Figure 1. Editing mechanisms during tRNA aminoacylation. A cognate amino acid proceeds through the aminoacylation pathway, indicated by the double-headed arrows, to form AA-tRNA. A noncognate amino acid enters the aminoacylation pathway but is rejected at points indicated by single-headed arrows; as a result, continuous hydrolysis of ATP occurs, a diagnostic feature of editing.

Figure 2. The synthetic/editing active site of E. coli MetRS: Hydrophobic and hydrogen bonding interactions provide specificity for the cognate substrate l-methionine. Superimposition of a-carbon chain backbones for the MetRS·Met complex (burly-wood) and free MetRS (light grey), solved at 1.8 Å resolution, shows movements of active site residues upon binding of methionine. Residue colours are red in the MetRS·Met complex and green in free MetRS, and l-methionine is magenta. Reprinted with permission from Serre et al. (2001).

Figure 3. Editing of homocysteine, aminoacylation of thiols, and synthesis of S-nitroso-Hcy-tRNA by MetRS. (a) The MetRS-catalysed cyclization of homocysteinyl adenylate to form Hcy-thiolactone and AMP, which are subsequently released from the synthetic/editing active site of MetRS. (b) The MetRS-catalysed reaction of a thiol (mimicking the side-chain of Hcy, R- CH₂SH) with methionyl-tRNA to form a methionine thioester, which is subsequently released from the synthetic/editing active site of MetRS. (c) MetRS-catalysed aminoacylation of tRNA with S-nitroso-Hcy. Adapted with permission from Jakubowski (2001)2001.

Figure 4. (a) Conformations of pretransfer substrate analogue, NvaAMS, bound in the synthetic and editing sites of T. thermophilus LeuRS. (b) Locations of NvaAMS in the synthetic and editing active sites of LeuRS. Reprinted with permission from Lincecum et al. (2003).

Figure 5. Metabolism of Hcy in a human endothelial cell. Hcy arises from methionine as a by-product of cellular transmethylation reactions. When its re-methylation to methionine, catalysed by methionine synthase (MS), is impaired, for example by inadequate folate supply, Hcy is metabolized to Hcy-thiolactone by MetRS. Hcy-thiolactone freely diffuses out and into the cell, reacts with protein lysine residues or is hydrolysed in serum to Hcy by an HDL-associated Hcy-thiolactonase. Hcy forms a mixed disulfide with serum albumin (Hcy-S-protein). Reaction with nitric oxide, produced by endothelial nitric oxide synthetase, converts Hcy to S-nitroso-Hcy (S-NO-Hcy), which is then incorporated translationally into protein following formation of S-nitroso-Hcy-tRNA catalysed by MetRS. Hcy-N-protein contains Hcy in amide or peptide bonds. Reprinted with permission from Jakubowski)(2002b)2002B.

Figure 6. Homocysteinylation of protein lysine residues by Hcy-thiolactone.