Mode of substrate interaction and energetics of carbon-oxygen bond formation of the dopamine beta-monooxygenase reaction
Alliston, Kevin R.
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Biochemistry. 1999 Nov 9; 38(45): 14916-26.
Previous studies have shown that the dopamine beta-monooxygenase (DbetaM; E.C. 18.104.22.168)/1-(2-aminoethyl)-1,4-cyclohexadiene (CHDEA) reaction partitions between side chain and ring H-abstraction to produce the side-chain-hydroxylated product, 2-amino-1-(1, 4-cyclohexadienyl)ethanol, and the aromatized product, phenylethylamine, and that the two pathways do not crossover. [Wimalasena, K., and May, S. W. (1989) J. Am. Chem. Soc. 111, 2729-2731; Wimalasena, K., and Alliston, K. R. (1995) J. Am. Chem. Soc. 117, 1220-1224]. We now report that the ring H-abstraction pathway of the reaction further partitions to produce the ring hydroxylated product, CHDEA-6OH, and the aromatized product, PEA, at the carbon-oxygen bond formation step. The ring hydroxylation is shown to be stereospecific, exclusively producing the (S) product. The absolute stereospecificity of the ring and side-chain hydroxylations of the DbetaM/CHDEA reaction suggests that the side-chain pro-R hydrogen of the enzyme-bound substrate is close to perpendicular to the aromatic ring of the phenylethylamine substrate or cyclohexadiene ring of CHDEA. The relative activation energy parameters suggest that the partitioning of the ring H abstraction pathway between aromatized and ring hydroxylated products is due to the partitioning of the high-energy intermediates, the cyclohexadienyl radical and the Cu(II)-O(*) species, between carbon-oxygen bond formation and direct electron transfer. The relatively high activation enthalpic favorability and entropic unfavorability for the carbon-oxygen bond formation strongly suggest that the critical balancing of these two opposing forces is mandatory for the desired product formation.
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