Transcript Document
BIOC 801 - Dr. Tischler Lecture 20 – February 10, 2006 METABOLISM: NUCLEOTIDE SYNTHESIS & DISORDERS CLINICAL PREMISE S.G., a 45-year-old Caucasian male, presented to your office complaining of foot pain. The pain began approximately one week ago when he noticed one morning that his right big toe was swollen and painful to touch. He attributed the pain to “stubbing” his toe two days earlier on a coffee table. He initially took aspirin and Tylenol with some minimal improvement in the pain, but over the past week the pain has increased and now the big toe is red. Further history reveals that S.G. is an accountant, has had a weight problem “most of his life”, doesn’t exercise and is a wine connoisseur. Physical examination reveals an obese, middle-aged white male. His vital signs are normal with the exception of elevated BP. His right toe is swollen and the skin over the joint is inflamed and tender to touch. His left big toe is swollen but is not inflamed. A complete blood count is normal. His blood chemistry values were normal except for uric acid of 14.5 mg/dl (nl: 3-9 mg/dL). Urinalysis reveals a crystalluria. Analysis of RBC enzymes relevant to uric acid metabolism shows normal activity and normal regulation of the metabolic pathway. ROLES OF NUCLEOTIDES building blocks for DNA and RNA “second messengers” in signal transduction cascades energy “currency” of the cell Glucose-6-P pentose phosphate pathway ATP Ribose-5-phosphate inosine monophosphate (IMP) AMP 5-phosphoribosylpyrophosphate (PRPP) PRPP synthetase Gln PRPP Amino acids: amidotransferase Glu Gly + Gln + Asp 5-phospho--ribosylamine Cofactors: N10-formyl THF Figure 1. Synthesis of inosine monophosphate (IMP) NAD+ aspartate + GTP IMP GDP + Pi NADH Gln + ATP fumarate Glu+AMP+PPi guanosine monophosphate (GMP) adenosine monophosphate (AMP) GDP ADP GTP ATP Figure 2. Formation of AMP and GMP from IMP PRPP synthetase PRPP amidotransferase IMP allosteric inhibition GMP AMP GDP ADP Figure 3. Allosteric inhibition of purine biosynthesis; also ATP stimulates stimulates formation of AMP. GMP, GDP or GTP AMP, ADP or ATP Adenosine Guanosine NH3 adenosine deaminase Inosine Ribose-1-P purine nucleoside phosphorylase +PRPP Guanine HGPRT Hypoxanthine Xanthine xanthine oxidase GMP Ribose-1-P xanthine oxidase +PRPP HGPRT IMP Uric acid Figure 4. Degradation of purines to uric acid and salvage of purine bases via hypoxanthine-guanine phosphoribosyl transferase (HGPRT). Glucose-6-P pentose phosphate pathway ATP Ribose-5-phosphate inosine monophosphate (IMP) AMP X1a 5-phosphoribosylpyrophosphate (PRPP) PRPP synthetase Gln PRPP Amino acids: amidotransferase Glu Gly + Gln + Asp 5-phospho--ribosylamine Cofactors: N10-formyl THF Figure 1. Hyperuricemia: Gout: X1a = PRPP synthetase defects associated with a superactive enzyme characterized by an increased Vmax or an enzyme with a reduced Km for ribose-5-P. X1b PRPP synthetase PRPP amidotransferase IMP GMP AMP GDP ADP Figure 3. Gout: X1b = PRPP synthetase defect associated with resistance to feedback inhibition. GMP, GDP or GTP AMP, ADP or ATP Adenosine Guanosine NH3 adenosine deaminase Inosine Ribose-1-P purine nucleoside phosphorylase +PRPP Guanine HGPRT GMP Hypoxanthine Xanthine X2 Ribose-1-P Inhibited by allopurinol xanthine oxidase xanthine oxidase +PRPP HGPRT X2 IMP Uric acid Figure 4. X2 = moderate defect (>50% activity) leading to gout; severe defect (very low activity) leads to Lesch-Nyhan syndrome. GOUT AND LESCH-NYHAN SYNDROME Gout = hyperuricemia due to a variety of causes Lesch-Nyhan syndrome = excessive hyperuricemia; leads to self-mutilation Urate crystals appearing in a diaper – often found in synovial fluid of joints Incan Mask Depicting An Individual Presumably with Lesch-Nyhan as Evidenced by the Self-Mutilation ATP, PRPP carbamoyl phosphate synthetase II (gln) Glutamine + 2ATP + CO2 - UDP UTP Glu + Pi + ADP CTP Glu + Pi + 2 ADP Gln + ATP +2 ADP UTP carbamoyl phosphate aspartate transcarbamoylase carbamoylaspartate aspartate +2 ATP orotate orotate phosphoribosyl transferase UMP OMP orotidylic acid decarboxylase Figure 5. Biosynthesis of the pyrimidine nucleotides UTP and CTP. +PRPP ADP GDP UDP CDP ribonucleotide reductase reduced thioredoxin dADP dGDP dUDP dCDP oxidized thioredoxin dUMP thymidylate synthase N5,N10-methylene THF dihydrofolate TMP NADP + thioredoxin reductase +NADPH + H+ Figure 6. Biosynthesis of deoxyribonucleotides by ribonucleotide reductase and of thymidine monophosphate (TMP) by thymidylate synthase. PRPP N5,N10-Methenyl THF Purines: AMP + GMP N10-Formyl THF N5,N10Methylene THF Glycine Suicide inhibitor F4 dUMP 5-F-uracil X thymidylate synthase N5-Methyl THF TMP Dihydrofolate (DHF) OH-B12 folate trap dUMP Serine X3 X5 THF DHF reductase MTX Methyl B12 Figure 7. Metabolism of dietary folic acid to its various cofactor forms. DIETARY FOLIC ACID X34 = site at which cobalamin (B12) deficiency causes X 5= fluorodeoxyuridylate (F-dUMP), synthesized from 5-flurouracil, acts as folate to be as N5-methyl(MTX) THF acts as a chemotherapeutic agent; X = site attrapped which agent methotrexate chemotherapeutic