PENTOSE PATHWAY & ANTIOXIDANTS BIOC DR. TISCHLER LECTURE 30

OBJECTIVES 1.Pentose-phosphate pathway: a) oxidative and non-oxidative branches b) cofactor with each branch c)how oxidative branch is regulated d)three modes of the pentose phosphate pathway in terms of roles of the potential endproducts of each mode.

2. Antioxidant functions: a)major active oxygen species; rank according to relative reactivity b)enzymes that remove peroxides and superoxide radicals from a cell and name their cofactor. c)why a defect of glucose-6-phosphate dehydrogenase in the red blood cell might lead to loss of membrane integrity. d)relationships between components of antioxidant cascade including the reactions involved

Functions of Pentose Phosphate Pathway 1)NADPH for biosynthetic pathways (e.g., synthesis of fatty acids and cholesterol); 2) NADPH for maintaining glutathione in its reduced state (see discussion of glutathione later); 3) Pentose sugar for synthesis of nucleic acids

glycolytic intermediates Glucose 6-P 6-Phosphogluconate NADPNADPH Glucose-6-P-DH Xylulose 5-P Ribose 5-P Oxidative Branch Non- oxidative Branch Nucleic acids Sedoheptulose-7-P Erythrose 4-P Transketolase Transaldolase Glyceraldehyde 3-P Fructose 6-P Glyceraldehyde 3-P TPP Transketolase Ribulose 5-P CO 2 NADPH NADP 6-Pgluconate DH Figure 1. The pentose phosphate pathway containing an oxidative and a non-oxidative branch

Ribulose 5-P Xylulose 5-P Ribose 5-P Sedoheptulose 7-P Erythrose 4-P Transketolase Transaldolase Glyceraldehyde 3-P Fructose 6-P Glyceraldehyde 3-P Transketolase Figure 2. Using the non-oxidative branch of the pentose pathway to produce ribose-5-phosphate for the nucleic acid pathways (Mode 1). Ribose-5-P is the sugar required for the synthesis of nucleic acids Nucleic acids Non- oxidative Branch

Glucose 6-P Ribulose 5-P 6-Phosphogluconate Ribose 5-P NADP NADPH CO 2 NADPH NADP Figure 3. Using the oxidative branch of the pentose pathway to produce NADPH for biosynthetic reactions and ribose-5- phosphate for producing nucleic acids (Mode 2). Nucleic acids Oxidative Branch

Oxidative Branch Non- oxidative Branch back to glucose-6-P or to glycolysis Glucose 6-P (3) Ribulose 5-P (3) 6-Phosphogluconate Xylulose 5-P (2) Ribose 5-P (1) Sedoheptulose 7-P (1) Erythrose 4-P (1) NADP NADPH CO 2 NADPH NADP Glyceraldehyde 3-P (1) Fructose 6-P (1) Glyceraldehyde 3-P (1) back to glucose-6-P or to glycolysis Figure 4. Using the oxidative branch to produce NADPH for biosynthesis and returning ribulose-5-P to glycolytic intermediates (mode 3)

used by transketolase, PDH,  KgDH deficiency affects nucleic acid synthesis/energy metabolism Wernicke-Korsakoff syndrome – observed in alcoholics due to poor diet thiamine deficiency in individuals on high CHO diet (e.g., rice) causes beriberi patients tire easily cardiac decompensation energy depletion on high CHO diet NUTRITIONAL PREMISE: THIAMINE (VITAMIN B 1 ) Carl Wernicke Sergei Korsakoff Patient with beriberi

Table 1. ACTIVE OXYGEN SPECIES ReactivityAO Species Leastsinglet oxygen superoxide radical anion (O 2 -  ) Moderatehydrogen peroxide (HOOH) lipid peroxyl radical (LOO  ) Most hydroxyl radical (OH  )

H2O2H2O2 glutathione peroxidase 2 H 2 O 2 GSH GSSG glutathione reductase NADPH + H + NADP + pentose pathway Figure 5. Reactions of glutathione reduction and oxidation

SUMMARY OF ANTI-OXIDANT ENZYMES Glutathione peroxidase: 2 GSH + H 2 O 2 GSSG + 2 H 2 O Uses selenium as a cofactor Catalase : 2 H 2 O 2 H 2 O + O 2 Superoxide dismutase: 2 O 2 -  + 2H +  H 2 O 2 + O 2 Mitochondrial - Mn 2+ cofactor Cytoplasmic – Cu 2+ -Zn 2+ cofactors; mutations associated with familial amyotrophic lateral sclerosis (FALS) Lipid Peroxidase: removes LOOH

Vit E red VIT E ox Vit C red VIT C ox LOOH lipid peroxyl radical LOO  Glutathione red (GSH) NADP + NADPH + H + Glucose-6-P Ribulose-5-P Pentose phosphate pathway +ROOH Glutathione ox (GSSG) H2O2H2O2 2H 2 O hydroxyl radical (OH  ) superoxide radical (O 2 -  ) reduced products Figure 6. Antioxidant cascade Reduced forms/reduction Oxidized forms/oxidation