ISSN:
1573-6881
Keywords:
Chromatophores
;
membrane-bound pyrophosphatase
;
phosphatepyrophosphate exchange
;
pyrophosphate hydrolysis
;
pH
;
divalent cations
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
,
Physics
Notes:
Abstract The relation that exist between the Pi-PPi exchange reaction and pyrophosphate hydrolysis by the membrane-bound pyrophosphatase of chromatophores ofRhodospirillum rubrum was studied. The two reactions have a markedly different requirement for pH. The optimal pH for hydrolysis was 6.5 while the Pi-PPi exchange reaction was at 7.5; the pH affects mainly theK m of Mg2+ or Pi for the enzyme; Mn2+ and Co2+ support the Pi-PPi exchange reaction partially (50%), but the reaction is slower than with Mg2+; other divalent cations like Zn2+ or Ca2+ do not support the exchange reaction. In the hydrolytic reaction, Zn2+, at low concentration, substitutes for Mg2+ as substrate, and Co2+ also substitutes in limited amount (50%). Other cations (Ca2+, Cu2+, Fe2+, etc.) do not act as substrates in complex with PPi. The Zn2+ at high concentrations inhibited the hydrolytic reaction, probably due to uncomplexed free Zn2+. In the presence of high concentration of substrate for the hydrolysis (Mg-PPi) the divalent cations are inhibitory in the following order: Zn2+〉Mn2+〉Ca2+≧Co2+〉Fe2+〉Cu2+〉Mg2+. The data in this work suggest that H+ and divalent cations in their free form induced changes in the kinetic properties of the enzyme.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00762416
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