Structural Biochemistry/The Pentose Phosphate Pathway
As we all have known that cellular energy is inevitably important for metabolism of the body in all organisms, one important factor controlling all those metabolic pathways is enzymes. Most enzymes are proteins, which are synthesized by ribosomes in the cytoplasm. The precursor for protein synthesis are amino acids, which are obtains from the translation of RNA's, which come from the transcription of DNA. Therefore, DNA and RNA play major roles in metabolism. In order to synthesize DNA and RNA, there is one important precursor, which is Pentose Phosphate. There is a pathway which produces Pentose Phosphate and NADPH (which has a phosphate group at 2'-C of NADH instead of the hydroxyl group)
The pathway has 6 steps:
- It starts with Glucose 6-Phosphate, Glu 6-P, which comes from other pathways, glycolysis for example.
- Instead of using the enzyme glucose 6-phosphate isomerase to isomerize the original reactant into fructose 6-phosphate for glycolysis, cells use another enzyme, glucose 6-phosphate dehydrogenase, and one important cofactor, NADP+, to oxidize the Glu 6-P into 6-phosphoglucono-δ-lactone with NADP+ being reduced to NADPH.
- Next, the enzyme lactonase hydrolyzes 6-phosphoglucono-δ-lactone into 6-phosphogluconate.
- In this step, cofactor NADP+ is used again as an oxidizing agent to oxidize 6-phosphogluconate to ribulose 5-phosphate in a reaction catalyzed by the enzyme 6-phosphogluconate dehydrogenase with the reduced NADPH as another product and the release of carbon dioxide.
- Note that in every step of the pathway, an addition of Magnesium cation helps stabilizing the reactions, which involves releases of electrons and protons.
- In a ketose-aldose reaction catalyzed by the enzyme phosphopentose isomerase, ribulose 5-phosphate is isomerized into ribose 5-phosphate, a precursor for later important reactions, such as DNA synthesis.
The Pentose Phosphate pathway produces to important precursors for basic building blocks of life:
- The production of ribose for the synthesis of some cofactors, DNA, RNA.
- The production of NADPH for the synthesis of fatty acids, steroids, and other oxidation-reduction reactions (some occur in photosynthesis, especially in the Calvin cycle)