title: Volume changes upon addition of Ca2+ to calmodulin: Ca2+-calmodulin conformational states.
abstract: Measurement of the volume change by a rapid density method upon sequential addition of calcium ion to calmodulin showed relatively large, nonuniform increases for the first 4 moles Ca2+ per mole calmodulin. Substantially larger volume increases (approximately 15 ml/mol protein) were observed upon addition of the second and fourth moles Ca2+ relative to the first and third moles added per mole calmodulin. A total volume increase of approximately 170 ml/mol protein attended the addition of 4 moles Ca2+, as expected for multidentate carboxylate coordination to metal ion. Marginal changes in volume were observed upon further additions, the data showing a remarkably sharp transition after [Ca2+]/[calmodulin] = 4. The results are consistent with an ordered binding of Ca2+ in which pair-wise additions produce similar volume changes; the volume change behavior, however, does not indicate an absence of distinct conformational states for a Ca2+(1)-calmodulin and a Ca2+(3)-calmodulin complex as has been proposed on the basis of 1H-NMR evidences.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002147:Calmodulin / Q000494:pharmacology*; D002417:Cattle; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008433:Mathematics; D011487:Protein Conformation; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D002147:Calmodulin; D002118:Calcium