bioc441 - lecture 25.Nucleotides

bioc441 - lecture 25.Nucleotides - 25 Biosynthesis of...

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25. Biosynthesis of pyrimidines & purines
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DNA and RNA contain five main purine and pyrimidine bases cytosine C NH 2 N N CH C O CH H uracil N CH C O HN CH C O H H CH 3 N C C O HN CH C O thymine pyrimidines H purines NH 2 C C N N N CH C N HC H adenine guanine C C C HN N N CH C O N H 2 N
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Nucleoside & nucleotide nomenclature NH 2 C C N N N CH C N HC H bases: adenine, guanine, cytosine, uracil thymine, nucleosides: adenosine, guanosine, cytidine, uridine, (deoxy)thymidine HOCH 2 OH OH NH 2 C C N N N CH C N HC O nucleotides: adenylate (AMP), guanylate, cytidylate, uridylate, (deoxy)thymidylate NH 2 C C N N N CH C N HC - O-P-O-CH 2 O - O OH OH O
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Pyrimidine biosynthesis starts with carbamoylphosphate carbamoyl- phosphate HCO 3 - 2 ATP 2 ADP + P i CO NH 2 CO 2 - + H 3 N C H CH 2 CH 2 Gln + + CO 2 - CO 2 - + H 3 N C H CH 2 CH 2 Glu This reaction resembles the first step of the urea cycle, except that Gln replaces NH 4 + as the nitrogen donor. It’s catalyzed by a cytosolic enzyme, carbamoylphosphate synthetase II. carbamoylphosphate synthetase II O - H 2 N - C -O-P-O - O O
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Bacterial carbamoylphosphate synthetase II has three catalytic sites connected by a channel Gln ATP + HCO 3 - ATP O O - H 2 N-C-O-P-O - O 1 2 3 Human carbamoylphosphate synthetase II is part of a larger, multifunctional protein. The bacterial enzyme has two subunits. Ammonia is produced from Gln in the small subunit, channeled to the large subunit, and converted to carbamate at the second active site. Carbamate moves to the third site, where it is converted to carbamoyl phosphate. The total length of the channel is about 100 Å.
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All the other atoms of the pyrimidine ring come from aspartate P i O-P-O - O - O O=C NH 2 H 2 N CH 2 CH-CO 2 - - O-C O - O-C O O=C NH 2 N CH 2 CH-CO 2 - H aspartate transcarbamoylase carbamoylphosphate N-carbamoyl- aspartate Asp H 2 O N C O O=C HN CH 2 CH-CO 2 - H orotate H N C O O=C HN CH C-CO 2 - dihydroorotate + NAD + NADH
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bioc441 - lecture 25.Nucleotides - 25 Biosynthesis of...

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