BIOCHEMISTRY: CARBOHYDRAT E METABOLISM 3 Page 1 ALTERNATIVE PATHWAY FOR CARBOHYDRATE METABOLISM (for clearer images, pls. refer to the ppt) Question: How many ATPs do you need in the conversion ofFRUCTOSE-6-PHOSPHATE to PYRUVATE? Answer: 1 ATP only since we have already bypassed the major regulatory pathway of glycolysis which is PHOSPHOFRUCTO KINASE 1 -It is not recommended to carbo load on fructose since it cannot be controlled (Phosphofructokinase regulates the amount that the body requires) Question: How many ATPs do you need in the conversion ofSUCROSE to PYRUVATE? (It will come out in the exam.) Answer: 3 ATPs. Sucrose is formed by glucose and fructose. From glucose to pyruvate, 2 ATPs are needed. For fructose to pyruvate, 1 ATP is required. In conclusion, 3 ATPs are required since 2 ATPs are needed for the breakdown ofglucose and 1 ATP is needed for the breakdown of fructose ALTERNATIVE PATHWAY OF METABOLISM-Pentose Phosphate Pathway -Uronic Acid Pathway -Fructose Metabolism -Metabolism of Galactose -Sorbitol Pathway -Ethanol Metabolism -Hexosamine Formation HEXOSE MONOPHOSPHATE SHUNT -aka PENTOSE PHOSPHATE PATHWAY-an ALTERNATE ROUTE for glucose oxidation -not the main main pathway -major pathway is GLYCOLYSIS -found in the CYTOSOL -active in: liver, adipose tissue, lactating mammary gland, adrenal cortex and RBC -common among these organs: all are secretory organs, except for RBC -5 carbons are produced in this pathway -the 1 carbon will be produced as CO2-the only pathway that produces CO2 in cytoplasm-carbon dioxide is normally produced in the mitochondria but in this pathway, the carbon dioxide is in the cytoplasm -NADPH is important in this pathway (PPP is the major source of NADPH in cells) -This pathway is used as an alternative when there is a high demand for NADPH which is required in FATTY ACID SYNTHESIS TISSUE DISTRIBUTION -Demand for NADPH •Biosynthetic pathways -fatty acid synthesis (liver, adipose, mammary) -cholesterol synthesis (liver) -steroid hormone synthesis (adrenal, ovaries, testes) •Detoxification (Cytochrome P450 system) liver •Reduced glutathione as an antioxidant in RBC to maintain the shape of RBC -preventing hemolysis •Generation of superoxide (neutrophils) **All these conditions will cause an INCREASE in NADPH demand which can lead to the hexose monophosphate shunt FUNCTIONS -Source of: 1. NADPH + H + for REDUCTIVE BIOSYNTHETIC PROCESSES (synthesis of cholesterol, fatty acids, steroid hormones) -more important function 2. Pentoses for nucleotides, nucleic acids and coenzymes CHARACTERISTICS -neither consumes nor produce ATP -that is why it is not the major pathway -branches off glycolysis at G6P (common to both glycolysis and HMP shunt) -most important molecule in this pathway is GLUCOSE-6- PHOSPHATE -glucose should first be converted to glucose-6- phosphate before entering the hexose monophosphate shunt -GLUCOKINASE in the liver and HEXOKINASE in the muscles enzymes needed for the conversion of glucose to glucose-6-phosphate -common enzyme in glycolysis and HMP shunt -re-entry is at FRUCTOSE-6-PHOSPHATE -PHOSOHEXOSE ISOMERASEenzyme required for the conversion of glucose-6-phosphate to fructose-6-phosphate SUBJECT: BIOCHEMISTRYTOPIC: CARBOHYDRATE METABOLISM 3 LECTURER: DR. ESPERANZA UYDATE: DECEMBER, 2010
METABOLISM (for clearer images, pls. refer to the ppt)
Question: How many ATPs do you need in the conversion of FRUCTOSE-6-PHOSPHATE to PYRUVATE?
Answer: 1 ATP only since we have already bypassed themajor regulatory pathway of glycolysis which isPHOSPHOFRUCTOKINASE 1
- It is not recommended to carbo load on fructose
since it cannot be controlled (Phosphofructokinaseregulates the amount that the body requires)
Question: How many ATPs do you need in the conversion of SUCROSE to PYRUVATE? (It will come out in the exam.)
