title: The allosteric modulation of thyroxine-binding globulin affinity is entropy driven.
abstract: BACKGROUND
  Thyroxine-binding globulin (TBG) is a non-inhibitory member of the serpin family of proteins whose main structural element is the reactive center loop (RCL), that, upon cleavage by proteases, is inserted into the protein core adopting a β-strand conformation (stressed to relaxed transition, S-to-R). After S-to-R transition thyroxine (T4) affinity decreases. However, crystallographic studies in the presence or absence of the hormone in different states are unable to show significant differences in the structure and interactions of the binding site. Experimental results also suggest the existence of several S states (differing in the number of inserted RCL residues), associated with a differential affinity.
  
  
  METHODS
  To shed light into the molecular basis that regulates T4 affinity according to the degree of RCL insertion in TBG, we performed extended molecular dynamics simulations combined with several thermodynamic analysis of the T4 binding to TBG in three different S states, and in the R state.
  
  
  RESULTS
  Our results show that, despite T4 binding in the protein by similar interactions in all states, a good correlation between the degree of RCL insertion and the binding affinity, driven by a change in TBG conformational entropy, was observed.
  
  
  CONCLUSIONS
  TBG allosteric regulation is entropy driven. The presence of multiple S states may allow more efficient T4 release due to protease activity.
  
  
  CONCLUSIONS
  The presented results are clear examples of how computer simulation methods can reveal the thermodynamic basis of allosteric effects, and provide a general framework for understanding serpin allosteric affinity regulation.
authors: Instituto Superior de Investigaciones Biológicas, Tucumán, Argentina. apetruk@fbqf.unt.edu.ar
authors: Ariel A
authors: AA
authors: Petruk
authors: María S
authors: MS
authors: Labanda
authors: Rosa M S
authors: RM
authors: Alvarez
authors: Marcelo A
authors: MA
authors: Marti
mesh_terms: D000494:Allosteric Regulation / Q000502:physiology; D001665:Binding Sites; D018360:Crystallography, X-Ray; D019277:Entropy; D006801:Humans; D017433:Protein Structure, Secondary; D013974:Thyroxine / Q000737:chemistry* / Q000378:metabolism; D056825:Thyroxine-Binding Globulin / Q000737:chemistry* / Q000378:metabolism
chemical_list: D056825:Thyroxine-Binding Globulin; D013974:Thyroxine