Aminoacylation at the Atomic Level in Class IIa Aminoacyl-tRNA Synthetases.

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• Aminoacyl-tRNA synthetases (aaRSs) are crucial enzymes in protein synthesis, responsible for attaching specific amino acids to their cognate tRNAs.
• Class II aaRSs, including histidyl- (HisRS), threonyl- (ThrRS), and glycyl-tRNA synthetases (GlyRS), share conserved structural features but exhibit mechanistic variations.
• Understanding the structural basis of aminoacylation steps is vital for comprehending translation fidelity and developing therapeutic strategies.

Purpose of the Study:

• To determine the crystal structures of HisRS, ThrRS, and GlyRS in various states of the aminoacylation reaction.
• To elucidate the structural mechanisms underlying amino acid activation and tRNA binding in these class II enzymes.
• To compare the catalytic strategies employed by different aaRSs for amino acid activation.

Main Methods:

• X-ray crystallography was used to determine the high-resolution structures of HisRS, ThrRS, and GlyRS.
• Enzyme complexes were crystallized with substrates, intermediates, and products, including ATP, amino acid analogs, adenylates, and tRNA.
• Structural analysis focused on ligand binding sites, active site residues, and domain interactions.

Main Results:

• Crystal structures revealed enzyme-substrate and enzyme-product states for the amino acid activation step in HisRS and GlyRS.
• HisRS utilizes an arginine residue, while GlyRS employs a magnesium ion for amino acid activation, highlighting mechanistic divergence.
• ThrRS structures showed tRNA binding to the active site, with the acceptor stem positioned for amino acid transfer and the anticodon loop interacting with a conserved C-terminal domain.

Conclusions:

• The study provides detailed structural insights into the two key steps of aminoacylation catalyzed by class II aaRSs.
• Conserved interactions with the adenosine-phosphate moiety and distinct catalytic mechanisms for amino acid activation were identified.
• The findings extend principles of tRNA binding across different aaRSs, contributing to a unified understanding of their function.