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Comparative Study
. 2009 Dec;1794(12):1831-7.
doi: 10.1016/j.bbapap.2009.08.022. Epub 2009 Sep 3.

The reaction mechanism of phenylethanolamine N-methyltransferase: a density functional theory study

Polina Georgieva  1 , Qian Wu, Michael J McLeish, Fahmi Himo
Affiliations
Comparative Study

The reaction mechanism of phenylethanolamine N-methyltransferase: a density functional theory study

Polina Georgieva et al. Biochim Biophys Acta. 2009 Dec.

Abstract

Hybrid density functional theory methods were used to investigate the reaction mechanism of human phenylethanolamine N-methyltransferase (hPNMT). This enzyme catalyzes the S-adenosyl-L-methionine-dependent conversion of norepinephrine to epinephrine, which constitutes the terminal step in the catecholamine biosynthesis. Several models of the active site were constructed based on the X-ray structure. Geometries of the stationary points along the reaction path were optimized and the reaction barrier and energy were calculated and compared to the experimental values. The calculations demonstrate that the reaction takes place via an SN2 mechanism with methyl transfer being rate-limiting, a suggestion supported by mutagenesis studies. Optimal agreement with experimental data is reached using a model in which both active site glutamates are protonated. Overall, the mechanism of hPNMT is more similar to those of catechol O-methyltransferase and glycine N-methyltransferase than to that of guanidinoacetate N-methyltransferase in which methyl transfer is coupled to proton transfer.

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Figures

Figure 1
Figure 1
Model structure of the active site of PNMT showing the two substrates, octopamine and AdoMet. Water molecules are in gray and the potential hydrogen bonding network is in magenta. As used in the text, N, C and S refer to the amine nitrogen, the methyl carbon, and the AdoMet sulfur, respectively. The AdoMet structure was from PDB 2G72 while octopamine and other active site residues were from PDB 2AN4. The figure was created using PyMol [44].
Figure 2
Figure 2
Optimized reactant (A), methyl transfer transition state (B), and product (C) structures for Model A. Arrows indicate centers that are locked to their crystallographic positions during the geometry optimizations. Distances are given in angstrom.
Figure 3
Figure 3
Optimized reactant (A), methyl transfer transition state (B), and product structures (C and D) for Model B.
Figure 4
Figure 4
Optimized structures of the reactant (A), transition state (B), and product (C) of Model C.
Figure 5
Figure 5
Optimized reactant (A), transition state (B) and product (C) structures of Model C in the case of a protonated Glu219 residue, called Model C(H+).
See this image and copyright information in PMC

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