Alexander M. Walker, Steven A. Sattler, Matt Regner, Jeffrey P. Jones, John Ralph, Wilfred Vermerris, Scott E. Sattler, ChulHee Kang

Caffeoyl-CoA 3-O-methyltransferase (CCoAOMT) is an S-adenosyl methionine (SAM)-dependent O-methyltransferase responsible for methylation of the meta-hydroxyl group of caffeoyl-CoA, on the pathway to monolignols with their ring methoxylation status characteristic of guaiacyl or syringyl units in lignin. In order to better understand the unique class of type-2 O-methyltransferases from monocots, we have characterized CCoAOMT from sorghum (Sorghum bicolor) (SbCCoAOMT), including the SAM binary complex crystal structure, steady state enzyme kinetics, isothermal titration calorimetry (ITC), inductively coupled plasma-optical emission spectroscopy (ICP-OES) and molecular docking. Key amino acid residues were validated with site-directed mutagenesis. ITC data indicated a sequential binding mechanism for SbCCoAOMT, wherein SAM binds prior to caffeoyl-CoA, and the enzyme showed allosteric behavior with respect to it. 5-Hydroxyferuloyl-CoA was not a substrate for SbCCoAOMT. We propose a catalytic mechanism in which Lys180 acts as a catalytic base and deprotonates the reactive hydroxyl group of caffeoyl-CoA. This deprotonation is facilitated by the coordination of the reactive hydroxyl group by Ca2+ in the active site, lowering the pKa of the 3'-OH group. Collectively, these data give a new perspective on the catalytic mechanism of CCoAOMTs and provide a basis for the functional diversity exhibited by type-2 plant OMTs that contain a unique insertion loop (residues 208-231) conferring affinity for phenylpropanoid-CoA thioesters.