© 2006 The Japanese Biochemical Society.
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Binding of Glucose to the D-Galactose/D-Glucose–Binding Protein from Escherichia coli Restores the Native Protein Secondary Structure and Thermostability That Are Lost upon Calcium Depletion
1 Istituto di Biochimica delle Proteine, CNR, Via P. Castellino, 111 80131 Napoli, Italy; 2 Istituto di Biochimica, Università Politecnica delle Marche, Via Ranieri, 60131 Ancona, Italy; 3 Laboratorio di Bioinformatica, Istituto di Scienze dell'Alimentazione, CNR, via Roma 52 A/C, 83100 Avellino, Italy; and 4 Centro di Ricerca Interdipartimentale di Scienze Computazionali e Biotecnologiche (CRISCEB), Seconda Universitá degli Studi di Napoli, Via Costantinopoli, 16, 80138 Napoli, Italy
* To whom correspondence should be addressed. Tel: +39-0816132250, Fax: +39-0816132277; E-mail: s.dauria{at}ibp.cnr.it
The effect of the depletion of calcium on the structure and thermal stability of the D-galactose/D-glucose–binding protein (GGBP) from Escherichia coli was studied by fluorescence spectroscopy and Fourier-transform infrared spectroscopy. The calcium-depleted protein (GGBP-Ca) was also studied in the presence of glucose (GGBP-Ca/Glc). The results show that calcium depletion has a small effect on the secondary structure of GGBP, and, in particular it affects a population of 
-helices with a low exposure to solvent. Alternatively, glucose-binding to GGBP-Ca eliminates the effect induced by calcium depletion by restoring a secondary structure similar to that of the native protein. In addition, the infrared and fluorescence data obtained reveal that calcium depletion markedly reduces the thermal stability of GGBP. In particular, the spectroscopic experiments show that the depletion of calcium mainly affects the stability of the C-terminal domain of the protein. However, the binding of glucose restores the thermal stability of GGBP-Ca. The thermostability of GGBP and GGBP-Ca was also studied by molecular dynamics simulations. The simulation data support the spectroscopic results. New insights into the role of calcium in the thermal stability of GGBP contribute to a better understanding of the protein function and constitute important information for the development of biotechnological applications of this protein. Mutations and/or labelling of amino acid residues located in the protein C-terminal domain may affect the stability of the whole protein structure.
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