Hexose Monophosphate Shunt

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Hexose Monophosphate Shunt

• BAHS 501
• October 23, 2001

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Real Physician Quotes (III)
Large brown stool ambulating in the
hall. The skin was moist and dry. Patient
has two teenage children, but no other
abnormalities. Healthy appearing decrepit 69
year-old male, mentally alert, but
forgetful. Rectal exam revealed a normal size
thyroid.

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(No Transcript)
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POLYSACCHARIDES (COMPLEX CARBOS)
glucose
Glycolysis
ATP
2 NADH
pyruvate
acetyl-CoA
FADH2 and NADH (reducing power)
O2
citric acid cycle
oxidative phosphorylation
ATP
ATP
ATP
ATP
ATP
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HMP Products

• NADPH for reductive biosynthesis
• Fatty acids and steroids
• Ribose-5-phosphate
• Nucleic acids
• Glycolytic Intermediates
• Glyceraldehyde-3-phosphate
• Fructose-6-phosphate

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Adenine nucleotides and energy charge globally
regulate metabolism and, in general,
reciprocally regulate catabolic and anabolic
pathways.
energy charge ATP ADP
ATP ADP AMP
NADH, ATP NAD
glycolysis
ATP
ADP
pyruvate dehydrogenase
OXPHOS
NADH, ATP NAD
NADH
NAD
citric acid cycle
NADH, ATP ADP
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glucose
Hexose Monophosphate Shunt
glucose-6-P
Glycolysis
NADH
NADPH (coenzyme for reductive biosynthesis)
pyruvate
acetyl-CoA
sugar interconversions (ribose-5-P for
nucleotides)
citric acid cycle
FADH2 and NADH
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Hexose Monophosphate Pathway
NADPH
NADP
NADPH
NADP
ribulose 5- phosphate
6-phospho- gluconate
Glucose-6P
G6PD
oxidative reactions (irreversible)
ribulose 5- phosphate
(5 carbons)
ribose 5- phosphate
xylulose 5- phosphate
erythrose 4-phosphate
(5 carbons)
(5 carbons)
(4 carbons)
Sedoheptulose 7-phosphate
glyceraldehyde 3-phosphate
(3 carbons)
(7 carbons)
erythrose 4-phosphate
fructose 6-phosphate
(6 carbons)
(4 carbons)
nucleotide metabolism
glycolysis
nonoxidative reactions (reversible)
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Glucose-6-Phosphate Dehydrogenase

• Catalyzes first step in HMP
• Rate-limiting step
• Uses NADP as cofactor (reaction generates NADPH)
• Highly regulated by NADPH/NADP ratio

NADPH NADP
NADPH NADP
HIGH
LOW
G6PD
X
G6PD
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NADP (Nicotinamide Adenine Dinucleotide
Phosphate )
reactive center
H
H
H
..
N
NADPH...
O-P-O32-
O-P-O32-
NADP
NADPH
1. differs from NADH by presence of phosphate on
one ribose which allows interaction with a
distinct set of enzymes 2. formed by HMP to
allow some energy from glucose to be shunted into
reductive biosynthetic reactions (e.g. fatty acid
and steroid biosynthesis) 3. used in reactions
that detoxify cells and even to kill bacteria
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NADPH vs NADH

• NADPH
• Primarily uses high
• energy electrons for
• making stuff
• Fatty acids
• Steroids

• NADH
• Uses high enery
• electrons to make
• energy/ATP via
• OX/Phos

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Functions of NADPH

• Provide high energy e- for reductive biosynthesis
• Used as a cofactor by enzymes that deal with
  reactive oxygen species (ROS)

These functions cannot be replaced by NADH
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H2O
O2
DNA
Protein
LIPIDS
genetic mutation
membrane function
enzyme inactivation
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Reducing O2 to H20

• Goal of Ox/Phos is to generate energy and in the
  course of this process O2 gets reduced to H2O

