BIOC 460 - DR. TISCHLER
LECTURE 26
PENTOSE PATHWAY
&
ANTIOXIDANTS
OBJECTIVES
1.
For the pentose phosphate pathway:
a. describe the oxidative and non-oxidative branches
b. describe how the oxidative branch is regulated
c. distinguish between the 3 modes in terms of the roles of the
potential endproducts of each mode.
2. Describe the consequences of thiamine deficiency
3. In relation to antioxidant function in the body:
a. list the major active (reactive) oxygen species, identify the
antioxidant which reduces that species.
b. describe the metabolism of glutathione
c. identify the enzymes that remove peroxides and superoxide
radicals from a cell and name their cofactor.
d. describe the relationships between the components of the
antioxidant cascade including the reactions involved.
e. discuss why a defect of glucose-6-phosphate dehydrogenase in
the red blood cell might lead to loss of membrane integrity.
PHYSIOLOGICAL PREMISE
Do you have a partial enzyme deficiency about which you are
unaware? There are circumstances where an individual may have such
a partial deficiency but be unaware of the fact until a physiological
event shifts the balance of metabolic processes. For example,
individuals with malaria are given a drug called primaquine. When the
body metabolizes primaquine it increases the demand for production of
NADPH in most cells. A major source of NADPH is the glucose-6phosphate dehydrogenase (G6PDH) reaction in the pentose phosphate
pathway. In the red blood cell, this pathway is essential for removing
peroxides, which can oxidize lipids in the plasma membrane causing
the cell to become more fragile. Stressing the system with primaquine
in an individual with a partial deficiency of G6PDH will lead to red cell
destruction and hence the individual becomes anemic.
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
Glucose-6-P-DH
NADP NADPH
Glucose 6-P
6-Phosphogluconate
NADP
6-Pgluconate DH
NADPH
glycolytic
Ribulose 5-P CO2
intermediates
Glyceraldehyde 3-P
Xylulose 5-P
Transketolase
Glyceraldehyde 3-P
Erythrose 4-P
Nucleic
acids
Ribose 5-P
TPP
Transketolase
Sedoheptulose-7-P
Transaldolase
Fructose 6-P
Oxidative
Branch
Fructose 6-P
Figure 1. The pentose phosphate pathway
containing an oxidative and a non-oxidative branch
Nonoxidative
Branch
Glyceraldehyde 3-P
Fructose 6-P
Transketolase
Ribulose 5-P
Nucleic acids
Ribose 5-P
Xylulose 5-P
Transketolase
Glyceraldehyde 3-P
Erythrose 4-P
Sedoheptulose 7-P
Transaldolase
Fructose 6-P
Nonoxidative
Branch
Ribose-5-P is the sugar required for the synthesis of nucleic acids
Figure 2. Using the non-oxidative branch of the pentose pathway to
produce ribose-5-phosphate for the nucleic acid pathways (Mode 1).
NADP
Glucose 6-P
NADPH
6-Phosphogluconate
NADP
Ribulose 5-P
CO
Oxidative
Branch
NADPH
2
Ribose 5-P
Nucleic
acids
Figure 3. Using the oxidative branch of the pentose pathway
to produce NADPH for biosynthetic reactions and ribose-5phosphate for producing nucleic acids (Mode 2).
NADPH
NADP
6-Phosphogluconate
NADP
Glucose 6-P
(3)
Ribulose 5-P (3)
NADPH
CO2
Oxidative
Branch
Glyceraldehyde 3-P (1)
Xylulose 5-P (2)
back to
glucose-6-P
or to glycolysis
Glyceraldehyde 3-P
(1)
Ribose 5-P (1)
Sedoheptulose 7-P
(1)
Erythrose 4-P (1)
Fructose 6-P (1)
Nonoxidative
Branch
Fructose 6-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)
NUTRITIONAL PREMISE: THIAMINE (VITAMIN B1)
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
Brain atrophy due to Wernicke’s encephalopathy
Slide to be shown in class
Table 1. Reactive Oxygen Species
and Antioxidants that Reduce Them
Reactive Species
Antioxidant
Singlet oxygen 1O2
Vitamin A, vitamin E
Superoxide radical (O2-) superoxide dismutase, vitamin C
Hydrogen peroxide
(H2O2)
Catalase; glutathione peroxidase
Peroxyl radical (ROO)
Vitamin C, vitamin E
Lipid peroxyl radical
(LOO)
Vitamin E
Hydroxyl radical (OH)
Vitamin C

Lipid
(LH)
L
OH
Fe2+
O2
H2O


LOO
H2O2
1O
2
UV light
heme Fe
CoQ



H2O, H+
H+
O2-
O2
HOO

NADPH
or CoQ

H+
Figure 5. Pathways for the formation of reactive oxygen species
 lipid radical
 Singlet oxygen
 Peroxyl radical
 Haber-Weiss
 lipid peroxyl
reaction;
 Superoxide

Superoxide
dismutase
radical
radical anion
 Fenton reaction
H2O2
2 GSH
glutathione
peroxidase
glutathione
reductase
NADP+
2 H2 O
GSSG
NADPH + H+
pentose pathway
Figure 6. Reactions of glutathione reduction and oxidation
SUMMARY OF ANTI-OXIDANT ENZYMES
Glutathione peroxidase: 2 GSH + H2O2  GSSG + 2 H2O
Uses selenium as a cofactor
Catalase : 2 H2O2  H2O + O2
Lipid Peroxidase: removes LOOH
Superoxide dismutase: 2 O2- + 2H+  H2O2 + O2
Mitochondrial - Mn2+ cofactor
Cytoplasmic – Cu2+-Zn2+ cofactors; mutations
associated with familial amyotrophic lateral sclerosis
(FALS)
NUTRITIONAL CORRELATE: SELENIUM
 selenocysteine in glutathione peroxidase
 intake may be related to lower cancer mortality
• cancer patients have lower plasma Se levels
• risk may be higher in those with low Se intake
• AZCC study – reduced incidence of prostate,
colon, lung cancers
 toxicity (> 1 mg/day) results in hair loss, GI upset,
nerve damage
lipid peroxyl radical
rxn 5
LOO
Vit Ered
VIT Cox
Glutathionered
(GSH)
rxn 6
LOOH
VIT Eox
Vit Cred
rxn 7
+ROOH
rxn 2
NADP+ rxn 1
Glucose-6-P
Figure 7. Antioxidant cascade
Reduced forms/reduction
Oxidized forms/oxidation
hydroxyl radical (OH)
superoxide radical (O2-)
rxn 9
reduced
products
Glutathioneox
(GSSG)
rxn 4
H 2 O2
2H2O
NADPH + H+
Pentose phosphate pathway (rxn 8)
Ribulose-5-P
Medical Scenario:
If the antioxidant protective system in the red blood cell becomes
defective, hemolytic anemia occurs; that is red blood cells
undergo hemolysis and their concentration in the blood decreases.
Such is the case if glucose 6-phosphate dehydrogenase is
defective in the pentose phosphate pathway. In individuals whose
glucose 6-phosphate dehydrogenase is defective, there is
insufficient NADPH produced in red blood cells to maintain the
ratio of reduced glutathione to oxidized glutathione at its normal
value of well over 100. Hence, peroxides destroy the red cell
membrane because of the limited protective mechanism in these
cells.