P 1, P 3-bis(5′-adenosyl)triphosphate (Ap 3A) as a substrate and a product of mammalian tryptophanyl-tRNA synthetase

Abstract

Bovine tryptophanyl-tRNA synthetase (TrpRS, E.C. 6.1.1.2) is unable to catalyze in vitro formation of Ap 4A in contrast to some other aminoacyl-tRNA synthetases. However, in the presence of l-tryptophan, ATP-Mg 2+ and ADP the enzyme catalyzes the Ap 3A synthesis via adenylate intermediate. Ap 3A (not Ap 4A) may serve as a substrate for TrpRS in the reaction of E·(Trp ∼ AMP) formation and in the tRNA Trp charging. The K m value for Ap 3A was higher than the K m for ATP (approx. 1.00 vs. 0.22 mM) and V max was 3 times lower than for ATP. The Zn 2+-deficient enzyme catalyzes Ap 3A synthesis in the absence of exogenous ADP due to ATPase activity of Zn 2+-deprived TrpRS. The inability of mammalian TrpRS to synthesize Ap 4A, might be considered as a molecular tool preventing the removal of Zn 2+ due to chelation by Ap 4A and therefore preserving the enzyme activity.