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Specific and Nonspecific Interactions in Ultraweak Protein–Protein Associations Revealed by Solvent Paramagnetic Relaxation Enhancements
journal contribution
posted on 2015-12-17, 03:25 authored by Helle Johansson, Malene
Ringkjøbing Jensen, Henrik Gesmar, Sebastian Meier, Joachim M. Vinther, Camille Keeler, Michael
E. Hodsdon, Jens J. LedWeak
and transient protein–protein interactions underlie
numerous biological processes. However, the location of the interaction
sites of the specific complexes and the effect of transient, nonspecific
protein–protein interactions often remain elusive. We have
investigated the weak self-association of human growth hormone (hGH, KD = 0.90 ± 0.03 mM) at neutral pH by the
paramagnetic relaxation enhancement (PRE) of the amide protons induced
by the soluble paramagnetic relaxation agent, gadodiamide (Gd(DTPA-BMA)).
Primarily, it was found that the PREs are in agreement with the general
Hwang-Freed model for relaxation by translational diffusion (J. Chem. Phys. 1975, 63, 4017–4025),
only if crowding effects on the diffusion in the protein solution
are taken into account. Second, by measuring the PREs of the amide
protons at increasing hGH concentrations and a constant concentration
of the relaxation agent, it is shown that a distinction can be made
between residues that are affected only by transient, nonspecific
protein–protein interactions and residues that are involved
in specific protein–protein associations. Thus, the PREs of
the former residues increase linearly with the hGH concentration in
the entire concentration range because of a reduction of the diffusion
caused by the transient, nonspecific protein–protein interactions,
while the PREs of the latter residues increase only at the lower hGH
concentrations but decrease at the higher concentrations because of
specific protein–protein associations that impede the access
of gadodiamide to the residues of the interaction surface. Finally,
it is found that the ultraweak aggregation of hGH involves several
interaction sites that are located in patches covering a large part
of the protein surface.