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e-
e-
e-
e-
O2
H2O2
H2O
O2-
H2O
oxygen
superoxide anion
hydrogen peroxide
hydroxyl radical
water
reactive oxygen species (ROS)
ROS are toxic to cells because they can react
with and damage cellular components (proteins,
lipids and DNA)
I. superoxide anion- produced biologically by a
variety of reactions, most notably by leaky
mitochondrial electron transfer. Electrons can
be transferred from the reduced form of Coenzyme
Q to oxygen thus generating superoxide.
O2-
II. hydrogen peroxide-produced by oxidase
enzymes (many of which are in oxidative
organelles called peroxisomes). Very
toxic organic peroxides can also be formed from
2e- reduction of O2 in compounds containing a
double bond (unsaturated fatty acids in membranes
for example)
H2O2
OH-
III. hydroxyl radical-produced from a
metal-catalyzed reaction of superoxide and
hydrogen peroxide. This is a very reactive
species that can take part in free radical chain
reactions.
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Glutathionea multifunctional tripeptide
g-glutamylcysteinylglycine (g-glu-cys-gly)
g-glu-cys-gly
S
S
SH
g-glu-cys-gly
g-glu-cys-gly
(GSH)
(GSSG)
glutathione peroxidase a selenium-containing
enzyme that detoxifies H2O2 and nasty organic
peroxides (lipid peroxides)
2GSH R-O-OH
GSSG R-OH
glutathione reductase regenerates reduced GSH
2 GSH
GSSG
NADPH
NADP
functions of glutathione a major cellular
reductant and sulfhydryl buffer, conjugated to
drugs to make them more soluble, amino
acid transport across membranes, disulfide
interchanges in proteins
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cellular defense against ROS
I. superoxide dismutase (SOD)-detoxifies
superoxide as follows
H2O2 2O2
2H 2O2-
Manganese SOD (Mn SOD)- mitochondrial enzyme
Copper-Zinc SOD- cytoplasmic enzyme, deficiency
of this enzyme causes the severe progressive
neurodegenerative disorder Amyotrophic Lateral
Sclerosis (ALS) or Lou Gherigs Disease
II. catalase- a heme-containing peroxisomal
enzyme that detoxifies hydrogen peroxide as
follows
2H2O O2
2H2O2
III. There are no known enzymatic systems that
deal directly with hydroxyl radicals. Cells
apparently rely on the above two reactions to
remove the precursors to this ROS (superoxide
and hydrogen peroxide).
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Cytochrome P-450 System

• Liver enzymes that detoxify many nasty compounds
• Drugs
• Steroids
• Alcohols
• These enzymes require NADPH as a co-factor

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antioxidant capacity
ROS production
antioxidant capacity
ROS production
OXIDATIVE STRESS !
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Glucose 6-P Dehydrogenase (G6PD) Deficiency
red blood cell (erythrocyte)
NADPH
X
X
glutathione reductase activity
increased H2O2 oxidative stress hemolysis
G6PD
lack of G6PD results in loss of NADPH production,
loss of glutathione antioxidant system, increased
oxidative stress, membrane damage and red blood
cell lysis
X-linked disease and is the most common
disease- causing enzymatic defect in humans (200
million people)
hemolytic anemia caused by this mechanism can be
precipitated by oxidant drugs (e.g., primaquine),
diet (FAVA beans), infection (induction of
NADPH oxidase)
However, for those without G6PD deficiency,
these are great with liver and a nice chianti (H.
Lechter, personal communication)
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G6PD Mutations

• Slightly decreased life span (complications from
  hemolysis)
• Many mutations alter 6GPD function
• Many alter the Km or Vmax of the enzyme
• Some mutations confer resistance to falciparum
  malaria

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Drugs that Exacerbate G6PD
AAA

• Antibiotics
• e.g. sulfamethoxazole
• Antimalarials
• e.g. primaquine, chloroquine
• Antipyretics
• e.g. acetanilid NOT aspirin or acetaminophen

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The Respiratory Burst generating ROS on purpose
to kill bacteria
internal storage granules
Neutrophil
O2-
O2
NADPH
NADP
The neutrophil is a circulating white blood cell
that is the first line of defense against
invading bacteria. The neutrophil can engulf
bacteria with its plasma membrane (phagocytosis)
to form a phagosome. The phagosome then fuses
with internal storage granules that contain
bacteriocidal enzymes (phagolysosome). A burst
of non-mitochondrial O2 consumption (the
respiratory burst) by NADPH oxidase forms large
amounts of superoxide which are then converted
into toxic ROS that kill bacteria.
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Take Home Message

• Hexose Monophosphate Shunt
• NADPH, Ribose-5-phosphate, glycolytic stuff
• Role of NADPH
• Reductive Biosynthesis
• Required for enzymes (glutathione reductase) that
  deal with reactive O2 species
• Mutations in the Rate-limiting enzyme 6GPD