Download Pentose Phosphate Pathway

Pentose Phosphate
Pathway
Dr. Abir Alghanouchi
Biochemistry department
Sciences college
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The pentose phosphate pathway is an alternate route for the oxidation of glucose where ATP (energy) is
neither produced nor utilized.
`
The pentose phosphate pathway takes place entirely within the cytoplasm (because NADP+ is used as a hydrogen acceptor) and is also known as the hexose
monophosphate shunt or phosphogluconate pathway
`
Is basically used for the synthesis of NADPH and D‐
ribose.
The PPP can be divided into two phases
¾
The irreversible oxidative phase: generates NADPH
which is required for many biosynthetic pathways and for detoxification of reactive oxygen species.
¾
The nonoxidative phase:
interconverts C3, C4, C5, C6
and C7 monosaccharides to produce ribose‐5P for nucleotide synthesis, and also to regenerate glucose‐
6P to maintain NADPH production by the oxidative phase. The irreversible oxidative phase
¾
Three enzymatic reactions in the oxidative phase
G6PD is the committed step in the Pentose Phosphate Pathway because 6‐Phosphoglucono‐lactone has no other metabolic fate except to be converted to 6‐phosphogluconate.
6CH OPO 2−
2
3
5
O
H
4
OH
H
OH
3
H
Glucose-6-phosphate
Dehydrogenase
+
NADPH + H
+
H
OH NADP
H
H
2
Mg2+
OH
6 CH OPO 2−
2
3
5
O
H
OH
H
O
H
1
3
H
OH
glucose-6-phosphate
¾
4
1
6-P
gl
lac
2
OH
6-phoshogluconolactone
e
Glucose‐6‐phosphate Dehydrogenase catalyzes oxidation of the aldehyde at C1 of glucose‐6‐phosphate, to a carboxylic acid
¾This enzyme requires
Mg2+ et NADP+ (serves as electron acceptor) as coenzymes
¾
NADPH is a potent competitive inhibitor of this enzyme NADPH/NADP+ increase Æ inhibits the reaction
NADPH/NADP+ decreaseÆ stimulate the reaction
6-Phosphogluconolactonase
phate
se
6 CH OPO 2−
2
3
+
H+H
5
O
H 2 O H+
H
4
O OH
H
OH
O
H
1
3
H
2
OH
O−
O
1C
HC
2
OH
HO 3CH
HC OH
4
HC
5
OH
CH2OPO32−
6
6-phoshogluconolactone
6-phosphogluconate
¾
6‐Phosphogluconolactonase catalyzes hydrolysis of 6‐
phosphogluconolactone
¾
The product is 6‐phosphogluconate
¾
It is irreversible but not rate‐limiting
O−
O
1C
HC
2
OH
Phosphogluconate
Dehydrogenase
NADP+
HO 3CH
HC OH
4
HC
OH
CH OH
NADPH + H+ 1 2
C O
2
CO2
HC
OH
HC
OH
3
5
4
6
5
CH2OPO32−
6-phosphogluconate
CH2OPO32−
ribulose-5-phosphate
¾
Phosphogluconate Dehydrogenase catalyzes oxidative decarboxylation of 6‐phosphogluconate, to yield the 5‐C ketose ribulose‐5‐phosphate
¾
The OH at C3 (C2 of product) is oxidized to a ketone
¾
This promotes loss of the carboxyl at C1 as CO2
¾
NADP+ serves as oxidant
The reversible non‐oxidative phase
`
In nonoxidative phase, ribulose 5‐P is converted back to G‐6‐P by a series of reactions involving especially two enzymes
1.Transketolase
2.Transaldolase
¾
CH2OH
Epimerase inter‐converts stereoisomers ribulose‐5‐P and xylulose‐5‐P
Epimerase
CH2OH
¾
Isomerase converts the ketose ribulose‐5‐P to ribose‐5‐P which is used in nucleotide, nucleic acid biosynthesis
Both reactions are reversible
C
C
O
HO
C
H
H
C
OH
CH2OPO32−
O
H
C
OH
H
C
OH
CH2OPO32−
xylulose-5phosphate
HC
O
H
C
OH
ribulose-5H
