What is the parent nucleotide for AMP and GMP?
What is the "pre" reaction before the 10 reactions that yield IMP?
The PRPP synthetase reaction.
R-5-P + ATP yields PRPP and AMP
What is PRPP?
What is the first step in the synthesis of purine ribonucleotides (IMP, AMP, GMP)
PRPP is converted to 5-phosphoribosylamine.
What is the source of nitrogen in the first step of purine ribonucleotide synthesis?
Glutamine. It becomes glutamate in the reaction and the amino group is added to PRPP.
What enzyme catalyzes the transfer of the amine group from glutamine to PRPP (to form 5-phosphoribosylamine)?
Glutamine PRPP amidotransferase
What is the numbering of atoms in a purine? (see figure)
See next figure for purine numbering.
What is the source of N-1 (the arrow in the next figure)?
What is the source of the C-2 (arrow)
What is the source of N-3 and N-9 (arrows)
What is the source of C-4, C-5, and N-7 (shaded)?
What is the source of C-8 (arrow)?
Identify the source of each atom in the purine ring (arrows)
1: Aspartate amine
3: Glutamine amide
9: Glutamine amide
IMP, GMP, XMP, and AMP regulate purine nucleotide biosynthesis by negative feedback. Where do they act?
At a site on glutamine PRPP amidotransferase
(The first step after PRPP synthetase)
***IMP, GMP, and XMP act at one site, and AMP acts at a separate site.
What happens to the activity of PRPP amidotransferase as [Glutamine] increases?
Since Glutamine is the nitrogen source for this enzyme, its activity goes up.
True/False: AMP, IMP, or GMP alone reduce PRPP amidotransferase activity, and a combination of AMP plus IMP or GMP reduces activity even further.
What is the "feed forward" aspect of purine nucleotide synthesis?
PRPP is an activator of PRPP amidotransferase (1st step). If inhibitors become defective this can lead to unrestrained synthesis.
Describe the negative feedback of purine nucleotide synthesis by end products.
IMP, GMP, and AMP inhbit PRPP amidotransferase (1st step).
True/False: The PRPP pool size has almost no regulatory effect on purine nucleotide biosynthesis.
False. The amount of PRPP is a MAJOR determinant of the overall rate of de novo purine nucleotide synthesis.
What factors increase the PRPP pool size (and thus increase the rate of purine nucleotide synthesis)?
Ribose-5-phosphate supply (PRPP precursor)
Activity of PRPP synthetase (converts R-5-P to PRPP)
What enzyme decreases the amount of available PRPP? (hint: by committing PRPP to the purine nucleotide synthesis pathway).
Glutamine PRPP amidotransferase
Purine ribonucleotides act as allosteric inhibitors of Glutamine PRPP amidotransferase. What else do these end products inhibit?
The "pre" step. The formation of PRPP from Ribose-5-phosphate by PRPP synthetase.
What is gout?
High levels of uric acid (the end product of purine catabolism) in blood.
How does Glucose-6-phosphatase deficiency (GSD Type I = Von Gierke's Disease) cause gout?
By trapping G-6-P in the cell, there is increased flux through the pentose phosphate pathway. This increases Ribose-5-phosphate levels (the precursor to PRPP). Increased PRPP leads to increase purine nucleotide synthesis, thus increased purine catabolism, and increased production of uric acid (gout).
What is the "salvage pathway" for purines?
Purines resulting from nucleic acid turnover, or from the diet, can be converted to nucleoside monophosphates and used by the body.
What two enzymes are involved in the purine salvage pathway?
APRT (adenine phosphoribosyl transferase)
HGPRT (hypoxanthine-guanine phosphoribosyltransferase)
Describe the action of the purine salvage enzymes.
They use PRPP as a source of ribose-5-phosphate, which is added to adenine in a "phosphoribosylation" reaction. The release of pyrophosphate makes this reaction irreversible.
What is the action of 5'-nucleotidase?
Converts a purine nucleotide to the nucleoside by removing Pi, e.g. AMP goes to adenosine and Pi
What condition can be detected by measuring 5'-nucleotidase activity?
Purines are degraded to uric acid. What compound immediately precedes uric acid?
Xanthine (and hypoxanthine before that)
What is the final enzyme of purine degradation, that converts xanthine to uric acid?
A treatment for gout is to decrease uric acid production by "preoccupying" xanthine oxidase with what xanthine analog?
Gout can be caused be defective PRPP synthetase. What three things can be defective in this enzyme that results in purine overproduction and overexcretion?
1. Superactive PRPP (increased Vmax)
2. Resistance to feedback inhibition
3. Low Km for Ribose-5-phosphate
What inherited disorder of purine metabolism is characterized by self-mutilation (biting of lips, fingers)
How does Lesch-Nyhan syndrome cause hyperuricemia and purine overproduction and overexcretion?
A complete deficiency of HGPRTase, an enzyme in purine salvage. Since xanthine and guanine are not salvaged, they are instead degraded, producing excess uric acid. Additionally, this lack of salvage increases PRPP levels and decreased IMP and GMP levels, which increases de novo purine synthesis.
We have been discussing hyperuricemia caused by increased uric acid production. What is the more common cause of hyperuricemia?
The underexcretion of uric acid.
Renal lithiasis (kidney stones) can be caused by a complete deficiency of what enzyme in purine metabolism?
Adenine phosphoribosyl transferase (APRT) - a purine salvage enzyme.
What is Xanthinuria?
An inherited disorder of purine metabolism characterized by a complete deficiency of Xanthine oxidase, resulting in HYPOuricemia and xanthine renal lithiasis.
How does gout cause arthritis?
Hyperuricemia leads to Tophi (pathology term for crystallization of sodium urate in soft tissues and joints), which causes inflammation, and acute gouty arthritis.
How does adenosine deaminase (ADA) deficiency cause severe combined immunodeficiency disease (SCID)?
A deficiency of ADA leads to an accumulation of adenosine, which is converted to its ribonucleotide or deoxyribonucleotide form by kinases. As dATP levels rise, ribonucleotide reductase is inhibited, preventing DNA synthesis.
What other enzyme deficiency can cause SCID?
Purine nucleoside phosphorylase
What is the significance of 6-mercaptopurine?
Chemotherapy for acute lymphocytic leukemia.
This antimetabolite (purine analog) blocks purine nucleotide biosynthesis by inhibiting PRPP-amidotransferase.
6-mercaptopurine is a substrate for xanthine oxidase. What can be done to extend its half-life and prevent inactivation?
Administer the xanthine oxidase inhibitor, allopurinol.
What is the significance of Azathioprine?
This purine analog is an immunosuppressant and can be used in renal transplant, rheumatoid arthritis, and IBS.