24 - Lecture Guide

Review of Lecture 24 Questions to think about…. Each of the following metabolic disorders results in the accumulation of a metabolite. Match each disorder with a metabolite from the list. Metabolic Disorder Metabolite 1. Maple syrup urine disease D [BECAUSE REMEMBER THAT VALINE, ISOLEUCINE, AND LEUCINE ARE TRANSAMINATED, BUT THEIR DEAMININATED ALPHA-KETO ACIDS CANNOT BE DEGRADED SINCE THE ENZYME THAT DEGRADES BRANCHES AMINO ACIDS IS NOT PRESENT] a. Phenylalanine 2. Alcaptonuria E [YOU HAVE AN INCREASE IN HOMOGENTISATE] b. NH4+ 3. Phenylketonuria A [YOU LACK PHENYLALANINE HYDROXYLASE] c. Sodium urate 4. Hyperammonemia B d. ?-keto acids 5. Porphyria G [YOU HAVE A BUILD UP OF UROPORPHORIA OR COPROPHORYIA] e. Homogentisate 6. Jaundice F [BILIRUBIN (A LINEAR TETRAPYRROLE) WHICH IS DERIVED FROM BILIVERDIN VIA THE ENZYME REDUCTASE BINDS TO CARBOHYDRATES BECAUSE IT IS INSOLUBLE IN H20.] f. Bilirubin 7. Gout C [AMP IMP [HYPOXANTHINE REDUCTASE] HYPOXANTHINE [HYPOXANTHINE REDUCTASE] XANTHINE URIC ACID SODIUM URATE g. Porphyrin 8. Lesch-Nyhan syndrome B…C [BECAUSE THIS SYNDROME IS CHARACTERIZED BY THE PRODUCTION OF GOUT] One of the nitrogen atoms and the carbon atom of urea enter the urea cycle as a single molecule, namely… nitrogen atoms comes from NH4+ and aspirate, while the carbon comes from carbonate. The urea cycle requires energy to form urea. The equivalent of THREE ATP molecules is required. (2 during Carbamoyl phosphate synthetase and 1 during the production of arginosuccinate). The exchange of glucose and alanine between muscle and liver is called the GLUCOSE-ALANINE CYCLE. What are the two major stages by which amino acids are degraded? Ketogenic and glycogenic, ketogenic consist of acetyl-coa and acetoacetyl-coa while the glycogenic are pyruvate, oxaloacetate, fumerate, succinyl-coa, and alpha ketoglutarate. TRANSAMINATION AND DEAMINATION [WHERE NITROGEN IS REMOVED] Discuss the regulation of the urea cycle. The urea cycle is regulated by the amount of ammonium that is present because the rate limiting enzyme (I think) is Carbamoyl phosphate synthetase… recall that this produces Carbamoyl phosphate, an essential metabolite in pyrimindine synthesis – aspartate Carbamoyl and ornithine Carbamoyl. It is also regulated by the amount of aspirate produced. Compare the function of the alanine-glucose cycle with that of the Cori cycle. Under what circumstances might each operate? IN THE GLUCOSE-ALANINE CYCLE, GLUCOSE IS DEGRADED INTO PYRUVATE THROUGH GLYCOLYSIS. PYRUVATE COMBINES WITH GLUTAMATE TO PRODUCED ALANINE AND ALPHA KETOGLUTARATE VIA ALANINE TRANSAMINASE: GLUCOSE [PYRUVATE] + GLUTAMATE ALANINE + ALPHA-KETOGLUTARATE THE ALANINE AMINO ACIDS THAT ARE PRODUCED THEN TRAVEL TO THE LIVER WHERE THEY ARE TRANSAMINATED AND CONVERTED INTO PYRUVATE. THIS IS DONE BY THE REVERSE REACTION SHOWN ABOVE; IN FACT, YOU WILL NOTICE THAT NH4+ ISN’T SHOWN IN THE REACTION. THIS IS BECAUSE GLUTAMATE IS CONVERTED INTO ALPHA-KETOGLUTARATE +NH4+ VIA DEAMINATION USING THE ENZYME GLUTAMATE DEHYDROGENASE. THE PYRUVATE IS CONVERTED BACK INTO GLUCOSE VIA GLUCONEOGENESIS; RECALL THAT DEAMINATION AND THE PRODUCTION OF UREA OCCURS IN THE LIVER: ALANINE + ALPHA-KETOGLUTARATE GLUTAMATE + [PYRUVATE] GLUCOSE THE CORI CYCLE IS SIMILAR TO THIS PROCESS BECAUSE RATHER THAN HAVING PYRUVATE PRODUCE ALANINE, PYRUVATE IS CONVERTED IN LACTATE, ESPECIALLY UNDER ANAEROBIC CONDITIONS. THE LACTATE TRAVELS TO THE LIVER WHERE IT IS CONVERTED BACK INTO PYRUVATE AND PYRUVATE INTO GLUCOSE VIA GLUCONEOGENESIS. THE GLUCOSE RETURNS BACK TO THE MUSCLE WHERE IT IS USED TO HARVEST ENERGY. The ammonium ion is unable to enter the brain, but several known defects in urea cycle enzymes are known to lead to mental retardation. What is the connection? The reason why ammonium is so toxic is because it inhibits the action of glutamate dehydrogenase. If glutamate dehydrogenase is inhibited, glutamate isn’t produced and without glutamate, transaminases won’t work because glutamate is required as a substrate to produce amino acids, such as alanine or aspartate. Moreover, glutamate, via glutamine synthetase, produces glutamine. Glutamine is responsible for numerous biosynthetic reactions, including histidine synthesis and purine (glutamine prpp synthetase) and pyrimidine synthese (glutamine + carbonate producing carbamoyl phosphate). Asparagine is synthesized in most species by an ATP-dependent transfer of the amide nitrogen of glutamine to aspartate in a reaction catalyzed by ASPARAGINE SYNTHETASE. As a matter of fact, this deserves some more mentioning. Once aspartate is produced via transamination or pyruvate and glutamate, it can become asparagine by combining with glutamine. In fact, we saw earlier