Anabolism III: Biosynthesis Amino Acids & Nucleotides

ANABOLISM III: Biosynthesis Amino Acids & Nucleotides 1 Biosynthesis Amino Acids & Nucleotides • Nitrogen (a-er H, O, and C) is a major element of living organisms • Most nitrogen is inert in the atmosphere • Making nitrogen useful is not easy Atmosphere is 80% N2 , but is chemically inert need N2 + 3 H2 à 2 NH3 Even though DG′°= –33.5 kJ/mol… breaking a triple bond has high activation energy (i.e., SLOW, kinetically stable). can be accomplished using non-biological processes: N2 and O2 à NO via lightning N2 and H2 à NH3 via the industrial Haber-Bosch process requires T>400 °C, P>300 atm Haber Process Dr. Kornberg: Lecture 04.07.17 (4:03-6:25) 2 min Industrial synthesis of NH3 via the Haber process is one of mankind’s most significant chemical processes. 2 1 Biosynthesis Amino Acids & Haber process Nucleotides 3 Haber Process Dr. Kornberg: Lecture 04.07.17 (4:03-6:25) 2 min Biosynthesis Amino Acids & Nucleotides Catabolism Anabolism Purines, pyrimidines, Nucleotides porphorins neurotransmitters hormones glutathione CO2 cofactors SAM/THF (C1) a- + NH4 [R,Q] [27 other Ns] a-Ketoglutarate N2 Aspartate a-Keto acids Glutamate Sugars Oxaloacetate Fatty acids 5 2 ANABOLISM III: Biosynthesis Amino Acids & Nucleotides 1) Nitrogen fixation: N2 à +NH4 2) Nitrogen assimilation: incorporation of ammonia into biomolecules 3) Biosynthesis of amino acids a) non-essential b) essential 4) Biosynthesis of nucleotides 5) Control of nitrogen metabolism 6) Biosynthesis and degradation of heme; other 2° products of amino acids 6 Biosynthesis Amino Acids & Nucleotides Chemistry of Molecular Nitrogen Review: Oxidation States of Nitrogen Compounds Nitrate • • • • NO3 – N+5 O3– Nitrate Also Nitric acid (HNO3) and Dinitrogen pentoxide (N2O5) • “ate” is the higher oxidation state. Nitrogen(IV) dioxide • NO2 • N+4 O2– • Nitrogen dioxide • gas Nitrite NO2– N+3 O2– Na-Nitrite Also Nitrous acid (HONO) • “ite” is light on oxygen and oxidation state. • • • • Nitric oxide Nitrogen(I) Nitrogen (Nitrogen(II)) oxide • • • • NO N+2 Non-salt Gas • Physiologically important 2° messenger and paracrine signal • • • • N2O N+1 Non-salt gas • N2 • N0 • Covalent triple bond • gas Ammonia • NH3 • N–3 H3+ • NH3: N has oxidation state of –3. • Ammonium cation salts • gas Biology of Molecular Nitrogen: The Nitrogen Cycle 8 3 Biosynthesis Amino Acids & Nucleotides Nitrogen Fixa-on versus Nitrogen Assimila-on • Converts N2 to NH3 • Requires multiple ATPs • Uses electrons from pyruvate • Converts NO3, NO2, and/or NH3 to amino acids • Uses electrons from NADH, NADPH, or photosynthetic transfer from ferrodoxin Both: • Are electron transfer processes • Use Mo cofactor • Involve multiple redox cofactors, such as Fe-S, NADH, NADPH, ferrodoxin, flavodoxin, and so on 9 Biosynthesis Amino Acids & Nucleotides Chemical transformations maintain a balance between N2 and biologically useful forms of nitrogen. The Nitrogen Cycle 1. Fixation. Bacteria reduce N2 to + NH3/NH4 .via Nitrogenase 2. Nitrification. Bacteria oxidize ammonia into nitrite (NO2–) and nitrate (NO3–). Organisms die, returning NH3 to soil. Nitrifying bacteria again convert NH3 to nitrite and nitrate. 3. Assimilation. a.Nitrate/nitrite: Plants and microorganisms reduce NO2– and NO3– to NH3 via nitrite reductases and nitrate reductases. Animals can only do à b. NH3 is incorporated into amino acids. 4. Denitrification. Nitrate is reduced to N2 under anaerobic conditions. NO3– is the ultimate electron acceptor instead of O2. 12 4