Contribution of proton linkage to the thermodynamic stability of the major cold-shock protein of Escherichia coli CspA

Abstract

The stability of protein is defined not only by the hydrogen bonding, hydrophobic effect, van der Waals interactions, and salt bridges. Additional, much more subtle contributions to protein stability can arise from surface residues that change their properties upon unfolding. The recombinant major cold shock protein of Escherichia coli CspA an all-β protein unfolds reversible in a two-state manner, and behaves in all other respects as typical globular protein. However, the enthalpy of CspA unfolding strongly depends on the pH and buffer composition. Detailed analysis of the unfolding enthalpies as a function of pH and buffers with different heats of ionization shows that CspA unfolding in the pH range 5.5–9.0 is linked to protonation of an amino group. This amino group appears to be the N-terminal α-amino group of the CspA molecule. It undergoes a 1.6 U shift in pK_a values between native and unfolded states. Although this shift in pK_a is expected to contribute ∼5 kJ/mol to CspA stabilization energy the experimentally observed stabilization is only ∼1 kJ/mol. This discrepancy is related to a strong enthalpy–entropy compensation due, most likely, to the differences in hydration of the protonated and deprotonated forms of the α-amino group.