Fast antibody fragment motion: flexible linkers act as entropic spring.

Laura R Stingaciu,Michael Ohl,Ralf Biehl,Oxana Ivanova,Dieter Richter

doi:10.1038/srep22148

Laura R Stingaciu, Michael Ohl + Show 3 more

Open Access

https://doi.org/10.1038/srep22148

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Abstract

A flexible linker region between three fragments allows antibodies to adjust their binding sites to an antigen or receptor. Using Neutron Spin Echo Spectroscopy we observed fragment motion on a timescale of 7 ns with motional amplitudes of about 1 nm relative to each other. The mechanistic complexity of the linker region can be described by a spring model with Brownian motion of the fragments in a harmonic potential. Displacements, timescale, friction and force constant of the underlying dynamics are accessed. The force constant exhibits a similar strength to an entropic spring, with friction of the fragment matching the unbound state. The observed fast motions are fluctuations in pre-existing equilibrium configurations. The Brownian motion of domains in a harmonic potential is the appropriate model to examine functional hinge motions dependent on the structural topology and highlights the role of internal forces and friction to function.

Highlights

A flexible linker region between three fragments allows antibodies to adjust their binding sites to an antigen or receptor
We report a detailed study of human IgG domain motions using Neutron Spin Echo (NSE) spectroscopy on a protein solution in order to resolve the motional pattern in space and time
This study focused on the fragment motion of IgG

Summary

Introduction

A flexible linker region between three fragments allows antibodies to adjust their binding sites to an antigen or receptor. Antibodies are large Y-shaped glycoproteins produced by the humoral immune system and are generically called immunoglobulins (Ig). They consist of three equal-sized fragments (see IgG1 in Fig. 1) connected by a flexible linker region: two antigen-binding fragments (Fab) and one constant fragment (Fc). The linker region is responsible for the high flexibility between the 3 fragments and allows Fab to bind to antigens of various shapes while the Fc fragment simultaneously can bind to a receptor or complement. The flexible upper and lower linker regions connect the Fab and the Fc fragments to the core, respectively. IgG may be considered as a good general model for studying immunoglobulins, as it is a model for the majority of immune drug developments

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