Uronic Acid Pathway

1. Uronic Acid Pathway Dr. Sadia Haroon Biochemistry department Peshawar Medical College GIT MODULE 2019

2. Overview • It is an alternative oxidative pathway for glucose that doesn’t lead to ATP generation. • It includes oxidation of glucose to • Glucuronic acid • Ascorbic acid • Pentose It occurs mainly in the liver cytoplasm.

3. Normal carbohydrate metabolism ,phosphate esters are involved – but in uronic acid pathway free sugars and sugar acids are involved. • Steps of reactions : • Formation of UDP-glucoronate • Conversion of UDP- glucoronate to L-gulonate • Synthesis of ascorbic acid in some animals • Oxidation of L-gulonate

4. MajorFunctionistoproduce D-Glucuronicacidwhichisrequiredfor: Detoxificationofforeignchemicals.SynthesisofMPS. Thereisnoenergyproduction.

5. Uronic acid Pathway of Glucose Importance in humans: • ProvidesUDP-glucuronic acid for conjugation[conjugation of bilirubin, steroids etc] and synthesis of glycosaminoglycans. • In lower animals (not in primates- deficiency of enzyme L-gulonolactone oxidase), this pathway leads to synthesis of Vit C. • Essential Pentosuria: one of Garrod’s tetrad [alkaptonuria, albinism, pentosuria, cystinuria- inborn error of metabolism]: *1 in 2500 births due to deficiency of xylitol dehydrogenase → L-xylulose excreted in urine gives + benedict’s test-not harmful. *Diffentiated from DM by + Bials test by pentose sugars.

7. GLUCOSE GlucokinaseATPADP Glucose6-phospate Phosphogluco-Mutase Glucose1-phosphate

8. UDP-Glu-pyrophos- Phorylase UTPPPi UDP-Glucose 2NAD+2NADH+H+ UDP-Glu- dehydrogenase UDP-Glucoronate phosphorylation D-Glucuronicacid1-P

9. GlucuronidaseH2OUDP D-Glucoronicacid LGDH L-Gulonicacid

10. L-Gulonicacid beta-HD L-Gulonolactone oxidase 3-KetogulonicL-Ascorbicacid Acid beta-ketoL-gulonatedecarboxylase L-Xylulose

11. Xylulosereductase Xylitol NAD+ NADH+H+ D-Xylulose XylulosekinaseATPADP Xylulose5-phosphate HMPShunt

12. Administrationofdrugsi.e Chlorobutanol& Barbitalsignificantlyincreasestheuronic acidpathway. Certaindrugsarealsofoundtoenhance thesynthesisofAscorbicacid.

13. Administrationofdrugs increasestheuronicacidpathwaytoachievemoresynthesisofglucuronatefromglucose. Certaindrugs(aminopyrine,antipyrine) werefoundtoenhancethesynthesisofascorbicacidinrats.

14. URONIC ACID PATHWAY G-6-P Phosphoglucomutase G-1-P + UTP [UDPG Phosphorylase] UDP- Glucose enters Uronic acid pathway

15. Relations with HMP SHUNT The unutilized Glucuronate produced in this pathway is converted to Xylulose-5 P which is further metabolized through HMP pathway

16. Uronicacidpathway

17. oESSENTIALPENTOSURIA:ItisanAutosomalrecessivedisorder. Metabolicdefect:- DeficiencyofXylitolReductase

18. . Some monosaccharide units found in glycosaminoglycans.

19. Hyaluronic acid

20. A. Relationship between GAG structure and function • Because of their large # of –ve charges, these heteropolysacch chains tend to be extended in solutions. They repel each other and are surrounded by a shell of water molecules. • When brought together, they “slip” past each other, much as two magnets with the same polarity seem to slip past each other. This produces “slippery” consistency of mucous secretions & synovial fluid • When a soln of GAGs is compressed, the water is “squeezed out” & the GAGs are forced to occupy a smaller volume. When the compression is released, the GAGs spring back to their original, hydrated volume because of repulsion of their –ve charges. This property contributes to the resilience of synovial fluid & the vitreous humor of the eye.

