Enhanced susceptibility to transglutaminase reaction of alpha-lactalbumin in the molten globule state.

The susceptibility of alpha-lactalbumin to transglutaminase reactions was studied using an enzyme from Streptoverticillium which can catalyze the reactions irrespective of the presence or absence of Ca2+. Transglutaminase-catalyzed polymerization of alpha-lactalbumin in the native state occurred to a very limited extent. Transformation from the native state to the molten globule state brought about by Ca(2+)-removal from holo-alpha-lactalbumin enhanced the polymerization of the protein catalyzed by transglutaminase. The incorporation of Carbobenzoxy-Gln-Gly into alpha-lactalbumin through the enzyme reaction was investigated to determine the amounts of lysine residues which are present at molecular surface and available to the enzyme. There was no significant difference in the amount of available lysine residues between the native and the molten globule molecule. However, the amount of surface glutamine residues incorporated with monodansylcadaverine by transglutaminase was remarkably higher in the molten globule state than that in the native state. The monodansylcadaverine-incorporated site of alpha-lactalbumin in the molten globule state was identified as Gln-54 by amino-acid sequence analysis of fluorescence-labeled peptides separated from chymotryptic digests of the protein. Possible reason for selective labeling of Gln-54 in molten globule alpha-lactalbumin was proposed.