Many of its properties and mechanisms are thought to be responsible for arginine’s action on protein stability and solubility (see section 1.7.1). Lysine Lysine is structurally similar to arginine in that it is a positively charged amino acid with an aliphatic chain in its side chain (Figure 113). It differs to arginine in that it has a longer aliphatic chain capped with an ammonium group rather than a guanidinium group. As explained earlier in the literature review it is commonly used in therapeutic protein formulations as a stabilising excipient, but is not naturally present as an osmolyte in cells (see section 1.3.3.1). This is similar to the situation for arginine, and its lack of use in cells brings to question whether it stabilises proteins. In the literature there are data to suggest it stabilises and data to suggest it destabilises (see Table 14). Figure . Structure of lysine The protein-specific effects of lysine are supported by the fact that it is preferentially excluded from certain proteins, but demonstrates neutral preferential interaction coefficients for others (Arakawa & Timasheff 1984b). Due to the lack of the guanidinium group, lysine forms less cation-π interactions with aromatic residues on proteins than arginine despite having a positively charged side chain at pH 7 (Shah & Shaikh 2015; Crowley & Golovin 2005). Lysine has also shown a lack of self-association in solution, giving support to the hypothesis that arginine stacks in solution via guanidinium-guanidinium interactions (Vondrásek et al. 2009). Histidine & Imidazole Histidine is the third positively charged amino acid, with an imidazole-based side chain (Figure 114). It has three ionisation sites and as such is commonly used as a buffer at pH 5.5-6.5, as well as occasionally being used as a stabilising excipient (Jorgensen et al. 2009). Most experimental data suggests that histidine destabilises proteins (Table 14),
Many of its properties and mechanisms are thought to be responsible for arginine’s action on protein stability and solubility (see section 1.7.1). Lysine Lysine is structurally similar to arginine in that it is a positively charged amino acid with an aliphatic chain in its side chain (Figure 113). It differs to arginine in that it has a longer aliphatic chain capped with an ammonium group rather than a guanidinium group. As explained earlier in the literature review it is commonly used in therapeutic protein formulations as a stabilising excipient, but is not naturally present as an osmolyte in cells (see section 1.3.3.1). This is similar to the situation for arginine, and its lack of use in cells brings to question whether it stabilises proteins. In the literature there are data to suggest it stabilises and data to suggest it destabilises (see Table 14). Figure . Structure of lysine The protein-specific effects of lysine are supported by the fact that it is preferentially excluded from certain proteins, but demonstrates neutral preferential interaction coefficients for others (Arakawa & Timasheff 1984b). Due to the lack of the guanidinium group, lysine forms less cation-π interactions with aromatic residues on proteins than arginine despite having a positively charged side chain at pH 7 (Shah & Shaikh 2015; Crowley & Golovin 2005). Lysine has also shown a lack of self-association in solution, giving support to the hypothesis that arginine stacks in solution via guanidinium-guanidinium interactions (Vondrásek et al. 2009). Histidine & Imidazole Histidine is the third positively charged amino acid, with an imidazole-based side chain (Figure 114). It has three ionisation sites and as such is commonly used as a buffer at pH 5.5-6.5, as well as occasionally being used as a stabilising excipient (Jorgensen et al. 2009). Most experimental data suggests that histidine destabilises proteins (Table 14),