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Functional Consequences of Alterations to Amino Acids at the M5S5 Boundary of the Ca 2+ -ATPase of Sarcoplasmic Reticulum

Journal of Biological Chemistry, ISSN: 0021-9258, Vol: 270, Issue: 2, Page: 908-914
1995
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Article Description

The roles of the hydrophobic side chains of residues Phe 750 , Ile 761 , Tyr 763 , Leu 764 , and Ile 765 located at the M5S5 boundary of the Ca 2+ -ATPase of sarcoplasmic reticulum were analyzed by site-directed mutagenesis. Substitution of Tyr 763 with glycine resulted in a new phenotypic variant of the Ca 2+ -ATPase that catalyzed a high rate of Ca 2+ -activated ATP hydrolysis without net accumulation of Ca 2+ in the microsomal vesicles. The ATPase activity of the Tyr 763 → Gly mutant displayed characteristics similar to the ATPase activity of the wild-type enzyme measured in the presence of calcium ionophore, and the mutant was able to form the ADP-insensitive phosphoenzyme intermediate. Mutants Phe 750 → Gly, Ile 761 → Gly, Leu 764 → Gly, and Ile 765 → Gly were able to accumulate Ca 2+ . In mutants Leu 764 → Gly and Ile 765 → Gly, the turnover rate was low due to inhibition of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate. On the other hand, mutant Leu 764 → Lys dephosphorylated rapidly. Mutants Phe 750 → Gly and Leu 764 → Lys displayed apparent Ca 2+ affinities that were reduced two and three orders of magnitude, respectively, relative to that of the wild-type.

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