Spectroscopic studies on the metal ion binding loop of alpha-lactalbumin

摘要

alpha-Lactalbumins(alpha-LAs),the small,globular calcium-binding milk proteins,consist of two lobes:an alpha-helical domain and a beta-sheet domain,connected by a calcium binding loop.The unfolding of a-LAs has been studied intensively because it adopts a classic molten globule state under mild denaturing conditions.In the absence of Ca~(2+)the native protein looses its structural stability(tertiary structure).The calcium binding loop is an aspartate-rich decapeptide(-K79FLDDDLTDD88-)connected to alpha-helix(started from residue 86-)as well as a 3_(10)-helix(ended with residue-80).This motif is analogous to helix-loop-helix motif of the classical EF-hand,but smaller two amino acid residues.According to the X-ray crystallography three aspartic acid side chains(Asp82,Asp87 and Asp88),two oxygen atoms of amide carbonyls of Lys79 and Asp84 and two water molecules coordinate to Ca~(2+).The other two Asp-residues(Asp83,Asp84)are not involved in complex formation with Ca~(2+)but significantly contribute to the local charge density.