Answer: 3 ATPs. Sucrose is formed by glucose and fructose.From glucose to pyruvate, 2 ATPs are needed. For fructoseto pyruvate, 1 ATP is required. In conclusion, 3 ATPs arerequired since 2 ATPs are needed for the breakdown of
glucose and 1 ATP is needed for the breakdown of fructose
-decarboxylates 6-phosphogluconate which RELEASESCARBON DIOXIDE from the cytoplasm
-product: ribulose-5-phosphate
Question: Why do we need 3 glucose-6-phophatemolecules?
Answer: To produce 2 fructose-6-phosphate molecules and1 glyceraldehyde molecule through the interconversion of the pentoses
NON-OXIDATIVE PHASE
-starts in the ribulose-5-phosphate molecule
-reversible (can go anywhere)
-rearrangement of sugars to enter glycolytic pathway
-provides a pathway for recycling excess pentoses
IMPORTANT ENZYMES: (refer to the diagram below as well)
1) 3-epimerase
-to convert ribulose-5-phosphate to xylulose-5-phosphate
**epimerase translocation in only carbon
2) Ketoisomerase
-to convert ribulose-5-phosphate to ribose-5-phosphate
3) Transketolase
-will transfer 2 carbon atoms from xylulose-5-phosphate to ribose-5-phosphate to form glyceraldehyde-3-phosphateand sedoheptulose-7-phosphate
-important coenzyme: THIAMIN (Vit. B1)
-important cofactor: MAGNESIUM (Mg +)
-source: unpolished rice
** blocking transketolase activity may lead to pentosuria (apentose sugar, usually xylulose, is found in urine
** thiamine deficiency affects sedoheptulose 7 phosphatesince it affects transketolase activity too
4) Transaldolase
-will transfer 3 carbon atoms
-sedoheptulose-7-phosphate is a very unstablecompound that is why it will be acted upon byTRANSALDOLASE to form erythrose-4-phosphate andfructose-6-phosphate which will then be converted toglucose-6-phosphate through the enzymePHOSPHOHEXOSE ISOMERASE
**Another TRANSKETOLASE (similar with the transketolasementioned above) will act on xylulose-5-phosphate to beconverted to FRUCTOSE-6-PHOSPHATE andGLYCERALDEHYDE-3-PHOSPHATE
5) Phosphotriose isomerise
-converts glyceraldehydes-3-phosphate to fructose-1,6-bisphosphate
6) Fructose-1,6-bisphosphatase
-also found in GLUCONEOGENESIS because it uses thefructose-1,6-bisphosphate as a substrate
-converts fructose-1,6-bisphosphate to fructose-6-phosphate
**all the other enzymes are also found in the glycolysispathway except the FRUCTOSE-1,6-BIPHOSPHATASE whichis found in the gluconeogenesis pathway
**3 glucose-6-phosphate molecules are required, 6molecules of NADPH + H + are produced and 3 CO2 from
the cytoplasm are released to have the product, xylulose-5-phosphate irreversible
**the reversible process will ultimately produce 2molecules of glucose-6-phosphate and 1 molecule of glyceraldehydes. All of the three molecules will enter theglycolytic pathway
-entry point GLYCERALDEHYDE-3-PHOSPHATEDEHYDROGENASE
**defiency in thiamine (VIt.B1) problem in transketolaseenzyme. This can cause hemolytic anemia and jaundice
OVERALL EQUATION:
Glucose -6- PO 4 + 2 NADP + + H 2O ↔
ribose -5- PO 4 + 2NADPH + 2 H + + CO 2
REGULATION
1.Glucose-6-phosphate dehydrogenase
-first step; where regulation acts most
-rate limiting step
-most important regulator
-abundant in the RBC
2.Allosteric regulation
-feedback inhibited by NADPH
-increase amounts of NADPH will cease the HMP shunt
-increased NADPH also entails and increased amountof H ion which decreases intracellular pH (becomes moreacidic); can shrink and wrinkle the cell
3.Inducible enzyme-induced by insulin
-also found in the glycolytic and glycogenesis pathway
-increased amounts of insulin can induce the HMP shunt(since much glucose needs to be distributed)
GLUCOSE-6-PHOSPHATE DEFICIENCY
-problem in glucose-6-phosphate dehydrogenase enzyme
-not evident in most cases
-common in the Philippines (usually affects males, whilewomen are carriers)
-induced by drugs: Antimalarials
sulfa antibiotics antipyretics (aspirins,etc.)
