Enzymatic deglycosylation as a tool for crystallization of mamalian binding proteins

Enzymatic deglycosylation has been used in attempts to crystallize several glycoproteins with the aim of overcoming the problems resulting from heterogeneity and flexibility of the attached glycan chains. An endoglycosidase preparation from Flavobacterium meningosepticum, comprising the enzymes endo F and PNGase-F, was used in experiments on the mammalian binding proteins lactoferrin and haemopexin. Significant differences were found in the susceptibility of different proteins to deglycosylation. For human lactoferrin (Lf) and its recombinant N-terminal half-molecule (Lf_N), deglycosylation was rapid and complete, and was essential for obtaining high-quality crystals of both apo-Lf and Lf_N; for bovine Lf, however, complete deglycosylation did not occur. Similarly, for rabbit haemopexin the carbohydrate chain on the C-terminal domain was easily removed, but the three chains on the N-terminal domain proved more resistant and their removal led to some fragmentation of the protein. Nevertheless, this approach provided the only means of crystallizing the C-terminal domain and is likely to be useful for other glycoproteins.