otherwise known as the hexose monophosphate or phosphogluconate pathway
it occurs in the CYTOPLASM; starts with one glycolytic intermediate and ends with another
What are the two major functions of the pentose phosphate shunt?
1) biosynthesis of pentose sugars
(required for synthesis of nucleotides/nucleic acids)
2) Formation of NADPH
(reduced coenzyme required for synthesis of fatty acids, steroids/cholesterol, NTs, nucleotides, regeneration of reduced glutathione, and generation of reactive oxygen)
What are the 3 stages of the pentose phosphate shunt?
1) irreversible oxidative conversion of glucose-6-phosphate to pentose-5-phosphate (1 carbon is released as CO2)
2) reversible interconversion of 3 pentose-phosphates: ribulose-5-phosphate, xylulose-5-phosphate, ribose-5-phosphate
3) reversible conversion of pentose-5-phosphates to glycolytic intermediates!
the lactone is spontaneously hydrolyzed by water, but lactonase increases the rate of hydrolysis
Which pentose phosphate pathway enzyme irreversibly hydrolyzes 6-phosphoglucono-delta-lactone to 6-phosphogluconate?
lactonase
It is the rapid removal of the _______ product that makes the overall conversion of glucose-6-phosphate to 6-phosphogluconate irreversible
LACTONE (cyclic ester)
What is the main allosteric inhibitor of glucose-6-phosphate dehydrogenase?
NADPH
the in vivo enzyme activity is a small fraction of the potentially maximal velocity b/c the inhibiting NADPH concentration is much higher than that of NADP+ (the substrate)
Which pentose phosphate pathway enzyme catalyzes the oxidative decarboxylation of 6-phosphogluconate to form ribulose-5-phosphate?
6-phosphogluconate dehydrogenase
What is the main allosteric inhibitor of 6-phosphogluconate dehydrogenase?
NADPH
Which enzymes generate NADPH in the pentose phosphate pathway?
glucose-6-phosphate dehydrogenase
6-phosphogluconate dehydrogenase
6-phosphogluconate dehydrogenase
oxidatively decarboxylates 6-phosphogluconate to form ribulose-5-phosphate (it's a ketose)
the enzyme uses NADP+ as a coenzyme, therefore NADPH is formed in addition to the ribulose
also CO2 is released
the reaction is IRreversible, and inhibited by the product NADPH
What are the possible fates of ribulose-5-phosphate once it's been synthesized from the oxidative phase of the pentose-phosphate shunt?
it can be converted into other pentose-phosphates, eg:
1) xylulose-5-phosphate: by phosphopentose epimerase (ribulose & xylulose = epimers)
2) ribose-5-phosphate: by phosphopentose isomerase
phosphopentose epimerase
converts ribulose-5-phosphate to xylulose-5-phosphate
ribulose and xylulose are epimers
phosphopentose isomerase
converts ribulose-5-phosphate to ribose-5-phosphate
True or False: in the pentose phosphate pathway, the conversion of ribose-5-phosphate to glycolytic intermediates generates NADPH?
FALSE, only the first stage of the pentose phosphate pathway (from glucose-6-phosphate to ribulose-5-phosphate) generates NADPH
the reversible part of the shunt allows for both the degradation of pentose-phosphates & the synthesis of pentose-phosphate from glycolytic intermediates WITHOUT production of NADPH
transketolase
enzyme that uses coenzyme TPP to transfer a 2-carbon fragment FROM a ketose (xylulose-5-phosphate) TO an aldose (ribose-5-phosphate)
What is the structural difference between NADH and NADPH?
NADPH has an additional phosphate group at the 2'-position of the ribose linked to adenine that allows enzymes to discriminate between NADH and NADPH (specificity handle)
NADP+ still accepts a pair of electrons & a proton in the nicotinamide ring like NAD+
If the NAD+/NADH ratio in a cell is very high, the NADP+/NADPH ratio in the same cell is:
Very LOW: a cell with a high concentration of NAD+ will therefore also have a high concentration of NADPH
NADPH __ NADP+
NAD+ __ NADH
NADPH is in much higher concentrations than NADP+ (NADPH >> NADP+)
On the other hand, NAD+ is in much higher concentrations than NADH (NAD+ >> NADH)
NADPH provides reducing power for biosynthetic work OUTSIDE the mitochondria
What are the tissues in which the pentose phosphate pathway is most active?
