NITROGEN BIOSYNTHETIC PATHWAYS BIOC 460 DR. TISCHLER LECTURE 39

OBJECTIVES 1.Outline the pathways for synthesis of the following nonessential amino acids by identifying the primary source of the carbons and the nitrogen for their synthesis (alan, arg, asp, glu, gln, and tyr) in humans. 1.General role of cofactors-derived from folic acid and how these are important in metabolism of ser, gly, met and cys 1.Discuss significance of S-adenosylmethionine in "one- carbon" metabolism

4.Identify two specific cofactor forms of vitamin B12 and list the amino acids whose metabolism depends on each form. 5.Concerning heme synthesis: a) describe the reactions involved in the formation of porphobilinogen b) discuss the key features of its regulation c) explain why lead poisoning would affect the oxygen carrying ability of the blood. 6.List the steps in the catabolism of heme to conjugated bilirubin and identify the general condition associated with the accumulation of bilirubin in the body. OBJECTIVES (cont.)

ornithine pyruvate  -ketoglutarate glutamate alanine A +aspartate arginino- succinate arginine B phenylalanine tyrosine F glutamate citrulline + carbamoyl phosphate NH 3 + CO 2 + ATP oxaloacetate aspartate C  -ketoglutarate glutamate ADP + P i NH 3 + ATP glutamine E glutamate  -ketoglutarate + NH 3 NAD(P)H NAD(P) + D glutamate Figure 1. Synthesis of the nonessential amino acids alanine (A), arginine (B), aspartate (C), glutamate (D), glutamine (E), and tyrosine (F).

Figure 2. Synthesis of the nonessential amino acids asparagine, serine, glycine. CH 2 -THF is methylene tetrahydrofolate aspartateasparagine glutamine glutamate glycine serine THF CH 2 -THF 3-phosphoglycerate NAD + + glutamate + H 2 O NADH +  -ketoglutarate + P i serine CH 2 -THF + NADH THF + NAD + NH 3 + CO 2 glycine glycine synthase

Methionine-CH 3 HomocysteineCH 3 group donation adenosine S-Adenosylmethionine ATP Figure 3. Metabolism of CH 3 -THF, CH 3 -Cbl, met and cys. CH 3 CysteineCystathionine N 5 -CH 3 THF THF Hydroxy B 12 CH 3 - B 12 Methionine synthase CH 3

Table 1. Summary of the roles of folate and cobalamin in nitrogen metabolism CofactorVitamin precursor Role Methyl-cobalamin Cobalamin (vitamin B 12 ) methionine metabolism Adenosyl-cobalaminisoleucine/valine metabolism Methylene- tetrahydrofolate Folic acid serine/glycine metabolism Methyltetrahydrofolatemethylcobalamin formation Tetrahydrofolatethymidine biosynthesis

MITOCHONDRIA CYTOPLASM Aminolevulinic acid dehydratase zinc-containing enzyme Pb-sensitive 4 molecules combine Uroporphyrinogen III Coproporphyrinogen III  -Aminolevulinate synthase pyridoxal phosphate Succinyl CoA + Glycine  -Aminolevulinic acid  Porphobilinogen  Coproporphyrinogen III Protoporphyrin IX Ferrochelatase Pb-sensitive HEME Fe 2+   -Aminolevulinic acid (two molecules) Figure 4. Pathway for synthesis of heme

REGULATION OF HEME SYNTHESIS: Heme: diminishes transport of  -ALA synthase from cytoplasm to mitochondria after its synthesis represses the production of  -ALA synthase by regulating gene transcription. inhibits activity of pre-existing  -ALA synthase

TEETH FROM PATIENT WITH PORPHYRIA DISORDER

PATIENT WITH PORPHYRIA DISORDER

BLOOD CELLS LIVER Bilirubin diglucuronide (water-soluble) 2 UDP-glucuronic acid Stercobilin excreted in feces Urobilinogen formed by bacteria KIDNEY Urobilin excreted in urine reabsorbed into blood via bile duct to intestines Bilirubin (water- insoluble) via blood to the liver INTESTINE Figure 5. Catabolism of hemoglobin Biliverdin reductase CO Biliverdin IX  Heme oxygenase Bilirubin (water- insoluble) NADP + NADPH Heme Globin Hemoglobin O2O2