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Introduction
The pentose phosphate pathway is primarily an anabolic pathway that utilizes the 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents. However, this pathway does oxidize glucose and under certain conditions can completely oxidize glucose to CO2 and water. The primary functions of this pathway are:
1. To generate reducing equivalents, in the form of NADPH, for reductive biosynthesis reactions within cells.
2. To provide the cell with ribose-5-phosphate (R5P) for the synthesis of the nucleotides and nucleic acids.
3. 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.
Enzymes that function primarily in the reductive direction utilize the NADP+/NADPH cofactor pair as co-factors as opposed to oxidative enzymes that utilize the NAD+/NADH cofactor pair. The reactions of fatty acid biosynthesis and steroid biosynthesis utilize large amounts of NADPH. As a consequence, cells of the liver, adipose tissue, adrenal cortex, testis and lactating mammary gland have high levels of the PPP enzymes. In fact 30% of the oxidation of glucose in the liver occurs via the PPP. Additionally, erythrocytes utilize the reactions of the PPP to generate large amounts of NADPH used in the reduction of glutathione (see below). The conversion of ribonucleotides to deoxyribonucleotides (through the action of ribonucleotide reductase) requires NADPH as the electron source, therefore, any rapidly proliferating cell needs large quantities of NADPH.
Although the PPP operates in all cells, with high levels of expression in the above indicated tissues, the highest levels of PPP enzymes (in particular glucose 6-phosphate dehydrogenase) are found in neutrophils and macrophages. These leukocytes are the phagocytic cells of the immune system and they utilize NADPH to generate superoxide radicals from molecular oxygen in a reaction catalyzed by NADPH oxidase. Superoxide anion, in turn, serves to generate other reactive oxygen species (ROS) the kill the phagocytized microorganisms. Following exposure to bacteria and other foreign substances there is a dramatic increase in O2 consumption by phagocytes. This phenomenon is referred to as the oxygen burst.
back to the topReactions of the Pentose Phosphate Pathway
The reactions of the PPP operate exclusively in the cytoplasm. From this perspective it is understandable that fatty acid synthesis (as opposed to oxidation) takes place in the cytoplasm. The pentose phosphate pathway has both an oxidative and a non-oxidative arm. The oxidation steps, utilizing glucose-6-phosphate (G6P) as the substrate, occur at the beginning of the pathway and are the reactions that generate NADPH. The reactions catalyzed by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase generate one mole of NADPH each for every mole of glucose-6-phosphate (G6P) that enters the PPP.
The non-oxidative reactions of the PPP are primarily designed to generate R5P. Equally important reactions of the PPP are to convert dietary 5 carbon sugars into both 6 (fructose-6-phosphate) and 3 (glyceraldehyde-3-phosphate) carbon sugars which can then be utilized by the pathways of glycolysis.
The primary enzymes involved in the non-oxidative steps of the PPP are transaldolase and transketolase:
Transketolase functions to transfer 2 carbon groups from substrates of the PPP, thus rearranging the carbon atoms that enter this pathway. Like other enzymes that transfer 2 carbon groups, transketolase requires thiamine pyrophosphate (TPP) as a co-factor in the transfer reaction.
Transaldolase transfers 3 carbon groups and thus is also involved in a rearrangement of the carbon skeletons of the substrates of the PPP. The transaldolase reaction involves Schiff base formation between the substrate and a lysine residue in the enzyme.
The net result of the PPP, if not used solely for R5P production, is the oxidation of G6P, a 6 carbon sugar, into a 5 carbon sugar. In turn, 3 moles of 5 carbon sugar are converted, via the enzymes of the PPP, back into two moles of 6 carbon sugars and one mole of 3 carbon sugar. The 6 carbon sugars can be recycled into the pathway in the form of G6P, generating more NADPH. The 3 carbon sugar generated is glyceraldehyde-3-phsphate which can be shunted to glycolysis and oxidized to pyruvate. Alternatively, it can be utilized by the gluconeogenic enzymes to generate more 6 carbon sugars (fructose-6-phosphate or glucose-6-phosphate).
back to the topMetabolic Disorders Associated with the PPP
Oxidative stress within cells is controlled primarily by the action of the peptide, glutathione, GSH. See Specialized Products of Amino Acids for the synthesis of GSH. GSH is a tripeptide composed of γ-glutamate, cysteine and glycine. The sulfhydryl side chains of the cysteine residues of two glutathione molecules form a disulfide bond (GSSG) during the course of being oxidized in reactions with various oxides and peroxides in cells. Reduction of GSSG to two moles of GSH is the function of glutathione reductase, an enzyme that requires coupled oxidation of NADPH.
The cysteine thiol of GSH plays the role in reducing oxidized thiols in other proteins. Oxidation of 2 cysteine thiols forms a disulfide bond. Although this bond plays a very important role in protein structure and function, inappropriately introduced disulfides can be detrimental. Glutathione can reduce disulfides nonenzymatically. Oxidative stress also generates peroxides that in turn can be reduced by glutathione to generate water and an alcohol, or 2 waters if the peroxide were hydrogen peroxide. Regeneration of reduced glutathione is carried out by the enzyme, glutathione reductase. This enzyme requires the co-factor NADPH when operating in the direction of glutathione reduction which is the thermodynamically favored direction of the reaction.
There are at least three inborn errors in the pentose phosphate pathway that have been identified. The most common being the result of mutations in glucose-6-phosphate dehydrogenase (G6PDH). Extremely rare occurrences of ribose-5-phosphate isomerase and transaldolase deficiency have also been documented. In the transaldolase deficiency individuals liver problems were the principal symptom in neonates. It should be clear that any disruption in the level of NADPH may have a profound effect upon a cells ability to deal with oxidative stress. No other cell than the erythrocyte is exposed to greater oxidizing conditions. After all it is the oxygen carrier of the body.
Because of the need for NADPH in phagocytic cells, by the NADPH oxidase system, any defect in enzymes in this process can result in impaired killing of infectious organisms. Chronic granulomatous disease (CGD; also known as Bridges-Good syndrome) is a syndrome that results in individuals harboring defects in the NADPH oxidase system. Individuals with CGD are at increased risk for specific recurrent infections. The most common are pneumonia, abscesses of the skin, tissues, and organs, suppurative arthritis (invasion of the joints by infectious agent leading to generation of pus), and osteomyelitis (infection of the bone). The majority of patients with CGD harbor mutations in a gene that encodes a component of the NADPH oxidase system. The encoded protein is the β-subunit of cytochrome b245 (gene symbol CYBB), also called p91-PHOX or NOX2. However, individuals with reduced ability to produce NADPH (such as those with G6PDH deficiencies) also manifest with CGD.
back to the topErythrocytes 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 the synthesis and role 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 wall 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.
Deficiency in the level of activity of glucose-6-phosphate dehydrogenase (G6PDH) is the basis of favism, primaquine (an anti-malarial drug) sensitivity and some other drug-sensitive hemolytic anemias, anemia and jaundice in the newborn, and chronic nonspherocytic hemolytic anemia. In addition, G6PDH deficiencies are 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.
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Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu
