The histidine biosynthetic pathway in all plants and bacteria is novel in several respects. Histidine is derived from three precursors (Fig. 21-17): PRPP contributes five carbons, the purine ring of ATP contributes a nitrogen and a carbon, and the second ring nitrogen comes from glutamine. The key steps are the condensation of ATP and PRPP (N-1 of the purine ring becomes linked to the activated C-1 of the ribose in PRPP) (step ① in Fig. 21-17), purine ring opening that ultimately leaves N-1 and C-2 linked to the ribose (step ③), and formation of the imidazole ring in a reaction during which glutamine donates a nitrogen (step ⑤). The use of ATP as a metabolite rather than a high-energy cofactor is unusual, but not wasteful because it dovetails with the purine biosynthetic pathway. The remnant of ATP that is released after the transfer of N-1 and C-2 is 5-aminoimidazole-4-carboxamide ribonucleotide, an intermediate in the biosynthesis of purines (see Fig. 21-27) that can rapidly be recycled to ATP.
Lehninger-Nelson-Cox: Principles of Biochemistry, 708.o.
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