Action of protein-glutaminase on α-lactalbumin in the native and molten globule states

The action of a novel protein-glutaminase from microorganisms on alpha -lactalbumin was investigated. When alpha -lactalbumin in the native state was incubated with protein-glutaminase, the deamidation proceeded gradually, i.e., the deamidation degree increased to 20% and 55% after 4 and 24 h, respectively. The transformation of alpha -lactalbumin from the native state to the molten globule state caused an increase in the rate of the enzyme-catalyzed deamidation, particularly in the early stage. The deamidation degree for the molten globule state reached 61% after 4 h, followed by a gradual increase to 66% after 24 h. CD spectral analyses of deamidated alpha -lactablumin revealed that the stability of the tertiary structure of alpha -lactablumin was closely related to the degree of deamidation, whereas the secondary structure was not affected by deamidation. Glutamine residues in alpha -lactalbumin to be modified by protein-glutaminase were identified as Gln[39], [43], [54], and [65]. Conformational characteristics of the amino acid sequence around these glutamine residues are discussed.