Albumin Binding to FcRn:  Distinct from the FcRn−IgG Interaction

Abstract

The MHC-related Fc receptor for IgG (FcRn) protects albumin and IgG from degradation by binding both proteins with high affinity at low pH in the acid endosome and diverting both from a lysosomal pathway, returning them to the extracellular compartment. Immunoblotting and surface plasmon resonance studies show that both IgG and albumin bind noncooperatively to distinct sites on FcRn, that the affinity of FcRn for albumin decreases approximately 200-fold from acidic to neutral pH, and that the FcRn-albumin interaction shows rapid association and dissociation kinetics. Isothermal titration calorimetry shows that albumin binds FcRn with a 1:1 stoichiometry and the interaction has hydrophobic features as evidenced by a large positive change in entropy upon binding. Our results suggest that the FcRn-albumin interaction has unique features distinct from FcRn-IgG binding despite the overall similarity in the pH-dependent binding mechanism by which both ligands are protected from degradation.

Kinetics of FcRn-mediated recycling of IgG and albumin in human: Pathophysiology and therapeutic implications using a simplified mechanism-based model

“…Our calculated in vivo K m values for both ligands are much higher (> 50-fold) than in vitro K D , < 1 μM for human IgG [41] and 5 μM for human albumin [30]. The values differ because they are referenced to ligand concentrations in different places.…”

“…Using Equation 3, three pieces of information for the albumin interaction with FcRn can be determined, i.e., J max , K m , and k int . First, the binding molar stoichiometric ratio between albumin and FcRn was unity [30], indicating that J max for albumin would be twice that for IgG; viz., 1.96 μmol/d/kg. We assume that FcRn recycles albumin and IgG simultaneously, as all evidence indicates that FcRn is bound to each ligand independently of the other [8].…”

“…However, because human studies are characterized by large variation and because the albumin parameters are derived from the IgG data, the k int values of both proteins may actually be quite close. The albumin J max was taken to be twice that for IgG based on the binding stoichiometries of FcRn for the two substrates [30]. The albumin K m was calculated to be 319 μM (21.4 mg/mL), more than twice that for IgG (140 μM), suggesting a reduction in the recycling efficiency of FcRn for albumin as compared with that for IgG.…”

The conserved histidine 166 residue of the human neonatal Fc receptor heavy chain is critical for the pH‐dependent binding to albumin

“…Bound IgG and albumin are recycled back to the surface and released from the cell, while unbound ligands are shuttled downstream to lysosomal degradation [4,5]. While the two ligands bind independently to FcRn, they both do so in a strictly pH-dependent manner, with binding at pH 6.0, but not at pH 7.4 [6][7][8]. The IgG-FcRn interaction is attributed to conserved amino acid residues located at the CH2-CH3 domain interface of IgG Fc with the residues I253, H310 and H435 as key players [9].…”

“…In addition, as albumin has an unusually long half-life, it is an attractive carrier of therapeutic drugs [11][12][13]. The mechanism responsible for the long half life has been given little attention, but recent evidence strongly suggests that FcRn is involved [6,7,14].…”