Nucleotide functions allosteric effects

The answer is D. Impaired immune function in severe combined immunodeficiency (SCID) is the direct result of blocked DNA synthesis due to inadequate supplies of de-oxyribonucleotides in B and T cells. This effect arises by dATP-induced allosteric inhibition of ribonucleotide reductase, which catalyzes reduction of the 2 -hydroxyl groups on ADP and GDP to form dADP and dCDP. The ultimate cause of many cases of SCID is adenosine deaminase deficiency, which leads to accumulation of dATP and consequent inhibition of ribonucleotide reductase. Although the other enzymes mentioned are also involved in purine nucleotide metabolism, their deficiencies do not lead to SCID. 

As with the bacterial reductases, a complex pattern of activation and inhibition by nucleoside triphosphates has been demonstrated for the tumor reductase dATP inhibits reduction of all four substrates (S2). A similar pattern of nucleotide regulatory effects was found with partly purified reductase from rat embryo extracts (S5). Thus, presuming that analogy with the bacterial ribonucleotide reductases is valid, it would appear that the animal reductases are allosteric enzymes subject to a complicated regulation by nucleotides again, the function of such regulation would seem to be that of ensuring a balanced supply of deoxyribonucleo-tides for DNA synthesis. 

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