This Article Full Text Full Text (PDF) All Versions of this Article: biophysj.106.092262v1 92/1/245    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ferreon, A. C. M. Articles by Rösgen, J. Search for Related Content PubMed PubMed Citation Articles by Ferreon, A. C. M. Articles by Rösgen, J.

Protein Phase Diagrams II: Nonideal Behavior of Biochemical Reactions in the Presence of Osmolytes

Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, Texas

Correspondence: Address reprint requests to Jörg Rösgen, Dept. of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, TX 77555-1052. Tel.: 409-772-0968; E-mail: jorosgen{at}utmb.edu.

In the age of biochemical systems biology, proteomics, and high throughput methods, the thermodynamic quantification of cytoplasmatic reaction networks comes into reach of the current generation of scientists. What is needed to efficiently extract the relevant information from the raw data is a robust tool for evaluating the number and stoichiometry of all observed reactions while providing a good estimate of the thermodynamic parameters that determine the molecular behavior. The recently developed phase-diagram method, strictly speaking a graphical representation of linkage or Maxwell Relations, offers such capabilities. Here, we extend the phase diagram method to nonideal conditions. For the sake of simplicity, we choose as an example a reaction system involving the protein RNase A, its inhibitor CMP, the osmolyte urea, and water. We investigate this system as a function of the concentrations of inhibitor and osmolyte at different temperatures ranging from 280 K to 340 K. The most interesting finding is that the protein-inhibitor binding equilibrium depends strongly on the urea concentration—by orders-of-magnitude more than expected from urea-protein interaction alone. Moreover, the m-value of ligand binding is strongly concentration-dependent, which is highly unusual. It is concluded that the interaction between small molecules like urea and CMP can significantly contribute to cytoplasmic nonideality. Such a finding is highly significant because of its impact on renal tissue where high concentrations of cosolutes occur regularly.