Determining calcium‐binding stoichiometry and cooperativity of parvalbumin and calmodulin by mass spectrometry

The calcium‐binding properties of parvalbumin and calmodulin were investigated by electrospray ionization mass spectrometry (ESI‐MS). The two calcium sites of parvalbumin were found to be strongly cooperative in binding Ca2+ ion. Up to four calcium ions were found to bind to calmodulin at high calcium concentration levels. Strong cooperativity was detected between the third and fourth Ca2+ binding sites of calmodulin (ordered by loading sequence). Strong interactions were also indicated between the two halves of the calmodulin molecule. Demetallation of the gas‐phase ions can occur during the desolvation process, especially for ESI of aqueous solutions. However, the ESI‐MS methodology has the potential to provide insight into the intricate processes involved in metal site communications for other metalloproteins.