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  • Title: Membrane-bound pyrophosphatase of Thermotoga maritima requires sodium for activity.
    Author: Belogurov GA, Malinen AM, Turkina MV, Jalonen U, Rytkönen K, Baykov AA, Lahti R.
    Journal: Biochemistry; 2005 Feb 15; 44(6):2088-96. PubMed ID: 15697234.
    Abstract:
    Membrane-bound pyrophosphatase of the hyperthermophilic bacterium Thermotoga maritima(Tm-PPase), a homologue of H(+)-translocating pyrophosphatase, was expressed in Escherichia coli and isolated as inner membrane vesicles. In contrast to all previously studied H(+)-PPases, both native and recombinant Tm-PPases exhibited an absolute requirement for Na(+) but displayed the highest activity in the presence of millimolar levels of both Na(+) and K(+). Detergent-solubilized recombinant Tm-PPase was thermostable and retained the monovalent cation requirements of the membrane-embedded enzyme. Steady-state kinetic analysis of pyrophosphate hydrolysis by the wild-type enzyme suggested that two Na(+) binding sites and one K(+) binding site are involved in enzyme activation. The affinity of the site that binds Na(+) first is increased with increasing K(+) concentration. In contrast, only one Na(+) binding site (K(+)-dependent) and one K(+) binding site were involved in activation of the Asp(703) --> Asn variant. Thus, Asp(703) may form part of the K(+)-independent Na(+) binding site. Unlike all other membrane and soluble PPases, Tm-PPase did not catalyze oxygen exchange between phosphate and water. However, solubilized Tm-PPase exhibited low but measurable PP(i)-synthesizing activity, which also required Na(+) but was inhibited by K(+). These results demonstrate that T. maritima PPase belongs to a previously unknown subfamily of Na(+)-dependent H(+)-PPase homologues and may be an analogue of Na(+),K(+)-ATPase.
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