2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)pyrimidine is a metabolite in the purine metabolism, formed by the hydrolysis of GTP by GTP cyclohydrolase II. Alternatively two separate enzymes can carry out this reaction, initially GTP cyclohydrolase IIa hydrolyses the 8,9 bond to form 2-Amino-5-formylamino-6-(5-phospho-D-ribosylamino)pyrimidin-4(3H)-one, followed by de-formylation by 2-amino-5-formylamino-6-ribosylaminopyrimidin-4(3H)-one 5'-monophosphate deformylase. 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)pyrimidine is deaminated by Diaminohydroxyphosphoribosylaminopyrimidine deaminase to form 5-amino-6-(5-phosphoribosylamino)uracil. The compound is a precursor to riboflavin when a reductase enzyme catalyses the reaction:
In plants, this compound functions in the early part of the riboflavin biosynthesis pathway. Riboflavin is later used to produce flavin cofactors such as FMN and FAD. In rice, this part of the pathway involves bifunctional RibA proteins that possess both GTP cyclohydrolase II activity and 3,4-dihydroxy-2-butanone-4-phosphate (3,4-DHBP) synthase activity. The 3,4-DHBP synthase activity converts D-ribulose 5-phosphate into L-3,4-dihydroxy-2-butanone 4-phosphate and formate, providing other early precursors needed for riboflavin synthesis. Studies of rice RibA proteins point towards the fact that the catalytic steps associated with this compound are important for regulating vitamin B₂ biosynthesis in plants. RibA activity contributes to the production of early intermediates in the riboflavin pathway, and changes in this part of the pathway could affect the amount of riboflavin produced by plant cells. 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)pyrimidine is relevant not only as a biochemical intermediate, but also as part of a metabolic pathway in plants that is involved in flavin cofactor production.
References