3. How do PRPP levels influence purine and pyrimidine nucleotide synthesis?

In de novo synthesis of purines (via IMP), PRPP allosterically ACTIVATES amidophophoribosyl transferase which catalyzes reaction #2 in the IMP synthetic pathway. (This is an example of feed forward activation). Thus, PRPP stimulates de novo synthesis of purines. An excess of PRPP in Lesch-Nyhan syndrome accelerates the synthesis of purine nucleotides and subsequent formation of their degradation product uric acid.

In salvage of purines PRPP combines with adenine, hypoxanthine or guanine to make AMP, IMP and GMP. (i.e., adds a ribose phosphorylated at C5 to the base to form a nucleotide).

In de novo synthesis of pyrimidines (via UMP) PRPP is added to orotate in reaction #5 to form OMP. The rate of OMP production varies with the availability of its precursor, PRPP (pg 703-704). PRPP also activates the rate limiting enzyme in pyrimidine synthesis carbamoyl phosphate synthetase II (Fig. 22-7). Deficiencies in the enzyme that catalyzes reactions 5 and 6 lead to a build up of orotate in the urine, decreased growth and anemia.

6. Describe the metabolic defects of Lesch-Nyhan syndrome, orotic aciduria, SCID and gout:

Lesch-Nyhan syndrome (pg 700) results from a severe HGPRT deficiency. (HGPRT catalyzes the transfer of ribose-5-phosphate from PRPP to hypoxanthine and guanine to form IMP and GMP). Without HGPRT, PRPP builds up and because of feed forward activation of reaction #2 in purine synthesis, accelerates synthesis of purines. Increased levels of purines leads to an increase in the purine degradation product (uric acid). Lesch-Nyhan syndrome includes neurological abnormalities such as spasticity, mental retardation, and highly aggressive and distructive behavior, including self mutilation. The mechanism responsible for the neurological symptoms is not understood.

Orotic aciduria (pg 704) results from a deficiency in the enzyme that catalyze reactions 5 and 6 in de novo synthesis of pyrimidines. An excess of orotate is present in the urine. Symptoms include suppressed growth and severe anemia.

Severe combined immunodeficiency disease (SCID) (pg 715) results from deficits in the gene coding for adenosine deaminase. Adenosine deaminase removes the N in adenosine and deoxyadenosine to form inosine and deoxyinosine. Deficient adenosine deaminase causes an accumulation of deoxyadenosine and subsequently elevated dATP. High [dATP] inhibits ribonucleotide reductase (the enzyme that removes the OH on C₂’ to form deoxynucleotides from nucleotides). Inhibition of ribonucleotide reductase prevents the synthesis of other dNTPs, choking off DNA synthesis and thus cell proliferation. Lymphocytes are especially susceptible because lymphocytes produce a lot of deoxyadenosine which then builds up in the absence of adenosine deaminase. SCID is a fatal genetic disorder that has led to failed as well as successful attempts at gene therapy. (See Anderson W.F., 2000, The Best of Times, the Worst of Times, Science, 288(5466), pg 627.)

Gout (pg 717) is caused by an excess of uric acid, the final product of purine metabolism. The most common cause of gout is impaired uric acid excretion. Symptoms include inflammation of the joints and kidney stones.

8. What compounds are produced by the degradation of purines and pyrimidines?

Purines ® uric acid (pg. 714) (humans) ® allantoin (other mammals) ® allantoic acid (teleost fish) ® urea (cartilaginous fish and amphibia) ® NH4+ (marine invertebrates)

Pyrimidines ® Malonyl-CoA (pg. 719; you can get energy out of pyrimidines)!