phosphate Isomerase
C
OH
H
C
OH
CH2OPO32−
ribose-5phosphate
Transketolase
¾
Transfers a 2‐C fragment
from xylulose‐5‐P to either ribose‐5‐P or erythrose‐4‐P. Isomerase
Epimerase
Transketolase
¾
Requires thiamine pyrophosphate (TPP), a derivative of vitamin B1
as coenzyme and Mg2+ as cofactor
Transaldolase
Epimerase
transketolase
CH2OH
Transketolase
CH2OH
HC
O
C
O
H
C
OH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
+
CH2OPO32−
xylulose5-phosphate
CH2OPO32−
HC
H
C
O
OH
+
CH2OPO32−
C
O
HO
C
H
H
C
OH
H
C
OH
H
C
OH
CH2OPO32−
riboseglyceraldehyde- sedoheptulose5-phosphate 3-phosphate
7-phosphate
¾
Transfer of the 2‐C fragment to the 5‐C ribose‐5‐
phosphate yields sedoheptulose‐7‐phosphate
¾
Transfer of the 2‐C fragment instead to 4‐C erythrose‐
4‐phosphate yields fructose‐6‐phosphate CH2OH
C
HO
Transaldolase
O
H2 C
CH
C
HC
OH
HC
OH
HC
OH
H2 C
+
OPO 32−
HC
O
HC
O
HC
OH
HC
OH
HC
OH
H2C
OPO32 −
H2 C
HO
+
OPO32 −
OH
O
CH
HC
OH
HC
OH
H2 C
OPO32 −
sedoheptulose- glyceraldehyde- erythrose- fructose3-phosphate
4-phosphate 6-phosphate
7-phosphate
Transaldolase catalyzes transfer of a 3‐C from sedoheptulose‐7‐phosphate to glyceraldehyde‐3‐P
SUMMARY: The balance sheet below summarizes flow of 15 C atoms Through PPP reactions by which 5‐C sugars are converted to 3‐C
and 6‐C sugars. TK
5 + 5 → 3 + 7
TA
7 +3 → 4 + 6
TK: Transketolase
TA: Transaldolase
TK
4 + 5→ 3 + 6
3 C5
Æ
Xylulose-5-PO4
Ribose-5-PO4
Glyceraldehyde-3-PO4
2 C 6 + C3
(Overall)
Sedoheptulose-7-PO4
Erythrose-4-PO4
Fructose-6-PO4
Differences between HMP shunt and
glycolysis
HMP pathway
Location
Glycolysis
-------------------- In all cells
------------------In certain cells
-
Oxidation of glucose Occurs in the first --------------------
Phosphorylation occurs
--------------------
reaction
first then oxidation Coenzyme
+
-------------------NADP
------------------+
NAD
Energy
-------------------No energy production
-------------------2 or 8 ATP
CO2
-------------------Produced -------------------Not produced Pentoses
-------------------Produced
-------------------Not produced
Importance of Pentose Phosphate Pathway
2 NADP+ 2 NADPH + CO2
glucose-6-P
ribulose-5-P
ribose-5-P
Pentose Phosphate Pathway producing
NADPH and ribose-5-phosphate
¾
Ribulose‐5‐P may be converted to ribose‐5‐phosphate, a substrate for synthesis of nucleotides, nucleic acids and coenzymes ¾The pathway also produces some NADPH
2 NADP+ 2 NADPH + CO2
glucose-6-P
ribulose-5-P
ribose-5-P
fructose-6-P, &
glyceraldehyde-3-P
Pentose Phosphate Pathway producing
maximum NADPH
¾
Glyceraldehyde‐3‐P and fructose‐6‐P may be converted to glucose‐6‐P, via enzymes of gluconeogenesis, for reentry to Pentose Phosphate Pathway, maximizing formation of NADPH, which is need for reductive biosynthesis.
¾
3‐C Glyceraldehyde‐3‐P and 6‐C fructose‐6‐P, formed from 5‐C sugar phosphates, may enter Glycolysis
for ATP synthesis. ¾
5‐C Ribose‐1‐phosphate generated during catabolism of nucleosides also enters Glycolysis in this way, after first being converted to ribose‐5‐phosphate
Thus the Pentose Phosphate Pathway serves as an entry into Glycolysis for both 5‐carbon & 6‐carbon sugars.
