ADA is expressed in a variety of tissues, but, in humans, lymphocytes have the highest activity of this cytoplasmic enzyme. A deficiency of ADA results in an accumulation of adenosine, which is converted to its ribonucleotide or deoxyribonucleotide forms by cellular kinases. As dATP levels rise, ribonucleotide reductase is inhibited, thereby preventing the production of all deoxyribose-containing nucleotides (see p. 297). Consequently, cells cannot make DNA and divide. [Note: The dATP and adenosine that accumulate in ADA deficiency lead to developmental arrest and apoptosis of lymphocytes.] In its most severe form, this autosomal-recessive disorder causes a type of severe combined immunodeficiency disease (SCID), involving a decrease in T cells, B cells, and natural killer cells. ADA deficiency accounts for ~14% of cases of SCID in the United States. Treatments include bone marrow transplantation, enzyme replacement therapy, and gene therapy (see p. 501). Without appropriate treatment, children with this disorder usually die from infection by age 2 years. [Note: Purine nucleoside phosphorylase deficiency results in a less severe immunodeficiency primarily involving T cells.]
VI. PYRIMIDINE SYNTHESIS AND DEGRADATION
Unlike the synthesis of the purine ring, which is constructed on a pre-existing ribose 5-phosphate, the pyrimidine ring is synthesized before being attached to ribose 5-phosphate, which is donated by PRPP. The sources of the atoms in the pyrimidine ring are glutamine, CO2, and aspartate (Fig. 22.19).
A. Carbamoyl phosphate synthesis
The regulated step of this pathway in mammalian cells is the synthesis of carbamoyl phosphate from glutamine and CO2, catalyzed by carbamoyl phosphate synthetase (CPS) II. CPS II is inhibited by uridine triphosphate (the end product of this pathway, which can be converted into the other pyrimidine nucleotides) and is activated by PRPP. [Note: Carbamoyl phosphate, synthesized by CPS I, is also a precursor of urea (see p. 253). Defects in ornithine transcarbamylase of the urea cycle promote pyrimidine synthesis because of increased availability of carbamoyl phosphate. A comparison of the two enzymes is presented in Figure 22.20.]
B. Orotic acid synthesis
The second step in pyrimidine synthesis is the formation of carbamoylaspartate, catalyzed by aspartate transcarbamoylase. The pyrimidine ring is then closed by dihydroorotase. The resulting dihydroorotate is oxidized to produce orotic acid (orotate), as shown in Figure 22.21. The human enzyme that produces orotate, dihydroorotate dehydrogenase, is a flavin mononucleotide-containing protein of the inner mitochondrial membrane. All other enzymes in pyrimidine biosynthesis are cytosolic. [Note: The first three enzymic activities in this pathway (CPS II, aspartate transcarbamoylase, and dihydroorotase) are actually three different catalytic domains of a single polypeptide known as CAD from the first letter in the name of each domain. (See p. 18 for a discussion of domains.) This is an example of a multifunctional or multicatalytic polypeptide that facilitates the ordered synthesis of an important compound. Synthesis of the purine nucleotide IMP also involves multifunctional proteins.]
C. Pyrimidine nucleotide synthesis