Apparent cooperativity of Ca2+ binding associated with crystallization of Ca2+-binding protein from sarcoplasmic reticulum |
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| Authors: | M Tanaka T Ozawa A Maurer J D Cortese S Fleischer |
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| Affiliation: | 1. Department of Biomedical Chemistry, Faculty of Medicine, University of Nagoya, Nagoya 466, Japan;2. Department of Molecular Biology, Vanderbilt University, Nashville, Tennessee 37235 USA;1. School of Biological Sciences, Queen’s University Belfast, Medical Biology Centre, 97 Lisburn Road, Belfast BT9 7BL, UK;2. Institute for Global Food Security, Queen’s University Belfast, 18-30 Malone Road, Belfast BT9 5BN, UK;3. School of Pharmacy and Biomolecular Sciences, University of Brighton, Huxley Building, Lewes Road, Brighton BN2 4GJ, UK;1. The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory of Oral Biomedicine Ministry of Education, School & Hospital of Stomatology, Wuhan University, Wuhan, Hubei, People’s Republic of China;2. Department of Stomatology, Renmin Hospital of Wuhan University, 238#, Jiefang Road, Wuhan, Hubei, People’s Republic of China;3. Department of Prosthodontics, Stomatological Hospital, College of Medicine, Xi’an Jiaotong University, Xi’an, Shaanxi, People’s Republic of China;1. School of Biological Sciences, Queen''s University Belfast, Medical Biology Centre, 97 Lisburn Road, Belfast, BT9 7BL, UK;2. Institute for Global Food Security, Queen''s University Belfast, 18-30 Malone Road, Belfast, BT9 5BN, UK;3. Department of Pathology, University of Cambridge, Cambridge, CB2 1QP, UK;1. Tianjin Key Laboratory of Cognitive Computing and Application, School of Computer Science and Technology, Tianjin University, 300350, No.135 Yaguan Road, Jinnan District, Tianjin, PR China;2. The Computing and Communications Department, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom;1. Department of Physiology, Otago School of Medical Sciences and HeartOtago, University of Otago, Dunedin, Otago, New Zealand |
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| Abstract: | Needle-shaped crystals of the Ca2+-binding protein (CBP) isolated from rabbit skeletal muscle sarcoplasmic reticulum were studied with regard to the influence of Ca2+, K+, and H+ on its solubility and cation binding. The solubility of CBP is sharply decreased with concentration of Ca2+, whereas K+ increased it. Aggregation of the CBP and crystal formation is correlated with the binding of Ca2+. The Ca2+ bound to the crystalline CBP is two to three times higher than that of the soluble form. A strong apparent positive cooperative behavior of Ca2+ binding by CBP was observed concomitant with the shift in equilibrium from the soluble to the crystalline form. From the steepest Hill slope we obtained Hill coefficients of 3.3 for soluble CBP and 14 for the transition between soluble and crystalline forms of CBP. A detailed treatment is presented to validate the applicability of Hill plots for the combined binding and crystallization process. Two-thirds of the Ca2+-binding sites were K+ sensitive and one-third were K+ insensitive. An increase in H+ concentration decreased the Ca2+ binding by crystalline CBP without affecting its solubility, with a pK value of 6.2 determined for this process. These results indicate that the equilibrium between the soluble and crystalline forms of CBP is determined by the amount and nature of the bound cations, Ca2+, K+, and H+. They suggest the possibility that a cycle of aggregation and solubilization of CBP attends the uptake and release of Ca2+ in the sarcoplasmic reticulum, respectively. |
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