25 - Lecture Guide

Review of Lecture 25 Questions to think about…. Folic acid deficiency, believed to be the most common vitamin deficiency, causes a type of anemia in which haemoglobin synthesis is impaired and erythrocytes do not mature properly. What is the metabolic relationship between haemoglobin synthesis and folic acid deficiency? We know that hemoglobin is contains heme. Heme is derived from catalytic reaction performed by the protein ferrochelatase where it incorporates an iron ion (Fe2+) into protoporphyrin. The precursor for protoporphyrin is porphobilinogen, which is produced via the amino acid glycine and succinyl-coA. Glycine belongs to the serine family of amino acids. In order for serine to be produced from its precursor glycerol-3-phosphate, tetrahydrofolate (THF) is a required cofactor for the enzyme that drives this reaction my acting as a methyl carrier – THF is derived from a vitamin B6. Interestingly, pyridoxal phosphate and biotin are also vit. B6 derivatives. Serine also gives rise to glycine using an enzyme that also required THF. Therefore, without sufficient amounts of folate in one’s diet, haemoglobin synthesis could be impaired due to a lack of glycine and or serine. SUCCINYL-COA + GLYCINE AMINOLEVULINATE PORPHOBILINOGEN PROTOPORPHYRIN HEME [VIA FERROCHELATASE AND FE2+]. Suppose that a person is found who is deficient in an enzyme required for IMP synthesis. How might this person be treated? IMP (inosine monophosphate) gives rise to AMP and GMP. The nucleotide base of inosine is hypoxanthine and HPRT is responsible for regenerating hypoxanthine to IMP. IMP is broken down to inosine and inosine is broken down to hypoxanthine, so is xanosine and guanine. Since this is true, you can treat the person with hypoxanthine or inosine. Nucleotides play a variety of roles in the cell. Give an example of a nucleotide that acts in each of the following roles or processes: Second messenger CAMP Phosphoryl-group transfer ATP Activation of carbohydrates UDP-GLUCOSE [recall that glucose 1 phosphate, produced by glycogen phosphorylase is converted to uridine diphosphate glucose via the enzyme UDP-glucose pyrophosphorylase.] Activation of acetyl groups ACETYL-COA… coenzyme A. Transfer of electrons NAD+ Chemotherapy FLUOROURACIL Allosteric effector CTP INHIBITS ASPARTATE CARBAMOYLTRANSFERASE… [recall that CTP is derived from UTP; UTP also does this.] Muscle can metabolize AMP by using the purine nucleotide cycle. The intial step in this cycle, catalyzed by AMP deaminase, is the conversion of AMP into IMP. Why might the deamination of AMP facilitate ATP formation in the muscle? IMP is a precursor for AMP. IMP is converted to adenylsuccinate using aspartate and GTP via the enzyme adenylsuccinate synthetase. Adenylsuccinate that is converted to AMP via the enzyme adenylsuccinate lyase and fumerate is produced as a biproduct. Fumerate is a component in the Kreb cycle. In fact, fumerate can be converted to malate and malate, via malate dehydrogenase, can be converted into oxaloacetate, which is an intermediate of the Kreb cycle as well. Oxaloacetate via citrate synthase and acetyl-CoA produces citrate-acetyl. How does the purine nucleotide cycle assist the aerobic generation of ATP? So, in aerobic condition, acetyl-CoA is produced as a biproduct of glycolysis and this combines with oxaloacetate in the kerb cycle. Lesch-Nyhan syndrome is a disease caused by severe deficiency in HPRT activity. In the absence of HPRT, hypoxanthine and guanine are not converted to IMP and GMP, respectively, in the salvage reactions. The disease is characterized by the accumulation of excessive amounts of uric acid, a product of nucleotide degradation, which causes neurological abnormalities and destructive behaviour, including self-mutilation. Explain why the absence of HPRT causes uric acid to accumulate. HPRT is required to convert hypoxanthine and guanine back into inosine monophosphate and guanosine monophosphate. If this does not occur, IMP and GMP will continue to be produced because their production is ultimately regulated by their concentrations inside the cell. Recall that both IMP and GMP inhibit glutamine PRPP amidotransferase and PRPP synthetase when their concentrations are high. Once produced, IMP and GMP will be degraded in inosine and guanosine, respectively, and then hypoxanthine and guanine. This unregulated cycle will increase the concentration of hypoxanthine because all purines are converted to xanthine. Xanthine is converted into uric acid by xanthine oxidase. Explain how 5-fluorouracil combats some types of cancer. 5-fluorouracil is an analogue of uracil. When UMP is converted to dUMP via ribonucleotide reductase and subsequently into dUTP, it is converted to dTTP via the enzyme thymidylate synthase into thymine triphosphate. Since 5-fluorouracil is an analogue, dfUMP will inhibit the enzyme that produce the thymine derivative and it will not be able to get incorporated in DNA synthesis. Name the two thioesters of central metabolism that are formed from the catabolism of pyrimidines. Why is deoxyadenosine toxic to mammalian cells? It binds to the regulatory site (specificity site) of ribonucleotide reductase and inhibits it catalysis by preventing allosteric effectors from binding. Why might multifunctional enzymes such as those of the pyrimidine biosynthetic pathway of mammals be advantageous to an organism? In eukaryotes, the synthesis of pyrimidines is highly dependent on a multifunction protein complex which consists of all the essential enzymes to produce orotate. This enzyme is called dihydrorotate synthase and it consists of aspartate transcarbamoylase, Carbamoyl phosphate synthetase and dihydroorotase. In bacteria (prokaryotes), these are all monofunctional proteins separate from one another (in trans). Therefore, the synthesis of pyrimidines in more complex organisms is more organized and the possibility of one gene being defected is lower than having one of three genes being defected. Moreover, in terms of energy use, less energy is being invested to produce one multifunctional gene than three distinctive separate proteins – also, less targets for inhibition. Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) a salvage enzyme of nucleotide metabolism uses 5’-phsophoribosylpyrophosphate (PRPP) to convert hypoxanthine to IMP and guanine to GMP. A deficiency of this enzyme can lead to an increased level of purine synthesis, excess formation of uric acid and hyperuricemia, or gout. How might a deficiency in HGPRT stimulate purine synthesis? Already answered up top. Under what conditions might one expect a deficiency of hypoxanthine-guanine phosphoribosyl transferase to affect the rate of pyrimidine nucleotide synthesis? How could you estimate the rate of pyrimidine nucleotide synthesis in humans? Pyrimidine synthesis can be regulated by the biproducts of purines. In fact, in the rate limiting enzyme, aspartate transcarbamoylase is activated by ATP while CTP synthetase is activated by GTP. Therefore, if hypoxanthine is not being reconverted into IMP or guanine is not being converted into GMP, this could affect the rate of pyrimidine synthese. Likewise, ATP is required in the production of orotate and is required by CTP synthetase to drive the reaction. To test the rate, limit the amount of ATP and the amount of GTP and see how fast the reactions take place. Questions for extra review: Page 589: #17, 32 Page 590: #47, 53