that glutamine synthetase produces glutamine via glutamate and ammonium. Here, we don’t use ammonium, instead we use glutamine’s amino group nitrogen. Know the urea cycle. Know how the urea cycle is linked to the citric acid cycle. Via fumerate malate [malate dehydrogenase] oxaloacetate. Describe hyperammonemia. This is where you have a defect in glutamate dehydrogenase; it is characterized by an increased level of ammonium in the bloodstream. What is bilirubin? This is an antioxidant that is formed via biliverdin and NADPH using the enzyme biliverdin reductase. It is insoluble in water so it attached to sugars, giving a yellow colour. What are porphyrias? These are defects in hemoglobin metabolism; usually people with porphyria have urine contained uroporphoria which is red. Coproporyia slightly differs from urophoryia because it is methylated. Moreover, these people are usually sensitive to light. Know the conversion of arginine to nitric oxide and citrulline. Know that nitric oxide synthase is the enzyme involved. ARGININE HYDROXYARGININE CITRULLINE + Nitric oxide VIA THE NITRIC OXIDE SYNTHASE ENZYME HAS BOTH AN OXYGENASE DOMAIN AND A REDUCTASE DOMAIN. Individuals with the protein deficiency disease kwashiorkor suffer loss of skin pigmentation. Suggest a biochemical basis for this observation. PERHAPS A LACK OF PROTEIN IN ONE’S DIET YIELD LITTLE PHENYLALINE. PHENYLALANINE IS AN ESSENTIAL ENZYME TO PROTEIN TYROSINE. TYROSINE IS REQUIRED IN MELANOCYTES (PIGMENT PRODUCING CELLS) TO PRODUCE MELANIN via the enzyme tyrosinase. THERE IS NOT ENOUGH TYROSINE FOR TYRISINASE TO PRODUCE MELONIN. Why is NH4+ toxic in higher animals? This is because it deamination in high animals takes place solely in the liver. Therefore, ammonium can quickly accumulate and inhibit rate limiting enzymes including glutamate dehydrogenase. The carbohydrates and fatty acids are important fuel molecules because they yield metabolic energy in the form of ATP when they are catabolized. Are the purines and pyrimidines important fuel molecules? Explain. No, because they require energy to be produced. Do you expect birds to have the enzyme arginase? NO BECAUSE BIRDS ARE AVIAN REPTILES THAT PRODUCE URIC ACID INSTEAD OF UREA. ARGINASE IS USED CONVERT ARGININE INTO UREA AND ORNITHINE, therefore, it is unlikely to find arginase in a bird. The list below contains enzymes of the urea cycle. Place them in the correct order for the cycle. Urease [never heard of this one, but I am assuming is cleaves urea into carbon dioxide and ammonia?] Arginase Argininosuccinate synthetase Carbamoyl phosphate synthetase Ornithine transcarbamoylase Argininosuccinate lyase D E C F B A Which three mammalian enzymes can potentially ‘mop up’ excess NH4+. GLUTAMATE DEHYDROGENASE, CARBAMOYL PHOSPHATE SYNTHETASE, and glutamine synthetase. Write a one sentence answer to each question: Why does liver damage cause jaundice? BILIRUBIN GETS CONCENTRATED IN THE BLOOD, Which FORMS A YELLOW COLOUR WHEN BOUND TO CARBOHYDRATES CAUSING JAUNDINE Why do individuals with congenital erythropoietic porphyria become anemic? THEY ARE UNABLE TO MAKE PORPHORYIN EFFICIENTLY AND THERE IS AN INCREASE IN UROPORPHOYRIA. Why does a genetic deficiency of 5,10-methylenetetrahydrofolate reductase cause homocystinuria? Homocystinuria is caused by the enzyme deficiency cystathionine beta synthase that uses serine and homocysteine [homocysteine is produced from methionine and s-adenosylmethionine] to produce cystathionine and cystathionine goes on to become cysteine. The main factor here is serine and we know that in order to produce serine, THF is required – the same goes with glycine. So, if 5,10-methylenetetrahydrofolate reductase is deficient, than THF is not being produced and serine is not present. I’m not sure which view is right? Another way to look at this is when glycine and 5,10 – methylenetetrahydrofolate come together, serine is produced along with THF. However, if the enzyme that produces 5,10 –methylenetetrahydrofolate is missing, it cannot combine with glycine to produce serine and THF. If your diet is rich in alanine but deficient in aspartate, will you show signs of aspartate deficiency? Explain. The question is, are aspartate and alanine linked? Both alanine and aspartate are non essential amino acids. Alanine is derived from pyruvate and glutamate [via alanine transaminase] and aspartate is derived from oxaloacetate and glutamate [via aspartate transaminase]. So, if alanine is missing, pyruvate and glutamate levels will decrease. Pyruvate via pyruvate decarboxylase becomes oxaloacetate. So, yes, to some degree, they are linked. Questions for extra review: Page 589: #12, 13, 16, 17, 18, 26, 27, 28, 29, 31 Page 590: #33, 40