Developing a strategy to enhance protein crystallization using metal chelate complexes

The ability to solve the structures of important proteins can be enhanced by increasing the probability of obtaining suitable crystals. Co-crystallization of the proteins with metal-chelate complexes (MCCs) show promise in achieving this goal. In this study, six lanthanide ligand complexes, [Tb(HDPA)3]3- , [Tb(DPA)3]3-, [Tb(HDPA)3-n(DPA)n]n=0-3, [Gd(DTPA)]2-, [Eu(DOTA-4Amc)]- and [Tm(DOTP)]5- were used in crystallization trials with eight test proteins, concanavalin A, glucose isomerase, beta amylase, alpha lactalbumin, thermolysin, lipase B, lysozyme and streptavidin under customized grid screens to observe their ability to enhance crystallization of the proteins. A new crystal form of the streptavidin-biotin complex (C222) was facilitated by co-crystallization with [Tb(DPA)3]3- under conditions that were not previously reported for the protein. The crystal composed of sheets of streptavidin molecules held together by [Tb(DPA)3]3- complexes along the vertical 2-fold axes. Likewise this type of symmetric, rigid complexes can favor the bridging of symmetry related protein molecules and provide a symmetry point for the crystal on which to build. Also, it is evident that the [Tb(DPA)3]3- complexes are involved in interactions important to expand the lattice in the third dimension. Similarly a novel crystal form for concanavalin A was obtained in the presence of [Tb(HDPA)3]3- in the tetragonal space group P43212. The 4-hydroxyl groups of the ligands in [Tb(HDPA)3]3- could allow these complexes to interact with each other in addition to protein molecules. These inter-complex Interactions formed a network of [Tb(HDPA)3]3- complexes in the concanavalin A crystal, providing a stronger surface for the protein molecules to interact with and to build into a lattice. In addition, [Tb(HDPA)3-n(DPA)n]n=0-3, [Gd(DTPA)]2-, [Eu(DOTA-4Amc)]- and [Tm(DOTP)]5- promoted crystallization of the proteins under conditions, different from that of previously reported crystal structures. Co-crystallization with the metal-chelate complexes has the added advantage of incorporating lanthanides, which are strong anomalous scatterers that can be used to solve the structure by anomalous dispersion. Additionally, fluorescence microscopy could be used to separate protein crystals from small molecule crystals of certain MCCs. In conclusion, the lanthanide chelate complexes can enhance protein crystallization via introducing crystal contacts and/or by promoting nucleation.