Sandbox Reserved 660: Difference between revisions
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The mechanism begins with the binding of ATP and hydroxycinnamate substrates. The binding results in 4CL1 adopting the catalytic conformation for the adenylate-forming partial reaction, in which the side chain of Lys-523 interacts with and directs the carboxylate group of the bound hydroxycinnamates for the nucleophilic attack of the α-phosphate of ATP, resulting in an AMP-hydroxycinnamate conjugate and a PPi molecule. The release of PPi then propels 4CL1 to the catalytic conformation of the thioester-forming partial reaction. In this conformation, the side chain of His-234 swings aside to allow access of CoA to the AMP-hydroxycinnamate conjugate. The AMP-hydroxycinnamate conjugate and CoA are then catalyzed by side chains of Lys-438 and Gln-443 to form the final thioester product. The C-domain rotates again to expose the substrate binding site, and the thioester and AMP are released. | The mechanism begins with the binding of ATP and hydroxycinnamate substrates. The binding results in 4CL1 adopting the catalytic conformation for the adenylate-forming partial reaction, in which the side chain of Lys-523 interacts with and directs the carboxylate group of the bound hydroxycinnamates for the nucleophilic attack of the α-phosphate of ATP, resulting in an AMP-hydroxycinnamate conjugate and a PPi molecule. The release of PPi then propels 4CL1 to the catalytic conformation of the thioester-forming partial reaction. In this conformation, the side chain of His-234 swings aside to allow access of CoA to the AMP-hydroxycinnamate conjugate. The AMP-hydroxycinnamate conjugate and CoA are then catalyzed by side chains of Lys-438 and Gln-443 to form the final thioester product. The C-domain rotates again to expose the substrate binding site, and the thioester and AMP are released. | ||
==Implication== | |||
4-Coumarate-coenzyme A ligase (4CL) is an enzyme that functions early in the general phenylpropanoid pathway by producing the monolignol precursor p-coumaroyl-CoA. 4CL silencing in angiosperm species such as tobacco, Arabidopsis and Populus tremuloides causes lignin reductions in the range of 25% to 45%. However, the impacts of these manipulations on lignin composition varied. 4CL silencing in tobacco preferentially depleted syringyl (S) lignin units, which are prominent in wood fibers. 4CL silencing in Arabidopsis depleted only guaiacyl (G) lignin units, which are enriched in vessel elements, whereas silencing of 4CL in P.tremuloides had no impact on the S-G ratio. An increase in the S-G ratio in P.tremuloides was only recorded when 4CL silencing was combined with the over expression of coniferaldehyde 5-hydroxylase. These differences in lignin composition could be the consequence of silencing 4CL isoforms with different substrate preferences, or they could reflect the in adequacies or limitations of analytical procedures used for lignin analysis. | |||
