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Yeast inorganic pyrophosphatase: studies on metal binding.

A study has been made of the interaction of yeast inorganic pyrophosphatase with two activating cations, Mg2+ and Zn2+, and one inhibitory cation, Ca2+ at pH 6.5 and ionic strength of unity. These cations were found to partially protect the enzyme against inactivation by iodine. Dissociation constants of enzyme metal complexes estimated from protection studies were 11, 500 and 4000 μM for Zn2+, Mg2+ and Ca2+, respectively. An additional, lower affinity site, was found for Zn2+ (KD= 3.4 mM). Competition studies showed that magnesium and calcium ions compete for the same site as the tighter bound zinc ion and do not compete for the lower affinity site. Binding of Zn2+ to both sites is pH-dependent; the interaction with the high-affinity site is controlled by a group with pK= 6.6. The binding constant for Mg2+ is nearly the same as the lower activation constant for this cation in enzymatic hydrolysis of pyrophosphate. Two activating and one inhibitory site were found for Zn2+ in ATP hydrolysis. The inhibitory site was identified as the lower affinity site deduced from protection studies. Both activation constants for Zn2+ are higher than the dissociation constant for the tightly bound zinc deduced from protection studies.