-acute haemolytic anemia is due to decreased NADPHproduction
-HMP is the major pathway of NADPH production in the redblood cell (G6PD deficient RBCs = more prone to hemolysis)
-NADPH is responsible for maintaining glutathione in itsreduced state
-reduced glutathione is necessary for the integrity of theerythrocyte membrane, thus rendering enzyme-deficientred cells more susceptible to hemolysis by a wide range of compounds
-exposure to anti-malarial drugs (Primaquine) results inincreased cellular production of superoxide and hydrogenperoxide (Primaquine sensitivity)
-other chemicals known to increase oxidant stress
-sulfonamides (antibiotic)
-aspirin and NSAIDs
-quinidine and quinine
-naphthalene (mothballs)
-fava beans (vicine and isouramil)
ROLE OF NADPH IN THE RBC
1.Production of superoxide
2.Hb-Fe 2+ -O2 Hb-Fe 3+ + O 2 -
3.O 2 - + 2H 2 O 2H 2O2
4.Both O 2 - and H 2 O2 can produce reactive free radicalspecies, damage cell membranes and cause hemolysis
DETOXIFICATION OF SUPEROXIDE ANION AND HYDROGENPEROXIDE (H20 2 )
-Antioxidant enzymes-superoxide dismutase
-glutathione peroxidise
-if the glutathione is in the reduced state, hydrogenperoxide will be converted to water
-glutathione reductase
Question: In the formation of lactate from glucose-6-phosphate, how many ATPs are needed?
-also encountered in HEME METABOLISM (for more details,see Bilirubin Conjugation)
-catalyzes the conversion of glucose to glucuronic acid,ascorbic acid and pentoses
**GLUCORONIC ACID to conjugate bilirubin so that itcan be more soluble in water
-also an alternative oxidative pathway for glucose
-does not lead to ATP formation
**conversion of glucose-6-phosphate to glucose-1-phosphate was also encountered in glycogenolysis
**PENTOSURIA blocks the L-xylulose xylitol pathway
-causes the presence of xylulose in the urine
**xylulose-5-phosphate will then enter the HMP shunt
**The L-gulonate has 2 pathways: 1 for the formation of xylulose-5-phosphate and the other for the formation of ascorbic acid.
-However, the formation of ascorbic acid is not possiblein higher forms of life since the L-gulonolactone 2-ketogulonolactone pathway is BLOCKED. So in humans, theonly possible route is for it to be converted to xylulose-5-phosphate which will now enter the HMP shunt
-this means that Vitamin C cannot be produced by the
body and supplements should be taken in to supply thebody with Vit.C
-high amounts of Vit.C in the body can lead to stones
-glucuronide formation is important during detoxification,steroid excretion, and bilirubin metabolism
-the reaction is catalyzed by UDP-glucuronyl transferase,which may take several days to 2 weeks after birth tobecome fully active in humans
-babies are always exposed to the sun to hasten theprocess of bilirubin solubility
FRUCTOSE METABOLISM
ESSENTIAL FRUCTOSURIA
-fructokinase is deficient in essential fructosuria, a benignasymptomatic metabolic anomaly; autosomal recessive
-following intake of fructose, blood and urinary fructoselevels of affected individuals are unusually high; however,90% of their fructose intake is eventually metabolized
**Fructose can also be a source of glucose through theenzyme PHOSPHOFRUCTOISOMERASE
**hexokinase is still needed to form fructose-6-phosphate**Aldolase A used in glycolysis
**Aldolase B used in fructose metabolism
**Fructokinase deficient in essential fructosuria
-no formation of fructose-1-phosphate and instead,fructose from diet will be converted in the glycolysispathway (not that significant)
G6P G1P UDP-Glucose UDP- GlucoronicAcid Glucoronic Acid L-Gulonic Acid (a directVit. C precursor as well) 3-Keto-L-Gulonic Acid
