Pentose phosphate pathway (PPP or HMPP) Ferchmin 2018 Pentose phosphate pathway (PPP or HMPP) Index by slide numbers 2) Pathway description 3) NADP and NADP 4) Oxidative step of PPP 5) Glucose-6-Phosphate Dehydrogenase Deficiency 6) Nonoxidative step of PPP 7) Regulation of PPP 8) Function of PPP 9-11) Glutathione, and its role as antioxidant. 11) Role of Selenocysteine

Pentose phosphate pathway (PPP) or shunt is also known as the hexose monophosphate pathway and consists of two parts: 1) Glucose-6-P undergoes two oxidations by NADP+, and the second is an oxidative decarboxylation that forms a pentose-P. 2) The P-pentoses that are formed during the first part are either rearranged into glucose-6-P or used for nucleotide synthesis. The PPP is entirely different from glycolysis, it does not generate ATP but produces reduced NADPH+H+. In glycolysis, there was no net oxidation/reduction only rearrangement of the redox state of the carbons of glucose, but in PPP the carbon #1 of glucose is oxidized to CO2. The primary regulatory enzyme is glucose-6-phosphate dehydrogenase which uses NADP+

Comparison of NAD with NADP Although both coenzymes look deceptively similar, they have a different metabolic role. NADP is involved in anabolic functions while NAD in catabolic pathways.The third figure shows the shared molecular mechanism of oxidation-reduction of their nicotinamide moiety.

GPDH deficiency is relatively common in persons with roots from regions with endemic malaria. In glycolysis followed by TCA (Kreb’s) cycle the first Cs to be converted to CO2 are # 3 and 4. In PPP is C #1.

Glucose-6-Phosphate Dehydrogenase Deficiency Glucose-6-phosphate dehydrogenase (G6PDH) deficiency is an inherited condition caused by a defect or defects in the gene that codes for glucose-6-phosphate dehydrogenase (G6PD). It can cause hemolytic anemia, variable in severity from life-long anemia, to rare bouts of anemia to total unawareness of the condition. The episodes of hemolytic anemia can be triggered by common medicines, oxidants, infection, or by eating fava beans or other foods. G6PD deficiency is the most common enzyme deficiency in the world, with about 400 million people living with it. It is most prevalent in people of African, Mediterranean, and Asian ancestry. The incidence in different populations varies from zero in South American Indians to less than 0.1% of Northern Europeans to about 50% of Kurdish males. In the United States, it is most common among African American males; about 11 to 14% are G6PD-deficient. G6PD deficiency is a sex-linked recessive trait. Thus, males have only one copy of the G6PD gene, but females have two copies. Recessive genes are masked in the presence of a gene that encodes normal G6PD. Accordingly, females with one copy of the gene for G6PD deficiency are usually normal, while males with one copy express the phenotype G6PD is present in all human cells but is particularly important to red blood cells. It is required to make NADPH in red blood cells and maintain the RBC reduced. It is also required to make glutathione. Glutathione and NADPH both help protect red blood cells against oxidative damage. Thus, when G6PD is defective, and the demand for NADPH is too high oxidative damage to red blood cells readily occurs causing hemolysis and hemolytic anemia. As of 1998, there are almost 100 different known forms of G6PD enzyme molecules encoded by defective G6PD genes, yet not one of them is completely inactive. Suggesting that the activity of G6PD is indispensable for supporting life. Many G6PD defective enzymes are deficient in their stability rather than their initial ability to function. Since red blood cells lack nuclei, they, unlike other cells, cannot synthesize new enzyme molecules to replace defective ones. Hence, we expect young red blood cells to have new, functional G6PD and older cells to have non-functioning G6PD. This explains why episodes of hemolytic anemia are frequently self-limiting; new red blood cells are generated with enzymes able to afford protection from oxidation.

2) Nonoxidative steps of pentose phosphate shunt transketolase requires thiamine pyrophosphate (vitamin B1) transketolase requires thiamine pyrophosphate (vitamin B1) It seems counterintuitive that ylulose-5-P would regulate fatty acid synthesis! Positive regulator of lipid synthesis

GSSG is oxidized and GSH reduced glutathione GSSG is oxidized and GSH reduced glutathione. This will be discussed in the next page Although glutathione is not part of the PPP it is a regulator of glucose-6-P dehydrogenase and is reduced by the NADPH+H+ generated in the PPP. Glutathione is a tripeptide comprised of three amino acids (cysteine, glutamic acid, and glycine) present in most mammalian tissues. Glutathione acts as an antioxidant, a free radical scavenger, and a detoxifying agent. Glutathione is also essential as a cofactor for the enzyme glutathione peroxidase, in the uptake of amino acids, and in the synthesis of leukotrienes. As a substrate for glutathione S-transferase, this agent reacts with many harmful chemical species, such as halides, epoxides, and free radicals, to form harmless inactive products. In erythrocytes, these reactions prevent oxidative damage through the reduction of methemoglobin and peroxides. Glutathione is also involved in the formation and maintenance of disulfide bonds in proteins and the transport of amino acids across cell membranes.

Notice the unusual bonds between cysteine and glutamate. Glutathione synthesis is not ribosomal and glutathione is not directly encoded in DNA. Notice the unusual bonds between cysteine and glutamate. What is wrong with this bond?

Link between hexose monophosphate pathway and reduction of peroxides Hydrogen peroxide (H2O2) is one member of the family of reactive oxygen species (ROS). Reactive oxygen species (ROS) are formed from partial reduction of molecular O2 i.e. adding electrons to oxygen leading to the formation of superoxide, hydrogen peroxide and hydroxyl radical. ROS are formed continuously from aerobic metabolism of drugs and environmental toxins or diminished antioxidants. All these lead to oxidative stress. ROS cause damage to DNA, protein and unsaturated lipids of the cells including cell membranes.. They are implicated in cancer, chronic inflammatory disease and aging.

Proposed mechanism of glutathione peroxidase Glutathione peroxidase has selenocysteine a rare amino acid that contains selenium. The story of selenocysteine incorporation into proteins is unusual. There are many antioxidants that neutralize the oxygen reactive species (ROS). Among them are: vitamins C and E and recently the tomato red pigment, lycopene. Lycopene, became notorious for reportedly preventing prostate cancer and retinal macula degeneration. Proposed mechanism of glutathione peroxidase Reactive oxygen species, such as superoxide and hydrogen peroxide, are generated in all cells by mitochondrial and enzymatic sources. Left unchecked, these reactive species can cause oxidative damage to DNA, proteins, and membrane lipids. Glutathione peroxidase is an intracellular antioxidant enzyme that enzymatically reduces peroxides and limits its harmful effects. However, certain reactive oxygen species, such as hydrogen peroxide, are also essential for growth factor-mediated signal transduction, mitochondrial function, and maintenance of normal thiol redox-balance. Therefore, reductive stress has been known for some time and lack of cellular oxidants can diminish cell growth. Newer evidence points to additional cellular and physiological effects caused by lack of cellular oxidants and accumulation of excess reducing equivalents, including changes in protein disulfide bond formation, diminished mitochondrial function, and decreased cellular metabolism. Although, to date, a complete understanding of physiological conditions that may create reductive stress have not been elucidated