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Kinetic proofreading is a reaction scheme with a structure more complicated than that of Michaelis kinetics, which leads to a proofreading for errors in the recognition of a correct substrate by an enzyme. We have measured the stoichiometry between ATP hydrolysis and tRNAIle charging, using the enzyme isoleucyl-tRNA synthetase [L-isoleucine:tRNAIle ligase (AMP-forming), EC 6.1.1.5] and the amino acids isoleucine (correct) and valine (incorrect). The enzymatic deacylation of charged tRNA, which would normally prevent meaningful stoichiometry studies, was eliminated by the use of transfer factor Tu-GTP, (which binds strongly to charged tRNA) in the reaction mixture. For isoleucine, 1.5 ATP molecules are hydrolyzed per tRNA charged, but for valine, 270. These stoichiometry ratios are fundamental to kinetic proofreading, for the energy coupling is essential and proofreading is obtained only by departing from 1:1 stoichiometry between energy coupling and product formation. Within the known reaction pathway, these ratios demonstrate that kinetic proofreading induces a reduction in errors by a factor of 1/180. An overall error rate of about 10(-4) for tRNA charging is obtained by a kinetic proofreading using a fundamental discrimination level of about 10(-2), and is compatible with the low in vivo error rate of protein synthesis.

We report a rare case of a mucocele occuring in a naturally compartmentalized maxillary sinus in a 50-year-old woman. Sinus computerized tomography (CT) showed the antrum to be divided by a vertical bony septum, producing a mucocele within an isolated and non-draining anterolateral compartment. Accidental trauma to the thin bony shell precipitated rapid swelling of the pre-existing mass, leading to her initial consultation. A wide sublabial surgical approach allowed complete excision with a good cosmetic result.