4PGG

Caffeic acid O-methyltransferase from Sorghum bicolor

Summary for 4PGG

• Entry DOI: 10.2210/pdb4pgg/pdb
• Related: 4PGH
• Descriptor: Caffeic acid O-methyltransferase (2 entities in total)
• Functional Keywords: sam-dependent o-methyltransferase, transferase
• Biological source: Sorghum bicolor (Sorghum)
• Total number of polymer chains: 2
• Total formula weight: 78864.34

Authors

Green, A.R.,Lewis, K.M.,Kang, C. (deposition date: 2014-05-01, release date: 2014-07-02, Last modification date: 2023-12-27)

Primary citation

Green, A.R.,Lewis, K.M.,Barr, J.T.,Jones, J.P.,Lu, F.,Ralph, J.,Vermerris, W.,Sattler, S.E.,Kang, C.
Determination of the Structure and Catalytic Mechanism of Sorghum bicolor Caffeic Acid O-Methyltransferase and the Structural Impact of Three brown midrib12 Mutations.
Plant Physiol., 165:1440-1456, 2014

PubMed Abstract: Using S-adenosyl-methionine as the methyl donor, caffeic acid O-methyltransferase from sorghum (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 mutants that carry COMT gene missense mutations, we determined the apo-form and S-adenosyl-methionine 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 histidine-267. Additionally, despite the presence of other phenylpropanoid substrates in vivo, sinapaldehyde is the preferential product, as demonstrated by its low K for 5-hydroxyconiferaldehyde. Unlike its acid and alcohol substrates, the aldehydes exhibit product inhibition, and we propose that this is due to nonproductive 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.

PubMed: 24948836
DOI: 10.1104/pp.114.241729

Experimental method

X-RAY DIFFRACTION (2.015 Å)