(a) What is the role of p-aminobenzoate in these bacteria? (Hint: See Fig. 18-16).
(b) Why does AICAR accumulate in the presence of sul-fanilamide?
(c) Why are the inhibition and accumulation reversed by addition of excess p-aminobenzoate?
12. Pathway of Carbon in Pyrimidine Biosynthesis
Predict the locations of 14C in orotate isolated from cells grown on a small amount of uniformly labeled [14C]succinate. Justify your prediction.
13. Nucleotides As Poor Sources of Energy Under starvation conditions, organisms can use proteins and amino acids as sources of energy. Deamination of amino acids produces carbon skeletons that can enter the glycolytic pathway and the citric acid cycle to produce energy in the form of ATP. Nucleotides, on the other hand, are not similarly degraded for use as energy-yielding fuels. What observations about cellular physiology support this statement? What aspect of the structure of nucleotides makes them a relatively poor source of energy?
14. Treatment of Gout Allopurinol (see Fig. 22-47), an inhibitor of xanthine oxidase, is used to treat chronic gout. Explain the biochemical basis for this treatment. Patients treated with allopurinol sometimes develop xanthine stones in the kidneys, although the incidence of kidney damage is much lower than in untreated gout. Explain this observation in the light of the following solubilities in urine: uric acid, 0.15 g/L; xanthine, 0.05 g/L; and hypox-anthine, 1.4 g/L.
The diazo compound 0-(2-diazoacetyl)-L-serine, known also as azaserine (see Fig. 22-48), is a powerful inhibitor of glutamine amidotransferases. If growing cells are treated with azaserine, what intermediates of nucleotide biosynthesis would accumulate? Explain.
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