02
Importance of PPP in RBC
Aerobic respiration
Drugs, fava beans
02‐
Superoxide radicals
Detoxification
H2O2 Hydrogen peroxide glutathione
peroxidase
2 GSH
glutathione
reductase
2 H2O
When erythrocytes are exposed to chemicals that generate high levels of superoxide radicals, GSH
(Reduced Glutathione) is required to reduce these damaging compounds
¾
GSSG
¾Glutathione Peroxidase
catalyzes degradation of organic hydroperoxides by reduction, as two glutathione molecules are oxidized to a disulfide GSSG
NADP+
NADPH + H+
¾The PPP is responsible for pentose pathway
maintaining high levels of NADPH in red blood cells for use as a reductant in the glutathione reductase reaction. `
The entry of glucose 6‐phosphate into the pentose phosphate pathway is controlled by the cellular concentration of NADPH
`
NADPH is a strong inhibitor of glucose 6‐phosphate dehydrogenase (Rate Limiting Reaction)
`
As NADPH is used in various pathways, inhibition is relieved, and the enzyme is accelerated to produce more NADPH
`
The synthesis of glucose 6‐phosphate dehydrogenase is induced by the increased insulin/glucagon ratio after a high carbohydrate meal
`
Insulin, which secreted in response to hyperglycemia, induces the synthesis of G6P dehydrogenase and 6‐phosphogluconate dehydrogenase Æ increasing the rate of glucose oxidation by PPP
`
The synthesis of glucose 6‐phosphate dehydrogenase is repressed during fasting
`
Mutations present in some populations causes a deficiency in glucose 6‐phosphate dehydrogenase, with consequent impairment of NADPH production
`
Detoxification of H2O2 is inhibited, and cellular damage results ‐ lipid peroxidation leads to erythrocyte membrane breakdown and hemolytic anemia.
`
Most G6PD‐deficient individuals are asymptomatic ‐ only in combination with certain environmental factors (sulfa antibiotics, herbicides, antimalarials, *divicine) do clinical manifestations occur.
*toxic ingredient of fava beans
Uronic Acid Pathway
Dr. Abir Alghanouchi
Biochemistry department
Sciences college
¾ It is an alternative oxidative pathway for glucose in which glucose is converted into glucuronic acid
¾ Like the pentose phosphate pathway, Uronic
Pathway does not lead to generation of ATP
¾ It occurs in cytoplasm (mainly liver)
Acid Importance of Uronic Acid Pathway
This pathway produces glucuronic acid which is important for: ¾
Synthesis of substrates
1‐ Glycosaminoglycans (heparin, hyaluronic acid…)
2‐ Vitamin C, L ascorbic acid (not in human)
¾
Conjugation Reactions: UDP‐glucuronic acid is used for conjugation with many compounds to make them more soluble before excretion eg. steroid hormones and bilirubin
¾
Detoxification reactions: UDP‐glucuronic acid is used for conjugation with toxic compounds to make them less toxic eg. phenols
`
UDP‐glucuronate is converted to glucuronate then to L‐xylulose to D‐xylitol to D‐xylulose which then join PPP to complete its oxidation
Formation of UDP‐glucuronic acid
¾ Glucose 6‐phosphate is isomerized to glucose 1‐phosphate, which then reacts with uridine triphosphate (UTP) to form uridine
diphosphate glucose (UDPGlc) in a reaction catalyzed by UDPGlc
pyrophosphorylase
¾ UDPGlc is oxidized at carbon 6 by NAD‐dependent UDPGlc
dehydrogenase in a two‐step reaction to yield UDP‐glucuronate
Formation of UDP‐glucuronic acid
Glucose‐6‐(P)
Hexokinase
Glucose
Phospho‐
glucomutase
UDPGlc
pyrophosphorylase
UDPGlc dehydrogenase
`
www.rpi.edu/dept/bcbp/molbiochem
`
www.med.ufl.edu/biochem/rcohen/rcohen.html