PEDS Advance Access originally published online on August 10, 2005
Protein Engineering Design and Selection 2005 18(9):425-433; doi:10.1093/protein/gzi051
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How to improve nature: study of the electrostatic properties of the surface of 
-lactalbumin
1Institute for Biological Instrumentation, Russian Academy of Sciences, Pushchino, Moscow Region, Russia, 2Department of Biochemistry and Molecular Biology, Penn State University, College of Medicine, Hershey, PA 17033, 3Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, 4Center for Computational Biology and Bioinformatics, Department of Biochemistry and Molecular Biology, School of Medicine, Indiana University – Purdue University, Indianapolis, IN 46202 and 6Department of Veterinary Biosciences, The Ohio State University, Columbus, OH 43210, USA
5 To whom correspondence should be addressed. E-mail: vuversky{at}iupui.edu
It was recently shown that 
-lactalbumin interacts with histones and simple models of histone proteins such as positively charged polyamino acids, suggesting that some fundamental aspects of the protein surface electrostatics may come into play. In the present work, the energies of charge–charge interaction in apo- and Ca2+-loaded -lactalbumin were calculated using a Tanford–Kirkwood algorithm with either solvent accessibility correction or using a finite difference Poisson–Boltzmann method. The analysis revealed two major regions of -lactalbumin that possessed highly unfavorable electrostatic potentials: (a) the Ca2+-binding loop and its neighboring residues and (b) the N-terminal region of the protein. Several individual mutants were prepared to neutralize specific individual surface acidic amino acids at both the N-terminus and Ca2+-binding loop of bovine -lactalbumin. These mutants were characterized by intrinsic fluorescence, differential scanning microcalorimetry and circular dichroism. The structural and thermodynamic data agree in every case with the theoretical predictions, confirming that the N-terminal region is very sensitive to changes in charge. For example, desMet D14N mutation destabilizes protein and decreases its calcium affinity. On the other hand, desMet E1M and desMet D37N substitutions increase the thermal stability and calcium affinity. The Met E1Q is characterized by a marked increase in protein stability, whereas desMet E7Q and desMet E11L display a slight increase in calcium affinity and thermal stability. Examination of the unfavorable energy contributed by Glu1 and the energetically favorable consequences of neutralizing this residue suggests that nature may have made an error with bovine -lactalbumin from the viewpoint of stabilizing structure and conformation.
Keywords: -lactalbumin/calcium binding/electrostatic interactions/site-directed mutagenesis/thermal stability
Received April 5, 2005; revised June 28, 2005; accepted June 30, 2005.
Edited by Gideon Schreiber
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