Abstract
This is a concluding part of the three-part article from a series of reviews on the abundance and roles of intrinsic disorder in milk proteins. In this paper, we describe the peculiarities of metal binding to a multifunctional milk protein, a-lactalbumin, which has two domains, a large alpha-helical domain and a small beta-sheet domain connected by a calcium binding loop. It is known that in addition to four disulfide bonds, the native fold of this protein is stabilized by binding of a calcium ion. In fact, although in various mammals, alpha-lactalbumins are rather poorly conserved possessing the overall sequence identity of similar to 16%, the positions of all eight cysteines and a calcium binding site (residues DKFLDDDITDDI in human protein) are strongly conserved. Curiously, this conserved calcium binding loop is located within a region with increased structural flexibility. Besides canonical calcium binding, alpha-lactalbumin is known to interact with other metals, such as zinc (for which it has a specific binding site), and, in its apo-form, it can bind other divalent and monovalent cations. The binding of Mg2+, Na+, and K+ to the Ca2+ site increases alpha-lactalbumin stability against action of heat and various denaturing agents, with the higher stabilization effects being imposed by the stronger bound metal ions.