Location: Grain, Forage, and Bioenergy ResearchTitle: Elucidation of the structure and reaction mechanism of Sorghum bicolor hydroxycinnamoyltransferase and its structural relationship to other CoA-dependent transferases and synthases) Author
Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/26/2013
Publication Date: 6/18/2013
Publication URL: http://handle.nal.usda.gov/10113/56866
Citation: Walker, A.M., Hayes, R.P., Youn, B., Vermerris, W., Sattler, S.E., Kang, C. 2013. Elucidation of the structure and reaction mechanism of Sorghum bicolor hydroxycinnamoyltransferase and its structural relationship to other CoA-dependent transferases and synthases. Plant Physiology. 162:640-651. DOI http://dx.doi.org/10.1104/pp.113.217836. Interpretive Summary: In the US, sorghum biomass (stalks and leaves) serves as an important forage crop for livestock. In addition, sorghum is being developed as a renewable bioenergy crop for liquid biofuels. Conversion technologies are being developed to convert the three main components of plant biomass (cellulose, hemicellulose and lignin) into a range of transportation fuels (jet, diesel and gasoline). Understanding lignin synthesis is critically important for developing plants with altered biomass composition to be used with these emerging conversion technologies. The sorghum hydroxycinnamoyltransferase (SbHCT) is a key enzyme that participates in an early step of lignin synthesis. Herein we examined the structure of this enzyme to understand how the enzyme functions in lignin synthesis. The structure of SbHCT was similar to other enzymes found in plants. Our observations explain how SbHCT and these other enzymes that share similar structural features can participate in different biochemical pathways in different plant species.
Technical Abstract: Hydroxycinnamoyltransferase (SbHCT) from Sorghum bicolor participates in an early step of the phenylpropanoid pathway, exchanging CoA esterified to p-coumaric acid with shikimic or quinic acid, as intermediates in the biosynthesis of the monolignols coniferyl alcohol and sinapyl alcohol. In order to elucidate the mode of action of this enzyme, we have determined the crystal structures of SbHCT in its apo-form and ternary complex with shikimate and p-coumaroyl CoA, which was converted to its product inside of the crystal. The structure revealed the roles of Thr36, Ser38, Tyr40, His162, Arg371 and Thr384 in catalysis and specificity. Based on the analysis of mutant forms of SbHCT in which these residues were replaced with alanine via site-directed mutagenesis, kinetic and thermodynamic data, we propose the catalytic mechanism of HCT. Comparison of the structure of sorghum HCT with that of the HCT involved in chlorogenic acid synthesis in coffee (Coffea canephora) revealed many shared features and a high degree of conservation for most critical residues, including those involved in the entrance site for the acyl-CoA donors. More variability in structure was observed for the entrance site of the acidic substrate. Taken together, these observations explain how CoA-dependent transferases with similar structural features can participate in different biochemical pathways in different species.