places where NADPH is used:
Liver (coenzyme for drug-metabolizing enzymes)
Adipose tissue (for synthesis of fatty acids/steroids)
adrenal cortex (for steroid hormone synthesis)
erythrocytes (protects from oxidative damage)
phagocytes (as substrate for NADPH oxidase that protects against neutrophil infection via respiratory bursts)
How can the pentose phosphate pathway be manipulated if you're under extreme stress (eg. oxidative attack)?
you'd want to make a lot of NADPH, so the shunt would be run to make NADPH, then the resulting product would be re-converted back into glucose-6-phosphate and the shunt would be run run again until all carbons you started with were exhausted
this is the most extreme form of needing the pathway
How can the pentose phosphate shunt be regulated?
glucose-6-phosphate dehydrogenase is the first committed step AND the rate limiting step (will only proceed if energetically favorable)
insulin induces the pathway
NADPH inhibits the pathway (allosteric feedback)
NADPH oxidase
generates a superoxide anion from oxygen
the generation of reactive oxygen species (ROS) by phagocytes is an important mechanism used to kill engulfed bacteria
What are examples of reactive oxygen species'?
superoxide, hydrogen peroxide & hydroxyl radicals
myeloperoxidase
enzyme that generates hypochlorite (bleach) from hydrogen peroxide and chloride anions during the oxidative burst of a neutrophil
congenital granulomatous disease
defects in forming ROS' (most importantly, the superoxide radical)
can result in the patient being subject to severe recurrent infection + chronic inflammatory conditions due to an inability to kill ingested pathogens
What are some ways reactive oxygen can be harmful? (4)
can cause damage to DNA, especially mtDNA
lipid peroxidation causes increases in permeability that change ion concentration across membranes
it can attack some amino acids and promote cross-linking or fragmentation
they can form in damaged cells/tissues, eg. those recovering from ischemia (problem associated with myocardial infarction, bypass surgery, transplantation)
when individuals w/ a defect are exposed to oxidants (from food or drugs) they cause denaturation/precipitation of hemoglobin within RBCs
this makes RBCs inflexible and subject to damage as they pass through narrow capillaries
breakdown of RBCs in the spleen leads to anemia & release of hemoglobin (which may be excreted in urine)
evidence of how NADPH plays an important role in detoxifying reactive oxygen & especially protecting red blood cells from oxidative damage
Which chromosome is the gene for glucose-6-phosphate dehydrogenase (G6PD) located on?
the X chromosome
When someone with a glucose-6-phosphate dehydrogenase (G6PD) deficiency takes primaquine what condition develops?
severe acute hemolytic anemia
A patient is developing severe acute hemolytic anemia after taking the antimalarial drug primaquine. What enzyme is likely deficient?
glucose-6-phosphate dehydrogenase (G6PD)
Why do gene mutations that affect the Km or Ki of glucose-6-phosphate dehydrogenase (G6PD) make a person more susceptible to acute hemolytic anemia than changes in the enzyme's Vmax?
because the Km and Ki determine whether the in vivo activity of the enzyme can be increased to meet an oxidative challenge
the in vivo activity is only a small fraction of the potential Vmax
if the Km and Ki of a mutant enzyme are such that the enzyme does not respond to a change in the NADP+/NADPH ratio in the in vivo range, severe oxidative damage and red cell destruction ensues
What are some foods/classes of drugs that can trigger episodes of hemolytic anemia in people with glucose-6-phosphate dehydrogenase deficiency?
Fava beans, antimalarial drugs, some antibiotics & antipyretics
What are some disease states that are caused by free radical injury? (6)
Athergenesis
Acute Renal Failure
Alcoholism
Emphysema
Ischemia
Neurodegenerative disease
malic enzyme
an enzyme NOT found in erythrocytes that produces NADPH in a pathway other than the pentose phosphate shunt
Malate + NADP+ --> Pyruvate + CO2 + NADPH + H+
also known as NADP-linked malate dehydrogenase
Why are erythrocytes at a greater risk of oxidative damage than other cells?
because RBCs lack alternative ways to generate NADPH, such as malic enzyme
What are the substrates and products of the reaction catalyzed by malic enzyme?