第14章核苷酸的代谢-2014.ppt

Figure 22-45 Figure 22-46 Figure 22-47b Figure 22-48 Figure 22-31-04 Figure 22-31-05 Figure 22-31-06 Figure 22-31-07 Figure 22-31-08 Figure 22-31-09 Figure 22-31-10 Figure 22-31-11 Figure 22-32 Figure 22-32-01 Figure 22-32-02 Figure 22-32-03 Figure 22-32-04 Figure 22-33 Figure 22-04a Figure 22-37 Figure 22-41 Figure 22-42 Figure 22-40 Figure 22-43-left Figure 22-04a Figure 22-44 Figure 22-41 Figure 22-44-01 Figure 22-43-left Figure 22-43-right Figure 22-04a Figure 22-47a Figure 22-48 Figure 22-44-02 Figure 22-44-03 Figure 22-44-04 * * * * * Figure 22-30 Figure 22-31-01 Figure 22-31-02 Figure 22-31-03 Thymidylate Synthesis. Thymidylate synthase catalyzes the addition of a methyl group (derived from N 5,N 10-methylenetertahydrofolate to dUMP to form TMP. The addition of a thiolate from the enzyme activates dUMP.Opening the five-membered ring of the THF derivative prepares the methylene group for nucleophilic attack by theactivated dUMP. The reaction is completed by the transfer of a hydride ion to form dihydrofolate. Control of Purine Biosynthesis. Feedback inhibition controls both the overall rate of purine biosynthesisand the balance between AMP and GMP production. 补 救 途 径 磷酸核糖转移酶 尿嘧啶+PRPP 尿嘧啶核苷酸+PPi 胞嘧啶 不能 核苷磷酸化酶、核苷磷酸激酶 尿嘧啶+1-磷酸核糖 尿嘧啶核苷+Pi 嘧啶核苷+ATP 嘧啶核苷酸+ATP In de Novo Synthesis, the Pyrimidine Ring Is Assembled from Bicarbonate, Aspartate,and Glutamine The pyrimidine ring is assembled first and then linked to ribose phosphate to form a pyrimidine nucleotide. PRPP is the donor of the ribose phosphate moiety. The synthesis of the pyrimidine ring starts with the formation of carbamoylaspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation yield orotate, which reacts with PRPP to give orotidylate. Decarboxylation of this pyrimidine nucleotide yiel