Leucyl tRNA synthetase (LeuRS) catalytic core domain. This class I enzyme is a monomer which aminoacylates the 2'-OH of the nucleotide at the 3' of the appropriate tRNA. The core domain is based on the Rossman fold and is responsible for the ATP-dependent formation of the enzyme bound aminoacyl-adenylate. It contains the characteristic class I HIGH and KMSKS motifs, which are involved in ATP binding. In Aquifex aeolicus, the gene encoding LeuRS is split in two, just before the KMSKS motif. Consequently, LeuRS is a heterodimer, which likely superimposes with the LeuRS monomer found in most other organisms. LeuRS has an insertion in the core domain, which is subject to both deletions and rearrangements and thus differs between prokaryotic LeuRS and archaeal/eukaryotic LeuRS. This editing region hydrolyzes mischarged cognate tRNAs and thus prevents the incorporation of chemically similar amino acids.