Lect_15_AAII

Lect_15_AAII - BIS 103 - 15 - Sulfur Amino Acids; Asp and...

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Unformatted text preview: BIS 103 - 15 - Sulfur Amino Acids; Asp and Glu Acid Families Sulfur incorporation into amino acids plants vs animals Cysteine and Methionine Folic Acid and Tetrahydrofolic Acid Aspartic Acid Family: Asp, Asn, Lys, Met, Thr, Ile Glutamic Acid Family: Glu, Gln, Pro, Arg, Orn nitrogen fixation [1] glutamine synthetase [2] glutamate synthase [3] glutamate transaminase N 2 NH 3 glutamate glutamate -keto acids NO 3- glutamine -ketoglutarate amino acids [1] [2] [3] nitrate assimilation NADPH pyridoxal phosphate The path of nitrogen to amino acids in plants and microorganisms Carbon fixation - photosynthesis Nitrogen fixation - amino acids; glutamine synthetase Sulfur fixation - amino acids; cysteine and methionine inorganic sulfur (SO 4 2- ) activated sulfur H 2 S organic sulfur Sulfur is found in many biocompounds cysteine, methionine, CoASH glutathione, thiamine, biotin heparin, sulfated polysaccharides lipoic acid The first step in sulfur assimilation is to take a water soluble form of sulfur, the sulfate ion, and activate it in two steps: 1. ATP sulfurylase SO 4 2- + ATP 2. APS kinase adenosine-5-phosphosulfate + PPi (APS) ATP sulfurylase APS + ATP adenosine-3-phosphate-5-phosphosulfate + ADP (PAPS) APS kinase 1. ATP sulfurylase SO 4 2- + ATP adenosine-5-phosphosulfate + PPi (APS) ATP sulfurylase 1. AMP is transferred to sulfate and PPi is released. 2. The free energy change of this reaction is highly unfavorable for the formation of APS, since the phosphate-sulfate anhydride linkage has a G of hydrolysis of about -80 kJ/mol. 3. However two reactions make the overall reaction go forward. a. PPi hydrolysis : G = -33.4 kJ/mol. b. The P-S linkage is further stabilized by a further activation involving ATP in the second activation reaction. HO- S-O-P-O-CH 2 O O O OH adenosine-5-phosphosulfate (APS) G o = +50 kJ/mol G o = -33.4 kJ/mol S O 4 2- + ATP PPi 2Pi ATP sulfurylase The first step in sulfate ion activation is carried out by ATP sulfurylase. C C C CH OH O OH H H Adenine H + AMP is transferred to sulfate and PPi is released. The free energy change of this reaction is highly unfavorable for the formation of APS, since the phosphate-sulfate anhydride linkage has a G of hydrolysis of about -80 kJ/mol. However, two reactions make the overall reaction go forward: 1. The PPi which is formed is hydrolyzed to two Pi by pyrophosphorylase. 2. The P-S linkage is further stabilized by a further activation reaction involving ATP. APS + ATP adenosine-3-phosphate-5-phosphosulfate + ADP (PAPS) APS kinase Step 2 of activation 1. A second ATP is used to activate APS to PAPS....
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Lect_15_AAII - BIS 103 - 15 - Sulfur Amino Acids; Asp and...

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