Mechanisms of protein stabilization at very low pH

摘要

Physicochemical properties of periplasmic loops exposed to low pH were determined for two membrane proteins from the bioleaching-proteobacterium Acidithiobacillus ferrooxidans. The selected proteins were an aquaporin and a potassium ( K+ ) channel and the properties evaluated were surface area, hydrophobicity and amino acid composition. Properties were mapped onto three dimensional protein models and subjected to Accessible Surface Area (ASA) analysis and Molecular Dynamics (MD) simulations. Results were compared to equivalent loops from homologous proteins derived from microorganisms that live in neutral pH environments. It was found that periplasmic loops of both the aquaporin and the K + channel protein of At. ferrooxidans have less surface area and exhibit greater hydrophobicity compared to their neutrophilic homologs. A reduction of the percentage of amino acids that are negatively charged ( aspartic and glutamic acid) and an increase in the percentage of positively charged amino acids ( histidine, arginine and lysine) was also observed in the periplasmic loops of the aquaporin and potassium ( K+ ) channel of At. ferrooxidans. Finally, an increase in the proline content of the loops of At. ferrooxidans was detected. Hypotheses for how these features could help stabilize the structure and function of proteins in extremely acidic condition are presented. It is noted that these proposed mechanisms are similar to those shown to be operational for the stabilization of thermophilic proteins. The data and hypotheses presented could help provide a theoretical foundation for designing proteins with improved molecular functions for bioleaching and acid mine drainage remediation. It is also valuable information for understanding the molecular underpinnings and evolution of protein function at low pH.