Abstract
Proteins comprise an extremely heterogeneous class of biological macromolecules. Proteins are multifunctional in the sense that their specific amino acid sequence simultaneously determines folding, function and degradation. Regarding stability, proteins, due to their delicate balance of attractive and repulsive weak interactions, are only marginally stable if physiological conditions are considered as standard state. Therefore, it seems appropriate to briefly summarize some of the relevant methods that are available to study the stability and folding of proteins (Table 1). In the vast majority of examples it is established that proteins have the intrinsic capacity to acquire their unique three-dimensional structure in a spontaneous and autonomous way, depending solely on their amino acid sequence and the (native-like) environment. They are often unstable when not in their native environments, which can vary considerably among cell compartments and extracellular fluids (Damaschun et al., 1999). The spectrum of experimental approaches suitable for the characterization of the structure-function relationship of proteins is described in Table 1. If certain buffer conditions are not maintained, extracted proteins may not function properly or remain soluble. Proteins can lose activity as a result of proteolysis, aggregation and suboptimal buffer conditions. Purified proteins often need to be stored for an extended period of time while retaining their original structural integrity and/or activity. The extent of storage ‘shelf life’ can vary from a few days to more than a year and is dependent on the nature of the protein and the storage conditions used. Optimal conditions for storage are distinctive to each protein; nevertheless, it is possible to suggest some general guidelines for protein storage and stability. Common conditions for protein storage are summarized and compared in Table 2. Generally, there are tradeoffs associated with each method. For example, proteins stored in solution at 4°C can be dispensed conveniently as needed but require more diligence to prevent microbial or proteolytic degradation; such proteins may not be stable for more than a few days or weeks. By contrast, lyophilization allows for long-term storage of protein with very little threat of degradation, but the protein must be reconstituted before use and may be damaged by the lyophilization process (Taschner et al., 2001).
