Creatinase in its collapsed A state shows properties of a molten globule with dimeric quaternary structure

In the past, the molten globule state at acidic pH (A state) has mainly been observed for small single-domain proteins. For more complex proteins such as immunoglobulin, alternatively folded states, with certain characteristics of the molten globule but different thermodynamic properties, were observed. In the present work, the acid-induced structural characteristics of a homodimer, creatinase from Pseudomonas putida, are described. The 91-kDa protein at pH 2 shows molten globule behavior in that there is (a) a high content of native-like secondary structure (monitored by far-ultraviolet circular dichroism), (b) changes in the solvent accessibility of intrinsic fluorophores (acrylamide quenching of protein fluorescence), (c) increased hydrophobic surface area (indicated by anomalous dye binding) and (d) a slight expansion of the hydrodynamic volume (calculated from S20,solv, obtained from analytical ultracentrifugation). The enzyme at pH 2 shows reversible cooperative transitions in guanidinium chloride or urea (at elevated ionic strength). Its quaternary structure remains unaltered, indicating that native-like subunit interactions are involved in the stabilization of the A state of the enzyme. Anions stabilize the compact conformation due to reduced intramolecular charge repulsion; for the same reason, the enzyme in its A state shows a strong tendency to aggregate at > 0.3 M NaCl.