Term
| Name the amino acids that contribute to the purine ring structure: |
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Definition
Aspartate (N1)
Glutamine (N3, N9)
Glycine (C4, C5, N7)
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Term
What amino acid adds the first nitrogen to PRPP to start the formation of a purine?
What is the enzyme? |
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Definition
Glutamine and water react to donate an amino group to the 1 position of PRPP, freeing pyrophosphate to drive the reaction.
This reaction is catalyzed by PRPP glutamyl amidotransferase (GPAT) Which is another name for Amidophosphoribosyl transferase, the first enzyme of the pathway. |
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Term
What is the precursor of AMP and GMP in de novo purine synthesis?
How is each differentially formed? |
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Definition
Inosinate.
AMP is formed by the addition of aspartate followed by the release of fumarate.
GMP is generated by the addition of water, dehydrogenation by NAD+, and the replacement of the carbonyl oxygen atom by -NH2 derived by the hydrolysis of glutamine. |
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Term
| What is the rate limiting step in purine synthesis? |
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Definition
| The reaction of Amidophosphoribosyl transferase is rate limiting. It is allosterically inhibited by AMP, ADP, ATP and GMP, GDP, GTP. |
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Term
| What disease is associated with a lack of feedback inhibition at PRPP synthetase? |
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Definition
Lack of feedback inhibition by IMP, AMP or GMP leads to overproduction of pruines resulting in a form of GOUT.
Mutant forms of synthetase have high Vmax or altered Km that also result in increased purine synthesis and gout. |
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Term
| At the inosinate 5'-monophosphate (IMP) branch point in purine synthesis, what factors modulate which purine will be formed? |
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Definition
| GTP inhibits further GMP synthesis, thus high GTP causes more formation of AMP, and vice versa. |
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Term
| How is pyrimidine synthesis fundamentally different from purine synthesis? |
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Definition
| Unlike purine synthesis which occurs while attached to a ribose (PRPP), pyrimidine skeletons are assembled before attachment to a ribose phosphate. |
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Term
How many enzymes are involved in the formation of pyrimidines?
Name the enzymes, their activities and where they are located in the cell. |
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Definition
3 enzymes, two of which with multiple activities form pyrimidines.
Enzyme 1 is CAD (located in the cytosol) which has the activities of:
1. Carbamoyl phosphate synthetase II
2. Aspartate carbamoyl transferase
3. Dihydroorotase
Enzyme 2 is dihydro orotate dehydrogenase, it forms Orotate from dihydroorotate. It is located on the outer surface of the inner mitochondrial membrane.
Enzyme 3 is UMP synthase (located in the cytosol) which has the activities of:
5. Orotate phosphoribosyl transferase
6. OMP decarboxylase
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Term
| What are the regulated steps of pyramidine synthesis? |
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Definition
1. Reaction 1 (carbamoyl phosphate synthetase) which forms carbamoyl phosphate using glutamine, 2ATP and CO2.
2. Reaction 2 (Aspartate transcarbamoylase) which adds aspartate to carbamoyl phosphate to form carbamoyl aspartate |
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Term
| What 2 enzymes are used to phosphorylate nucleosides? What are their specificities? |
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Definition
Nucleoside monophosphate kinase which uses ATP to donate a phosphate group to either a ribose or deoxyribose. There is a specific nucleoside monophosphate kinase for every base.
Nucleoside Diphosphate Kinase has broad specificity. Utilizes any triphosphate as a phosphate donor and will phosphorylate any diphosphate. Keeps the pool of nucleotides varied in a sense. |
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Term
| What is the precursor of pyrimidine synthesis? |
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Definition
| Orotidine 5'-monophosphate (aka Orotidylate). |
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Term
| What amino acids in the active site of ribonucleotide reductase (RR) are responsible for the reduction of ribonucleotides? |
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Definition
There are 3 Cystienes and a glutamate arranged in a circle in the R1 dimer active site.
NOTE: There is also a tyrosyl-radical site (uses tyrosine to accept electrons) in the R2 dimer that is necessary for radical formation in the reduction process. |
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