α-Lactalbumin, Amazing Calcium-Binding Protein

Abstract

α-Lactalbumin (α-LA) is a small (Mr 14,200), acidic (pI 4–5), Ca2+-binding protein. α-LA is a regulatory component of lactose synthase enzyme system functioning in the lactating mammary gland. The protein possesses a single strong Ca2+-binding site, which can also bind Mg2+, Mn2+, Na+, K+, and some other metal cations. It contains several distinct Zn2+-binding sites. Physical properties of α-LA strongly depend on the occupation of its metal binding sites by metal ions. In the absence of bound metal ions, α-LA is in the molten globule-like state. The binding of metal ions, and especially of Ca2+, increases stability of α-LA against the action of heat, various denaturing agents and proteases, while the binding of Zn2+ to the Ca2+-loaded protein decreases its stability and causes its aggregation. At pH 2, the protein is in the classical molten globule state. α-LA can associate with membranes at neutral or slightly acidic pH at physiological temperatures. Depending on external conditions, α-LA can form amyloid fibrils, amorphous aggregates, nanoparticles, and nanotubes. Some of these aggregated states of α-LA can be used in practical applications such as drug delivery to tissues and organs. α-LA and some of its fragments possess bactericidal and antiviral activities. Complexes of partially unfolded α-LA with oleic acid are cytotoxic to various tumor and bacterial cells. α-LA in the cytotoxic complexes plays a role of a delivery carrier of cytotoxic fatty acid molecules into tumor and bacterial cells across the cell membrane. Perhaps in the future the complexes of α-LA with oleic acid will be used for development of new anti-cancer drugs.

Molecular Analytical Assessment of Thermally Precipitated α-Lactalbumin after Resolubilization

“…already at temperatures as low as low as 10 to 30 °C [4]. Though, the results fai indicate whether the native calcium-binding site is restored or not.…”

“…With the peak denaturation temperature all being higher than 62 °C, it is expected that the refolded α-la samples have a Ca + bound. The apo-form is described to unfold already at temperatures as low as low as 10 to 30 °C [ 4 ]. Though, the results fail to clearly indicate whether the native calcium-binding site is restored or not.…”

“…Bovine α-la’s richness in tryptophan and its similarity to the human α-la makes it an ideal source for infant food formulations, allowing further adaption to the golden standard, mother’s milk [ 2 , 3 ]. Of special interest is its property to carry different minerals, such as Mg 2+ , Mn 2+ , Na + , K + , and of course Ca 2+ , which is natively bound in the holo-state [ 4 ]. α-la is reported to enhance the regeneration of cells [ 5 ], to possess antimicrobial properties against a broad spectrum of bacteria, including antibiotic resistant strains [ 6 , 7 ], and to have antitumor properties in a complex with oleic acid [ 8 , 9 ].…”

“…Already in the early stages of research in this field, the possibility to resolubilize the α-la precipitate by increasing the pH plus the addition of calcium was reported [ 30 , 31 ]. In fact, several studies proved that α-la is able to refold from a thermally unfolded apo-state to a native holo-conformation [ 4 , 6 , 21 ]. However, all of these studies have been performed under ideal and protected laboratory conditions, excluding any additional mechanical stress.…”

Characterization of Conjugates between α-Lactalbumin and Benzyl Isothiocyanate—Effects on Molecular Structure and Proteolytic Stability

“…Next to β-lactoglobulin (β-LG), α-lactalbumin (α-LA) is the second most abundant whey protein in cow’s milk [ 24 ]. α-LA is an acidic, monomeric protein with a molecular weight of 14.2 kDa, whose amino acids are well characterized, and its sequence is composed of 123 amino acids [ 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. The structure of α-LA is further characterized by four disulfide bonds involving all eight cysteines and a calcium-binding site, which ensures the correct folding and high molecular stability of α-LA [ 24 , 36 , 37 , 38 , 39 ].…”