Function of PPP: 1. NADPH production i)Reducing power carrier in biosynthetic pathways such as fatty acid and cholesteraol synthesis ii)Role in glutathione cellular antioxidant system 2. Ribose synthesis i) Nucleic acids and nucleotides 3. Gylcolytic intermediates
Characteristics of PPP: Oxidative and Non-oxidative Phases Pentose Phosphate Pathway has two Phases: 1. Oxidative phases – Reactions producing NADPH – Irreversible 2. Non-oxidative phases – Produces ribose-5-P (Pentose sugar) – Produces intermediates of glycolytic pathway such as Fructose 6-P & Glyceraldehyde 3-P – Reversible reactions
NADPH producing reactions
Pentose phosphate and glycolytic intermediates producing reactions Non-oxidative phase of Pentose Phosphate Pathway
Production of superoxide in RBC – Hb-Fe 2+ - O 2 Hb-Fe 3+ + O 2 -. (superoxide anion) 2O 2 -. + 2H + H 2 O 2 (Hydrogen peroxide) (Superoxide dismutase) Both O 2 -. & H 2 O 2 can produce reactive free radical species such as hydroxyl radicals (OH. ) and cause oxidative injury by damaging cellular proteins, DNA, cell membranes and lysis Reactive Oxygen Species Reactive oxygen species are highly reactive derivatives of oxygen that are formed by the transfer of electrons from redox active molecules (with high electron transfer potential) to molecular oxygen. Example : Superoxide anion, Hydrogen peroxide
Detoxification of Superoxide Anion and Hydrogen Peroxide Antioxidant enzymes – Superoxide dismutase (removes superoxide anion) – Catalase (removes hydrogen peroxide) – Glutathione peroxidase (removes hydrogen peroxide using reduced glutathione [ GSH)) – Glutathione reductase (regenerates reduced glutathione (GSH) from oxidized glutathine (GS-SG) using NADPH) Cells have a number of antioxidant enzymes which remove reactive oxygen species and therefore protect the cells from oxidative injury
G6PDH Deficiency and Hemolytic Anemia Hemolytic anemia is a disorder in which the red blood cells are destroyed prematurely. Hemolytic anemia is often inherited, such as Glucose-6-Phosphate Dehydrogenase deficiency (G6PD). G6PD enzyme functions in the Pentose-Monophosphate shunt and catalyzes the production of NADPH which is required in the detoxification of the harmful oxidant H 2 O 2 by glutathione antioxidant system. Red cells deficient in G6PD are unable to produce NADPH to neutralize hydrogen peroxide - H 2 O 2 converts to hydroxyl radicals and this can lead to oxidative damage/toxic injury. Heinz bodies, aggregates of cross-linked hemoglobin, form on the cell membranes and subject the cell to mechanical stress as it tries to go through small capillaries. The action of the ROS on the cell membrane as well as mechanical stress from the lack of deformability result in hemolysis.