The effects of spermine and Mg2+ on the catalytic mechanism of isoleucine: tRNA ligase.
Eur J Biochem, 1975/5;54(1):169-73.
Carr AC, Igloi GL, Penzer GR, Plumbridge JA
PMID: 238842
Abstract
Isoleucyl-tRNA formation catalysed by isoleucine: tRNA ligase is stimulated by both Mg2+ and spermine in the pH-range 7.0 to 8.0 at 310 K. At low [Mg2+] the acceleration caused by both cations together exceeds the sum of their individual effects. 2. The spermine-stimulated reaction has a steeper temperature-dependence than reaction in the presence of Mg2+. Two phases in the kinetics of isoleucyl-tRNA formation are detected in the presence of Mg2+ plus or minus spermine, but only a single step is observed in the presence of spermine alone. Thus the rate-limiting steps under normal assay conditions are different for the two cations. 3. Enzyme-bound isoleucyl-AMP can be formed in the absence of Mg-2+ and plus or minus spermine. 4. It is concluded that there is no evidence for cation-dependent differences in the reaction mechanism of isoleucine: tRNA ligase, though there are certainly differences in the relative rates of some of the individual steps.
MeSH terms
Enzyme Activation; Escherichia coli; Hydrogen-Ion Concentration; Isoleucine; Kinetics; Ligases; Magnesium; RNA, Transfer; Spermine; Temperature
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