237_DE_S_09_Assignment__3_ANSWERS

237_DE_S_09_Assignment__3_ANSWERS - Chem 237 DE Spring 2009...

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Chem 237 DE Spring 2009 Assignment #2 ANSWERS 1. Mechanism of aspartate transcarbamoylase (I’ve also seen “transcarbamylase”). I have based my answer on the detailed studies that have been done on the E. coli enzyme over the years. The properties of this enzyme have been studied in many respects, mechanism, structure, allosteric mechanism, genetic regulation, etc. Now, as for your answers, there are many possibilities, depending on which base(s) you use in the reaction. The main error here on your papers was that many of you did not show how the enzyme participates in the reaction. You treated it as a simple organic reaction. Anyway………… General comments: the “Arg”, “His” and “Lys” are amino acid residues on the enzyme and are part of the primary structure of the protein, that are participating in the reaction. Some of you did not seem to understand that. The substrates bind, and as that happens, the Asp loses a proton to the water solution. This is due to the very electropositive nature of the active site (lots of + charges there already). Next, nucleophilic attack of the deprotonated Asp on the CP The Lys in the active site is thought to be the base that deprotonates the tetrahedral intermediate. Note how the Arg and His stabilize the (-) charge on the oxygen Some side group will protonate the leaving phosphate, and the structure suggests that it is the Lys.
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237_DE_S_09_Assignment__3_ANSWERS - Chem 237 DE Spring 2009...

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