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Isoleucyl-tRNA synthetase from Baker's yeast. Catalytic mechanism, 2',3'-specificity and fidelity in aminoacylation of tRNAIle with isoleucine and valine investigated with initial-rate kinetics using analogs of tRNA, ATP and amino acids. 1983

W Freist, and F Cramer

The aminoacylation of three modified tRNAIle species with isoleucine and with valine by isoleucyl-tRNA synthetase has been investigated by initial rate kinetics. For aminoacylation of tRNAIle-C-C-3'dA with isoleucine, a bi-bi uni-uni ping-pong mechanism has been found by bisubstrate kinetics and inhibition by products and by 3'dATP; for aminoacylation with valine a bi-uni uni-bi ping-pong mechanism. For isoleucylation of tRNAIle-C-C-A(3'NH2) bisubstrate kinetics, inhibition by products and by isoleucinol show a random uni-bi uni-uni-uni ping-pong mechanism; for valylation of this tRNA a bi-bi uni-uni ping-pong mechanism is observed by bisubstrate kinetics and product inhibition. tRNAIle-C-C-2'dA was aminoacylated under modified conditions with isoleucine in a bi-bi uni-uni ping-pong mechanism with a rapid equilibrium segment as observed by bisubstrate kinetics, inhibition by AMP, by P[NH]P as product analog and by isoleucinol. Aminoacylation with valine is achieved in a rapid-equilibrium sequential random AB, ordered C mechanism indicated by bisubstrate kinetics and inhibition by 3'dATP and valinol. All six reactions exhibit orders of substrate addition and product release which are different from those observed in aminoacylation of the natural tRNAIle-C-C-A. The Km values of the three substrates and the kcat values of the six reactions are given. For aminoacylation at the terminal 2'OH group of the tRNA differences of 13.38 and 13.17 kJ in binding energies between valine and isoleucine have been calculated which result in discrimination factors of 181 and 167. For aminoacylation at the terminal 3'-OH group a difference of only 4.43 kJ and a low discrimination factor of only 6 is observed. Thus maximal discrimination between the cognate and the noncognate amino acid is only achieved in aminoacylation at the 2'-OH group and conclusions drawn from experiments with modified tRNAs concerning 2',3'-specificity have led to correct results in spite of different catalytic cycles in aminoacylation of the natural and the modified tRNAs. The stability of Ile-tRNAIle-C-C-2'dA and Val-tRNAIle-C-C-2'dA, the lesser stability of Val-tRNAVal-C-C-2'dA and the instability of Thr-tRNAVal-C-C-2'dA are consistent with postulations for a 'pre-transfer' proofreading step for isoleucyl-tRNA synthetase and a 'post-transfer' hydrolytic editing step for valyl-tRNA synthetase at the terminal 3'OH group of the tRNA.

UI MeSH Term Description Entries
D007532 Isoleucine An essential branched-chain aliphatic amino acid found in many proteins. It is an isomer of LEUCINE. It is important in hemoglobin synthesis and regulation of blood sugar and energy levels. Alloisoleucine,Isoleucine, L-Isomer,L-Isoleucine,Isoleucine, L Isomer,L-Isomer Isoleucine
D007533 Isoleucine-tRNA Ligase An enzyme that activates isoleucine with its specific transfer RNA. EC 6.1.1.5. Isoleucyl T RNA Synthetase,Isoleucyl- tRNA Synthetase ILS1,Isoleucyl-tRNA Synthetase 1,Isoleucyl-tRNA Synthetase ILES1,Ile-tRNA Ligase,Isoleucyl-tRNA Synthetase,1, Isoleucyl-tRNA Synthetase,ILES1, Isoleucyl-tRNA Synthetase,Ile tRNA Ligase,Isoleucine tRNA Ligase,Isoleucyl tRNA Synthetase,Isoleucyl tRNA Synthetase 1,Isoleucyl tRNA Synthetase ILES1,Isoleucyl tRNA Synthetase ILS1,Ligase, Ile-tRNA,Ligase, Isoleucine-tRNA,Synthetase 1, Isoleucyl-tRNA,Synthetase ILES1, Isoleucyl-tRNA,Synthetase, Isoleucyl-tRNA
D007700 Kinetics The rate dynamics in chemical or physical systems.
D008956 Models, Chemical Theoretical representations that simulate the behavior or activity of chemical processes or phenomena; includes the use of mathematical equations, computers, and other electronic equipment. Chemical Models,Chemical Model,Model, Chemical
D000604 Amino Acyl-tRNA Synthetases A subclass of enzymes that aminoacylate AMINO ACID-SPECIFIC TRANSFER RNA with their corresponding AMINO ACIDS. Amino Acyl T RNA Synthetases,Amino Acyl-tRNA Ligases,Aminoacyl Transfer RNA Synthetase,Aminoacyl-tRNA Synthetase,Transfer RNA Synthetase,tRNA Synthetase,Acyl-tRNA Ligases, Amino,Acyl-tRNA Synthetases, Amino,Amino Acyl tRNA Ligases,Amino Acyl tRNA Synthetases,Aminoacyl tRNA Synthetase,Ligases, Amino Acyl-tRNA,RNA Synthetase, Transfer,Synthetase, Aminoacyl-tRNA,Synthetase, Transfer RNA,Synthetase, tRNA,Synthetases, Amino Acyl-tRNA
D012346 RNA, Transfer, Amino Acyl Intermediates in protein biosynthesis. The compounds are formed from amino acids, ATP and transfer RNA, a reaction catalyzed by aminoacyl tRNA synthetase. They are key compounds in the genetic translation process. Amino Acyl tRNA,Transfer RNA, Amino Acyl,tRNA-Amino Acyl,Amino Acyl T RNA,Acyl tRNA, Amino,Acyl, tRNA-Amino,tRNA Amino Acyl,tRNA, Amino Acyl
D012441 Saccharomyces cerevisiae A species of the genus SACCHAROMYCES, family Saccharomycetaceae, order Saccharomycetales, known as "baker's" or "brewer's" yeast. The dried form is used as a dietary supplement. Baker's Yeast,Brewer's Yeast,Candida robusta,S. cerevisiae,Saccharomyces capensis,Saccharomyces italicus,Saccharomyces oviformis,Saccharomyces uvarum var. melibiosus,Yeast, Baker's,Yeast, Brewer's,Baker Yeast,S cerevisiae,Baker's Yeasts,Yeast, Baker
D013379 Substrate Specificity A characteristic feature of enzyme activity in relation to the kind of substrate on which the enzyme or catalytic molecule reacts. Specificities, Substrate,Specificity, Substrate,Substrate Specificities
D014633 Valine A branched-chain essential amino acid that has stimulant activity. It promotes muscle growth and tissue repair. It is a precursor in the penicillin biosynthetic pathway. L-Valine,L Valine

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