Ultrafiltration membrane cut-off impacts structure and functional properties of transmitted proteins: Case study of the metalloprotein α-lactalbumin

Abstract

Globular milk proteins have lots of potential applications in the form of highly purified fractions due to their numerous biological, nutritional and functional properties. Membrane filtration is now envisaged as a promising purification technology as it can be more environmental-friendly than chromatography and quite as efficient as this current technique. However, the impact of membrane filtration on the final properties of proteins still needs to be clarified and especially in case of metalloproteins as they have the specificity to bind one cofactor entity which is highly decisive for the preservation of their structure and functionality. In this study, the impact of dead-end ultrafiltration on the properties of the metalloprotein α-lactalbumin (α-LA) was described depending on the membrane cut-off. For membranes with a cut-off larger than α-LA molecular weight (30 or 100kgmol−1), structural and functional modifications evidenced after permeation of a mixture between the apo- (without cofactor) and holo-form (native form) were consistent with an increase in the holo-α-LA content but did not point out any physical impact on α-LA structure. The modification of α-LA properties was rather attributed to the displacement of the equilibrium between the apo- and holo-forms during filtration. With a membrane cut-off close to α-LA molecular weight (10kgmol−1), structural and functional properties of the protein observed after permeation could not be reasonably associated with the increase in the holo-α-LA content. Results suggested that shear stress inside membrane pores slightly impacted α-LA structure and thus dictated final properties of the permeated fraction especially in terms of solubility and thermal stability. The impact of dead-end UF through the 10kgmol−1 membrane was supposed to be moderate and to affect less than 30–40% of permeated proteins.