Transferrin's mechanism of interaction with receptor 1

The kinetics and thermodynamics of the interactions of transferrin receptor 1 with holotransferrin and apotransferrin in neutral and mildly acidic media are investigated at 37 degrees C in the presence of CHAPS micelles. Receptor 1 interacts with CHAPS in a very fast kinetic step (<1 micros). This is followed in neutral media by the interaction with holotransferrin which occurs in two steps after receptor deprotonation, with a proton dissociation constant (K(1a)) of 10.0 +/- 1.5 nM. The first step is detected by the T-jump technique and is associated with a molecular interaction between the receptor and holotransferrin. It occurs with a first-order rate constant (k(-1)) of (1.6 +/- 0.2) x 10(4) s(-1), a second-order rate constant (k(1)) of (3.20 +/- 0.2) x 10(10) M(-1) s(-1), and a dissociation constant (K(1)) of 0.50 +/- 0.07 microM. This step is followed by a slow change in the conformation with a relaxation time (tau(2)) of 3400 +/- 400 s and an equilibrium constant (K(2)) of (4.6 +/- 1.0) x 10(-3) with an overall affinity of the receptor for holotransferrin [(K'1)(-1)] of (4.35 +/- 0.60) x 10(8) M(-1). Apotransferrin does not interact with receptor 1 in neutral media, between pH 4.9 and 6, it interacts with the receptor in two steps after a receptor deprotonation (K(2a) = 2.30 +/- 0.3 microM). The first step occurs in the range of 1000-3000 s. It is ascribed to a slow change in the conformation which rate-controls a fast interaction between apotransferrin and receptor 1 with an overall affinity constant [(K(3))(-1)] of (2.80 +/- 0.30) x 10(7) M(-1). These results imply that receptor 1 probably exists in at least two forms, the neutral species which interacts with holotransferrin and not with apotransferrin and the acidic species which interacts with apotransferrin. At first, the interaction of the neutral receptor with holotransferrin is extremely fast. It is followed by the slow change in conformation, which leads to an important stabilization of the thermodynamic structure. In the acidic media of the endosome, the interaction of apotransferrin with the acidic receptor is sufficiently strong and rate-controlled by a very slow change in conformation which allows recycling back to the plasma membrane.