pentose-phosphate shunt

Definition

Target type: biologicalprocess

The metabolic process in which glucose-6-phosphate is oxidized to form carbon dioxide (CO2) and ribulose 5-phosphate, coupled to reduction of NADP+ to NADPH; ribulose 5-P then enters a series of reactions that can yield biosynthetic precursors (ribose-5-phosphate and erythrose-4-phosphate) and glycolytic intermediates (fructose-6-phosphate and glyceraldehyde-3-phosphate). [GOC:pde, ISBN:0198506732, MetaCyc:PENTOSE-P-PWY]

The pentose phosphate pathway (PPP), also known as the hexose monophosphate shunt, is an alternative glucose catabolic pathway that operates alongside glycolysis. Unlike glycolysis, which primarily focuses on ATP production, the PPP primarily produces NADPH, a reducing agent essential for anabolic reactions, and pentose sugars, primarily ribose-5-phosphate, crucial for nucleotide biosynthesis.

The PPP consists of two distinct phases: the oxidative phase and the non-oxidative phase.

**Oxidative Phase:**

1. Glucose-6-phosphate dehydrogenase (G6PD) catalyzes the first committed step, oxidizing glucose-6-phosphate to 6-phosphogluconolactone using NADP+ as an electron acceptor, generating NADPH.
2. 6-Phosphogluconolactonase hydrolyzes the lactone ring, yielding 6-phosphogluconate.
3. 6-Phosphogluconate dehydrogenase further oxidizes 6-phosphogluconate to ribulose-5-phosphate, again using NADP+ as an electron acceptor and producing NADPH.

This phase generates two molecules of NADPH and one molecule of ribulose-5-phosphate per glucose-6-phosphate molecule.

**Non-Oxidative Phase:**

1. Ribulose-5-phosphate isomerase converts ribulose-5-phosphate to ribose-5-phosphate, a precursor for nucleotide biosynthesis.
2. Ribose-5-phosphate epimerase converts ribose-5-phosphate to xylulose-5-phosphate.
3. Transketolase and transaldolase catalyze a series of complex rearrangements, interconverting sugars with 3, 4, 5, 6, and 7 carbons. These reactions allow the pathway to produce the necessary amounts of ribose-5-phosphate, as well as glycolytic intermediates like fructose-6-phosphate and glyceraldehyde-3-phosphate, enabling the pathway to be integrated with glycolysis.

**Functions of the Pentose Phosphate Shunt:**

1. **NADPH Production:** NADPH is essential for reducing power in various anabolic reactions, including fatty acid biosynthesis, steroid hormone biosynthesis, and detoxification reactions, particularly in red blood cells to protect against oxidative damage.
2. **Ribose-5-phosphate Production:** Ribose-5-phosphate is a precursor for nucleotide biosynthesis, crucial for DNA, RNA, and coenzyme synthesis.
3. **Intermediates for Glycolysis:** The non-oxidative phase can generate glycolytic intermediates, allowing the PPP to contribute to glucose metabolism and ATP production.
4. **Red Blood Cell Function:** The PPP is particularly active in red blood cells, where NADPH is essential for reducing oxidative stress caused by reactive oxygen species.

The PPP plays a vital role in maintaining cellular redox balance, providing building blocks for biosynthesis, and contributing to overall energy metabolism.'"

Proteins (4)

Compounds (37)