Purine and Pyrimidine Metabolism Flashcards
What is a nucleoside?
just the base and the sugar without any phosphate attached
What is a nucleotide?
base, sugar, and phosphate group
How many rings do purines have?
2
What is the first step in purine synthesis?
ribose-5-phosphate to phosphoribosyl pyrophosphate (PRPP) using PRPS1 and PRPS2 (use ATP)
regulated step
Where are the genes for PRPS1 and PRPS2 found?
on opposite ends of the X chromosome
What is the feedback inhibition for PRPS1 and PRPS2?
the end-products of the pathway, guanine and adenine nucleotides (ADP and GDP)
There is a disease in humans caused by mutants in PRPS1 that do not react to feedback inhibition. The enzyme is then called superactive and it overproduces PRPP and its products like uric acid, leading to a form of gout.
The first step requires what?
This first step in the pathway requires ATP converted to AMP, thus it consumes two ATP equivalents making it irreversible.
What is the second step in purine synthesis?
PRPP to 5-phosphoribosyl-1-amine via the enzyme GPAT
regulated step
What is the significance of the conversion of PRPP to 5-phosphoribosyl-1-amine?
it is the committed step of the pathway. Once you have 5-phosphoribosyl-1-amine, you have to go all the way to IMP. This is because the PRPP has additional uses in histidine and tryptophan biosynthesis and pyrimidine biosynthesis.
Once the amino group of glutamine is added, the pathway must go forward to IMP (inosinate)
What regulates the second step?
feedback inhibition via ADP and GDP
What can inhibit the action of GPAT?
Dawn and Azoserine are structural analogs of glutamine and have an azeto group to disrupt the enzyme
what stimulates the activity of GPAT?
increased levels of PRPP
What does the 9th step of purine synthesis do?
removes the carbon skeleton of aspartate that was attached to make SAICAR in step 8, just leaving behind the amino group and splitting off fumarate. The product is AICAR.
Enzyme: adenylosuccinate lyase (ADSL)
What then happens in step 10?
A formyl group is added to AICAR from N10-formyl tetrahydrofolate (THF) to form FAICAR
What then happens in step 11?
FAICAR undergoes ring closure by IMP synthase (i.e. no ATP) to make inosine monophosphate (IMP). IMP is the immediate precursor to AMP and GMP and represents a branch point in the pathway.
enzyme: IMP synthase
N10-formyl THF is needed for which steps of purine synthesis?
steps 4 and 10
What can block the action of N10-formyl THF at steps 4 and 10?
Inhibitors of dihydrofolate reductase (methotrexate- used in cancer treatment) can prevent these steps and block purine synthesis
Why are drugs like methotrexate that inhibit purine synthesis successful for fighting cancer?
Since bacteria and cancer cells are faster growing than normal human cells, methotrexate, methopterin and aminopterin are effective treatments for some cancers. These will harm faster growing tissues like the blood and the intestinal lining if given for too long a time.
How is AMP made from IMP?
1) Asparate is added to IMP (via GTP) to form adenylosuccinate (via adenylosuccinate synthetase)
2) Adenylosuccinate is acted upon by ADSL (same as used in step 9) to give off fumarate and form AMP (adenylate)
Homozygosity for mutations in the ADSL gene results in a clinical disorder called?
succinylpurinemic autism, a genetic autism. Caused by the buildup of SAICAR and adenylosuccinate which interfere with proper functioning of the brain
How is GMP made from IMP?
1) IMP dehydrogenase oxidizes the IMP to introduce oxygen to make xynthylate (through NAD+)
2) Glutamine is added to xynthylate to form GMP (guanylate) using GMP synthetase (uses ATP)
How many ATP equivalents does it take to form AMP de novo? GMP?
AMP- 8
GMP- 9
For this reason it is best for the cell to recover these nucleotides from used mRNA and recycle them rather than making them from the de novo pathway.
How is GTP and ATP made from GMP and AMP?
1) first adenylate kinase and guanylate kinase are used to make ADP (AK1-3) and GDP (GUK1-3). These reactions require ATP
2) Once the diphosphates are formed, a non-specific enzyme nucleoside diphosphate kinase (NME1-4) converts them on to the triphosphate forms. This enzyme also requires ATP, but is it not base specific and it is not sugar specific
What is the main way to form ATP?
oxidative phosphorylation, so the enzymes like NME1-4 tend to work on non-adenine forms of diphosphate nucleiosides to make the triphosphate forms