Pentose phosphate pathway

Pentose phosphate pathway

The pentose phosphate pathway (also called Phosphogluconate Pathway, or HexoseMonophosphate Shunt [HMP shunt] ) is a process that serves to generate NADPH and the synthesis of pentose (5-carbon) sugars. There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars. This pathway is an alternative to glycolysis. While it does involve oxidation of glucose, its primary role is anabolic rather than catabolic. For most organisms it takes place in the cytosol; in plants most steps take place in plastids. [cite journal |author=Kruger NJ, von Schaewen A |title=The oxidative pentose phosphate pathway: structure and organisation |journal=Curr. Opin. Plant Biol. |volume=6 |issue=3 |pages=236–46 |year=2003 |month=June |pmid=12753973 |doi= |url=]


The primary functions of the pathway are:

*To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells.
*To provide the cell with ribose-5-phosphate (R5P) for the synthesis of the nucleotides and nucleic acids.
*Although not a significant function of the PPP, it can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary carbohydrates into glycolytic/gluconeogenic intermediates.

Located exclusively in the cytoplasm, the pathway is one of the three main ways the body creates molecules with reducing power, accounting for approximately 60% of NADPH production in humans.

One of the uses of NADPH in the cell is to prevent oxidative stress. It reduces the coenzyme glutathione, which converts reactive H2O2 into H2O. If absent, the H2O2 would be converted to hydroxyl free radicals, which can attack the cell.

Significantly, erythrocytes utilize the reactions of the PPP to generate large amounts of NADPH used in the reduction of glutathione

It is also used to generate hydrogen peroxide for phagocytes. [GeorgiaImmunology|1/cytotox]


Oxidative phase

In this phase, two molecules of NADP+ are reduced to NADPH, utilizing the energy from the conversion of glucose-6-phosphate into ribulose 5-phosphate.

The entire set of reactions can be summarized as follows:

The overall reaction for this process is::Glucose 6-phosphate + 2 NADP+ + H2O → ribulose 5-phosphate + 2 NADPH + 2 H+ + CO2

Non-oxidative phase


Glucose-6-phosphate dehydrogenase is the rate-controlling enzyme of this pathway. It is allosterically stimulated by NADP+. The ratio of NADPH:NADP+ is normally about 100:1 in liver cytosol. This makes the cytosol a highly-reducing environment. Formation of NADP+ by a NADPH-utilizing pathway, thus, stimulates production of more NADPH.

ee also

* G6PDH deficiency - A hereditary disease that disrupts the pentose phosphate pathway
* thiamine deficiency

Erythrocytes and the Pentose Phosphate Pathway

The predominant pathways of carbohydrate metabolism in the red blood cell (RBC) are glycolysis, the PPP and 2,3-bisphosphoglycerate (2,3-BPG) metabolism (refer to discussion of hemoglobin for review of role of 2,3-BPG). Glycolysis provides ATP for membrane ion pumps and NADH for re-oxidation of methemoglobin. The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione. The inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H2O2, that in turn results in a weakening of the cell membrane and concomitant hemolysis. Accumulation of H2O2 also leads to increased rates of oxidation of hemoglobin to methemoglobin that also weakens the cell wall. Glutathione removes peroxides via the action of glutathione peroxidase. The PPP in erythrocytes is essentially the only pathway for these cells to produce NADPH. Any defect in the production of NADPH could, therefore, have profound effects on erythrocyte survival. Several deficiencies in the level of activity (not function) of glucose-6-phosphate dehydrogenase have been observed to be associated with resistance to the malarial parasite, Plasmodium falciparum, among individuals of Mediterranean and African descent. The basis for this resistance is the weakening of the red cell membrane (the erythrocyte is the host cell for the parasite) such that it cannot sustain the parasitic life cycle long enough for productive growth. [ [ Pentose Phosphate Pathway ] ]


External links

* [ The chemical logic behind the pentose phosphate pathway]

Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • pentose phosphate pathway — a major branching of the Embden Meyerhof pathway of carbohydrate metabolism: a pathway of hexose oxidation in which glucose 6 phosphate undergoes two successive oxidations by NADP, the final one being an oxidative decarboxylation to form a… …   Medical dictionary

  • pentose phosphate pathway — (= pentose shunt; hexose monophosphate pathway; phosphogluconate oxidative pathway) Alternative metabolic route to Embden Meyerhof pathway for breakdown of glucose. Diverges from this when glucose 6 phosphate is oxidized to ribose 5 phosphate by… …   Dictionary of molecular biology

  • pentose phosphate pathway — (pen tōs) The pathway that oxidizes glucose 6 phosphate to ribulose 5 phosphate and then converts it to a variety of three to seven carbon sugars; it forms several important products (NADPH for biosynthesis, pentoses, and other sugars) and also… …   Dictionary of microbiology

  • pentose phosphate pathway — noun an alternative series of biochemical reactions to glycolysis for oxidising glucose, important for the synthesis of pentoses …  

  • pentose shunt — pentose phosphate pathway …   Medical dictionary

  • Pentose — A pentose is a monosaccharide with five carbon atoms.They either have an aldehyde functional group in position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses).The aldopentoses have three chiral centres ( asymmetric… …   Wikipedia

  • pathway — 1. A collection of axons establishing a conduction route for nerve impulses from one group of nerve cells to another group or to an effector organ composed of muscle or gland cells. 2. Any sequence of chemical reactions leading from one …   Medical dictionary

  • Glucose-6-phosphate — Chembox new ImageFileL1=Glucose 6 phosphate skeletal.png ImageFileR1=Beta D glucose 6 phosphate 3D balls.png ImageSize= IUPACName=(3,4,5,6 tetrahydroxytetrahydropyran 2 yl) methoxyphosphonic acid OtherNames= Section1= Chembox Identifiers CASNo=56 …   Wikipedia

  • Metabolic pathway — In biochemistry, metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary… …   Wikipedia

  • Ribose 5-phosphate — IUPAC name (2,3,4 Trihydroxy 5 oxo pentoxy)phosphonic acid …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”