oxidative stress. It also produces pentoses, particularly ribose-5-phosphate, a key precursor for nucleotide and nucleic acid synthesis.

The pathway occurs in the cytoplasm of eukaryotic cells and is active in tissues with high biosynthetic activity, such as the liver, adipose tissue, adrenal cortex, and mammary glands. It can be divided into two phases: the oxidative phase and the non-oxidative phase. The oxidative phase, which is irreversible, involves the conversion of glucose-6-phosphate to ribulose-5-phosphate, producing two molecules of NADPH. The key enzymes in this phase are glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase.

The non-oxidative phase is reversible and interconverts various phosphorylated sugars, including pentoses, hexoses, and heptoses. This phase allows for the synthesis of ribose-5-phosphate for nucleotide synthesis and can also regenerate glucose-6-phosphate, thus linking the pentose phosphate pathway back to glycolysis. Transketolase and transaldolase are the key enzymes in the non-oxidative phase. The pentose phosphate pathway is tightly regulated, with NADPH serving as a key allosteric inhibitor. Its products are vital for reducing equivalents and building blocks, highlighting its central importance in cellular metabolism.