title: Thyroid purine nucleoside phosphorylase. II. Kinetic model by alternate substrate and inhibition studies.
abstract: Nucleoside analog inhibition studies have been conducted on thyroidal purine nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) which catalyzed an ordered bi-bi type mechanism where the first substrate is inorganic phosphate and the last product is ribose 1-phosphate. Heterocyclic- and carbohydrate-modified nucleoside inhibitors demonstrate mixed type inhibition suggesting such analogs show an affinity (Ki) for the free enzyme. A kinetic model is proposed which supports the observed inhibition patterns. These studies together with alternate substrate studies indicate that nucleoside binding requires a functional group capable of hydrogen bonding at the 6-position of the purine ring and that the orientation of the bound substrate may be syn. Proper geometry of the phosphate is dependent upon the 3'-substituent to the orientated below the furanose ring. The 5'-hydroxyl group is required for substrate activity. The proposed rate limiting step of the phosphorylase mechanism is the enzymatic protonation of the 7-N position of the nucleoside.
mesh_terms: D000818:Animals; D002417:Cattle; D007288:Inosine / Q000031:analogs & derivatives / Q000378:metabolism; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D010430:Pentosyltransferases / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D011683:Purine-Nucleoside Phosphorylase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013379:Substrate Specificity; D013961:Thyroid Gland / Q000201:enzymology*
chemical_list: D010710:Phosphates; D007288:Inosine; D010430:Pentosyltransferases; D011683:Purine-Nucleoside Phosphorylase