On the Catalytic Mechanism of Tryptophan Hydroxylase

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• 作者：Graham R. Moran ; Agnes Derecskei-Kovacs ; Patrick J. Hillas ; and Paul F. Fitzpatrick
• 刊名：Journal of the American Chemical Society
• 出版年：2000
• 出版时间：May 17, 2000
• 年：2000
• 卷：122
• 期：19
• 页码：4535 - 4541
• 全文大小：201K
• 年卷期：v.122,no.19(May 17, 2000)
• ISSN：1520-5126

文摘

Tryptophan hydroxylase catalyzes the hydroxylation of tryptophan using tetrahydrobiopterin andmolecular oxygen. With tyrosine as a substrate, the amount of C4a-hydroxypterin formed greatly exceeds theamount of dihydroxyphenylalanine formed, consistent with oxygen-oxygen bond cleavage occurring in astep prior to amino acid hydroxylation. With L-indole-²H₅-tryptophan, L-4-²H- or L-5-²H-tryptophan as substratethere is no isotope effect on the V/K value for tryptophan. There is an inverse isotope effect on the V_max valuewith L-indole-²H₅-tryptophan and L-5-²H-tryptophan, but no effect with L-4-²H-tryptophan. Comparison of themeasured isotope effects with values of calculated secondary equilibrium isotope effects for tryptophanhydroxylation indicate that the results are most consistent with the formation of a cationic species. Retentionof the isotopic label from L-5-²H-tryptophan in the product confirms that an NIH shift occurs in tryptophanhydroxylase and shows that the direction of shift is from carbon 5 to carbon 4. The degree of retention of thedeuterium is higher when the deuterium is initially on carbon 4 rather than carbon 5.