>Bovine tryptophanyl-tRNA synthetase (TrpRS, E.C. 6.1.1.2) is unable to catalyze in vitro formation of Ap4A in contrast to some other aminoacyl-tRNA synthetases. However, in the presence of class="smallCaps">l-tryptophan, ATP-Mg2+ and ADP the enzyme catalyzes the Ap3A synthesis via adenylate intermediate. Ap3A (not Ap4A) may serve as a substrate for TrpRS in the reaction of E·(Trp ∼ AMP) formation and in the tRNATrp charging. The Km value for Ap3A was higher than the Km for ATP (approx. 1.00 vs. 0.22 mM) and Vmax was 3 times lower than for ATP. The Zn2+-deficient enzyme catalyzes Ap3A synthesis in the absence of exogenous ADP due to ATPase activity of Zn2+-deprived TrpRS. The inability of mammalian TrpRS to synthesize Ap4A, might be considered as a molecular tool preventing the removal of Zn2+ due to chelation by Ap4A and therefore preserving the enzyme activity.
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机译：牛色氨酸tRNA合成酶（TrpRS，E.C. 6.1.1.2）与其他一些氨酰基tRNA合成酶相比，不能催化Ap 4 A的体外形成。然而，在存在 class =“ smallCaps”> l -色氨酸，ATP-Mg 2 + 和ADP的情况下，该酶催化Ap 3 A通过腺苷酸中间体合成。 Ap 3 A（不是Ap 4 A）可能在E·（Trp〜AMP）形成反应和tRNA Trp反应中充当TrpRS的底物充电。 Ap 3 A的 K m 值高于 K m 的 K ATP（约1.00比0.22 mM）和 V max 比ATP低3倍。缺乏Zn 2 + 的TrpRS的ATP酶活性导致缺乏外源ADP的Zn 2 + 缺乏酶催化Ap 3 的合成。哺乳动物TrpRS不能合成Ap 4 A，可能被认为是防止由于Ap 4 螯合而去除Zn 2 + 的分子工具。 sub> A，因此保留了酶活性。
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