The allosteric modulation of thyroxine-binding globulin affinity is entropy driven.

ARIEL A PETRUK; MARIA SOLEDAD LAVANDA; ROSA MARIA ALVAREZ; MARCELO A MARTI

BIOCHIMICA ET BIOPHYSICA ACTA-GENERAL SUBJECTS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2013

0304-4165

Background: Thyroxine-binding globulin (TBG) is a non-inhibitory member of the serpin family of proteinswhose main structural element is the reactive center loop (RCL), that, upon cleavage by proteases, is insertedinto the protein core adopting a β-strand conformation (stressed to relaxed transition, S-to-R). After S-to-Rtransition thyroxine (T4) affinity decreases. However, crystallographic studies in the presence or absenceof the hormone in different states are unable to show significant differences in the structure and interactionsof the binding site. Experimental results also suggest the existence of several S states (differing in the numberof 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 insertionin TBG,weperformed extendedmolecular dynamics simulations combinedwith several thermodynamic analysisof 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 correlationbetween the degree of RCL insertion and the binding affinity, driven by a change in TBG conformationalentropy, was observed.Conclusion: TBG allosteric regulation is entropy driven. The presence of multiple S states may allow moreefficient T4 release due to protease activity.General significance: The presented results are clear examples of howcomputer simulationmethods can reveal thethermodynamic basis of allosteric effects, and provide a general framework for understanding serpin allostericaffinity regulation.