Structural Basis for Specificity and Flexibility in a Plant 4-Coumarate:CoA Ligase

Area of Science:

• Biochemistry
• Structural Biology
• Plant Science

Background:

• Plant 4-coumarate:CoA ligase (4CL) is a crucial enzyme in the phenylpropanoid pathway, regulating carbon flow and providing precursors for diverse metabolites.
• Understanding 4CL's catalytic mechanism and substrate specificity is vital for metabolic engineering and understanding plant secondary metabolism.

Purpose of the Study:

• To elucidate the high-resolution crystal structures of Nicotiana tabacum 4CL isoform 2 (Nt4CL2) in complex with various ligands.
• To characterize the structural basis for dual catalytic conformations and substrate binding of Nt4CL2.
• To identify key residues involved in catalysis, ATP binding, and substrate specificity through kinetic studies.

Main Methods:

• X-ray crystallography was used to determine the structures of Nt4CL2 in complex with Mg(2+)-ATP, AMP-CoA, and hydroxycinnamate-AMP intermediates.
• Kinetic studies were performed on structure-based variants of Nt4CL2.
• A deletion mutant of Nt4CL2 was characterized for its enzymatic activity.

Main Results:

• High-resolution crystal structures of Nt4CL2 were obtained in both adenylate-forming and thioester-forming conformations.
• The structures revealed the binding determinants for coenzyme A (CoA) and hydroxycinnamate substrates.
• Kinetic analyses identified critical residues for catalysis, ATP binding, and substrate specificity.
• A mutant exhibited unusual sinapinate-utilizing activity.

Conclusions:

• The study provides a comprehensive molecular framework for understanding Nt4CL2 function and its role in the phenylpropanoid pathway.
• The structural insights pave the way for targeted engineering of 4CL enzymes for improved biocatalytic applications.
• This work offers a rare visualization of an ANL enzyme in two distinct functional conformations.