Protein crystallization in stirred systems--scale-up via the maximum local energy dissipation

Protein Nucleation and Crystallization Process with Process Analytical Technologies in a Batch Crystallizer

Protein crystallization has drawn great attention to replacing the traditional downstream processing for protein-based pharmaceuticals due to its advantages in stability, storage, and delivery. Limited understanding of the protein crystallization processes requires essential information based on real-time tracking during the crystallization process. A batch crystallizer of 100 mL fitted with a focused beam reflectance measurement (FBRM) probe and a thermocouple was designed for in situ monitoring of the protein crystallization process, with simutaneously record of off-line concentrations and crystal images. Three stages in the protein batch crystallization process were identified: long-period slow nucleation, rapid crystallization, and slow growth and breakage. The induction time was estimated by FBRM, i.e., increasing numbers of particles in the solution, which could be half of the time required for detecting the decrease of the concentration, by offline measurement. The induction time decreased with an increase in supersaturation within the same salt concentration. The interfacial energy for nucleation was analyzed based on each experimental group with equal salt concentration and different concentrations of lysozyme. The interfacial energy reduced with an increase in salt concentration in the solution. The yield of the experiments was significantly affected by the protein and salt concentrations and could achieve up to 99% yield with a 26.5 μm median crystal size upon stabilized concentration readings.

Crystal Contact Engineering Enables Efficient Capture and Purification of an Oxidoreductase by Technical Crystallization

Technical crystallization is an attractive method to purify recombinant proteins. However, it is rarely applied due to the limited crystallizability of many proteins. To overcome this limitation, single amino acid exchanges are rationally introduced to enhance intermolecular interactions at the crystal contacts of the industrially relevant biocatalyst Lactobacillus brevis alcohol dehydrogenase (LbADH). The wildtype (WT) and the best crystallizing and enzymatically active LbADH mutants K32A, D54F, Q126H, and T102E are produced with Escherichia coli and subsequently crystallized from cell lysate in stirred mL-crystallizers. Notwithstanding the high host cell protein (HCP) concentrations in the lysate, all mutants crystallize significantly faster than the WT. Combinations of mutations result in double mutants with faster crystallization kinetics than the respective single mutants, demonstrating a synergetic effect. The almost entire depletion of the soluble LbADH fraction at crystallization equilibrium is observed, proving high yields. The HCP concentration is reduced to below 0.5% after crystal dissolution and recrystallization, and thus a 100-fold HCP reduction is achieved after two successive crystallization steps. The combination of fast kinetics, high yields, and high target protein purity highlights the potential of crystal contact engineering to transform technical crystallization into an efficient protein capture and purification step in biotechnological downstream processes.

Continuous Crystallization of Proteins in a Tubular Plug-Flow Crystallizer

Protein crystals have many important applications in many fields, including pharmaceutics. Being more stable than other formulations, and having a high degree of purity and bioavailability, they are especially promising in the area of drug delivery. In this contribution, the development of a continuously operated tubular crystallizer for the production of protein crystals has been described. Using the model enzyme lysozyme, we successfully generated product particles ranging between 15 and 40 μm in size. At the reactor inlet, a protein solution was mixed with a crystallization agent solution to create high supersaturations required for nucleation. Along the tube, supersaturation was controlled using water baths that divided the crystallizer into a nucleation zone and a growth zone. Low flow rates minimized the effect of shear forces that may impede crystal growth. Simultaneously, a slug flow was implemented to ensure crystal transport through the reactor and to reduce the residence time distribution.

Large-scale crystallization of proteins for purification and formulation

Since about 170 years, salts were used to create supersaturated solutions and crystallize proteins. The dehydrating effect of salts as well as their kosmotropic or chaotropic character was revealed. Even the suitability of organic solvents for crystallization was already recognized. Interestingly, what was performed during the early times is still practiced today. A lot of effort was put into understanding the underlying physico-chemical interaction mechanisms leading to protein crystallization. However, it was understood that already the solvation of proteins is a highly complex process not to mention the intricate interrelation of electrostatic and hydrophobic interactions taking place. Although many basic questions are still unanswered, preparative protein crystallization was attempted as illustrated in the presented case studies. Due to the highly variable nature of crystallization, individual design of the crystallization process is needed in every single case. It was shown that preparative crystallization from impure protein solutions as a capture step is possible after applying adequate pre-treatment procedures like precipitation or extraction. Protein crystallization can replace one or more chromatography steps. It was further shown that crystallization can serve as an attractive alternative means for formulation of therapeutic proteins. Crystalline proteins can offer enhanced purity and enable highly concentrated doses of the active ingredient. Easy scalability of the proposed protein crystallization processes was shown using the maximum local energy dissipation as a suitable scale-up criterion. Molecular modeling and target-oriented protein engineering may allow protein crystallization to become part of a platform purification process in the near future.