Question

12a) Why does it make sense for high levels of GTP to allosterically activate CTP synthetase?

12b) What pathway exists for the synthesis of GTP, separate from ATP synthesis through energy metabolism and synthesis of other nucleoside triphosphates through nucleoside disphosphate kinase?

Answer 1

12a) Cytosine pairs with Guanine. So in double strand DNA, the number of Cytosine is equal to number of guanine. Guanine and cytosine are produced as guanine triphosphate (GTP) and cytosine triphospahte (CTP). So when GTP levels are high, the levels of CTP also should be high. CTP is produced by CTP synthase from UTP (uracil triphosphate). Thus, high levels of GTP increses CTP concentration by activating CTP synthase.

12b) GTP is produced from degraded guanine through salvage pathway. Guanine is converted to GMP which is converted to GDP by GMP kinase. Then GDP is converted to GTP by GDP kinase.