JBC, Vol. 251, Issue 1, 193-197, Jan, 1976

Membrane-associated thiamin triphosphatase. II. Activation by divalent cations

R. L. Barchi and R. O. Viale

Activation of membrane-associated thiamin triphosphatase from rat brain requires a divalent cation (Mg2+, Ca2+, or Mn2+). The optimum concentration of Mg2+ necessary for maximal enzyme activity varies with substrate concentration; conversely, the maximal rate of hydrolysis attainbale by increasing thiamin triphosphate concentration is directly proportional to [Mg2+] for all levels of Mg2+ below that of the substrate. Under appropriate conditions, the Km of the thiamin triphosphatase for Mg2+ and for thiamin triphosphate are shown to be identical. Dissociation constants (Kd) for the binding of Mg2+ to thiamin triphosphate, thiamin diphosphate, and thiamin were determined; kinetic data re-expressed in terms of [Mg2+-thiamin triphosphate] conform to simple single substrate predictions, suggesting that the true enzyme substrate may be the Mg2+-thiamin triphosphate complex. Excess free Mg2+ inhibits thiamin triphosphatase activity competitively while excess free thiamin triphosphate in concentrations up to 10 times Km has no effect on the membrane-bound enzyme.
CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

This article has been cited by other articles:

				J. Song, L. Bettendorff, M. Tonelli, and J. L. Markley Structural Basis for the Catalytic Mechanism of Mammalian 25-kDa Thiamine Triphosphatase J. Biol. Chem., April 18, 2008; 283(16): 10939 - 10948. [Abstract] [Full Text] [PDF]