4pgh

Publication Abstract from PubMed

Using S-adenosylmethionine as the methyl donor, caffeic acid O-methyltransferase from Sorghum bicolor (SbCOMT) methylates the 5-hydroxyl group of its preferred substrate, 5-hydroxyconiferaldehyde. In order to determine the mechanism of SbCOMT and understand the observed reduction in the lignin syringyl-to-guaiacyl ratio of three brown midrib12 (bmr12) mutants that carry COMT gene missense mutations, we determined the apo-form and S-adenosylmethionine-binary complex SbCOMT crystal structures, and established the ternary complex structure with 5-hydroxyconiferaldehyde by molecular modeling. These structures revealed many features shared with monocot ryegrass (Lolium perenne) and dicot alfalfa (Medicago sativa) COMTs. SbCOMT steady-state kinetic and calorimetric data suggest a random bi-bi mechanism. Based on our structural, kinetic and thermodynamic results, we propose that the observed reactivity hierarchy among 4,5-dihydroxy-3-methoxycinnamyl (and 3,4-dihydroxycinnamyl) aldehyde, alcohol and acid substrates arises from the ability of the aldehyde to stabilize the anionic intermediate that results from deprotonation of the 5-hydroxyl group by His267. Additionally, despite the presence of other phenylpropanoid substrates in vivo, sinapaldehyde is the preferential product, as demonstrated by its low Km for 5-hydroxyconiferaldehyde. Unlike its acid and alcohol substrates, the aldehydes exhibit product inhibition and we propose this is due to non-productive binding of the s-cis-form of the aldehydes inhibiting productive binding of the s-trans-form. The s-cis-aldehydes most likely act only as inhibitors because the high rotational energy barrier around the 2-propenyl bond prevents s-trans conversion, unlike alcohol substrates whose low 2-propenyl bond rotational energy barrier enables rapid s-cis/s-trans interconversion.

Determination of the structure and catalytic mechanism of Sorghum bicolor caffeic acid O-methyltransferase and the structural impact of three brown midrib12 mutations.,Green AR, Lewis KM, Barr JT, Jones JP, Lu F, Ralph J, Vermerris W, Sattler SE, Kang C Plant Physiol. 2014 Jun 19. pii: pp.114.241729. PMID:24948836^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.