21. Figure 14.3. Resilience of glycosaminoglycans.

22. FORMATION AND ROLE OF UDP GLUCOSE • Activated sugars are attached to nucleotide are converted into other sugars oxidized sugar acids and joined to proteins ,lipids aand other sugars through glycosidic bonds

23. Metabolic reactions • Glucose 6-phosphate is isomerized to glucose 1-phosphate • Glucose 1-phosphate reacts with uridine triphosphate (UTP) to form uridinediphosphate glucose (UDPGlc) in a reaction catalyzed by UDPGlcpyrophosphorylase • UDPGlcis oxidized at carbon 6 by NAD-dependent UDPGlc dehydrogenase in a two-step reaction to yield UDP-glucuronate 1 2 3

24. Functions of UDP Glucuronate • 1.formation of bilirubin diglucuruonide. • 2.urinary diglucuronide • Like xenobiotics,steroids etc • 3.udp xylose goes to pentose phosphate pathways • 4.formation of iduronate • 5.formation of proteoglycans and glycosamino glycans .

25. UDP-glucuronate/iduronate • Acidic group of glucuronate carries a negative charge at physiological pH • And increases water solubility of the compounds. • L-Iduronic acid (IdoA) is the major uronic acid component of the glycosaminoglycans (GAGs) dermatan sulfate, and heparin. It is also present in heparan sulfate ...

26. Formation of glucuronoids • Compounds degraded and excreted as urinary glucuronides • Steroids like estrogen,progesterones, • Triodothyronine (thyroid hormone • Mebrobamate (drugs for sleep) • Morphine (pain killer )

27. Glucuronides • A glycosidic bond is formed between anomeric OH of glucuronate and an OH of nonpolar compound . • The negative charge of carboxyl group of the glucuronate increases the water solubility of other wise non polar compound and it is exceted in urine and bile.

28. UDP Glucuronic acid (active form) is needed in: • Conjugation to less polar compounds as bilirubin, steroids and some drugs making them more water soluble (detoxicated) . • Synthesis of glycosaminoglycans (mucopolysaccharide) as heparin, hyaluronic acid. • In plants and some animals (not Human) glucuronic acid serves as a precursor of L-ascorbic acid. • The uronic acid pathway also provides a mechanism by which dietary D-xylulose enter the central pathway.

29. Alternative oxidative pathway for glucose. • synthesis of glucorinc acid,pentoses and vitamin (ascorbic acid). • Normal carbohydrate metabolism ,phosphate esters are involved – but in uronic acid pathway free sugars and sugar acids are involved. • Steps of reactions : • Formation of UDP-glucoronate • Conversion of UDP- glucoronate to L-gulonate • Synthesis of ascorbic acid in some animals • Oxidation of L-gulonate

30. Conjugation of bilirubin with glucuronic acid • Bilirubin is non-polar. • Hepatocytes convert bilirubin to a polar form by adding glucuronic acid to it (conjugation) • Enzyme: glucuronosyl transferase • Location:endoplasmic reticulum • Glucuronyl donor:UDP-GLUCURONIC ACID

31. Bilirubin monoglucuronide is an intermediate, subsequently converted to diglucuronide. Phenobarbital induces UDP-glucoronoyl-transferase activity

32. 2. Congugation process: • In liver: Bilirubin is conjugated with Glucouronic acid to produce bilirubin diglucuronides, which is water soluble and readily transported to bile. and thus out into the small intestine. Blilirubin + Glucouronic acid bilirubin diglucuronides • Then conjugated bilirubin is excreted in bile through bile duct to help in food digestion (mainly fat). UDP-glucuronyl transferase "water insoluble" "water soluble" "Conjugated BIL" Bile

36. L-gulonateistheprecursorforthesynthesisof ascorbicacid enzymeL-gulonolactoneoxidase,convertsgulonatetoascorbicacid. L-gulonolactoneoxidaseabsentinman, otherprimatesandguineapigs. VitaminChastobesupplementedinthediet fortheseanimals.

37. ClinicalAspects • Effects of drugs : increases the pathway to achieve more synthesis of glucaronate from glucose . • - barbital,chloro-butanol etc. • Essential pentosuria : deficiency of xylitol- dehydrogenase • Rare genetic disorder • Asymptomatic • Excrete large amount of L-xylulose in urine • No ill-effects

38. Essentialpentosuriaisasymptomatic.Barbiturates,aminopyrine,willinduceuronicacidpathwayandwillincreasetheexcretionofL-xylulose.Essentialpentosuriaisasymptomatic.Barbiturates,aminopyrine,willinduceuronicacidpathwayandwillincreasetheexcretionofL-xylulose.

39. Essentialpentosuriaisasymptomatic.Barbiturates,aminopyrine,willinduceuronicacidpathwayandwillincreasetheexcretionofL-xylulose.Essentialpentosuriaisasymptomatic.Barbiturates,aminopyrine,willinduceuronicacidpathwayandwillincreasetheexcretionofL-xylulose.