Expression of substrate specificity in facilitated transport systems

Abstract

In facilitated transport systems the carrier reorientation step is shown to be largely independent of the forces of interaction between the substrate and the carrier site, whereas in coupled systems (obligatory exchange or cotransport) reorientation proceeds at the expense of the binding force developed in the transition state. In consequence, the expression of substrate specificity is expected to differ in the two systems. In the facilitated transport of analogs no larger than the normal substrate, the affinity but not the maximum rate of transport can vary widely; with larger analogs, both the affinity and rate can vary if steric constraints are more severe in the translocation step than in binding. In coupled transport, by contrast, the translocation step can be highly sensitive to the structure of the substrate, and binding much less sensitive. The theory agrees with published observations on facilitated systems for choline and glucose in erythrocytes, as well as on Na(+)-coupled systems for the same substrates in other cells. The following mechanism, which could account for the behavior, is proposed. In facilitated systems, the transport site fits the substrate closely and retains its shape as the carrier undergoes reorientation. In coupled systems, the site is initially looser, but during carrier reorientation it contracts around the substrate. In both systems, the carrier encloses the substrate during the translocation step, though for a different reason: in coupled but not in facilitated systems the binding force enormously increases in the enclosed state, through a chelation effect. In both systems, steric interference with enclosure retards the translocation of bulky substrate analogs.

Anomeric preference of fluoroglucose exchange across human red-cell membranes. 19F-n.m.r. studies

“…experiment that was described previously [15], but with the following n.m.r. parameters: for each spectrum 4-16 transients (depending on the concentration of 3FG used) were averaged and 12 s (> 5 x T, where T7 is the longitudinal relaxation time) were allowed between the acquisition of each transient; the 7r/2 acquisition pulse was 22 Its; the spectral width was 5000 Hz, with the free-induction decay digitized into 12 k ('zero-filled' to 16 k) or 16 k data points and processed using I Hz of line-broadening; tm (mixing time) was 0.5 s, four evolution times were used and the times were chosen (a) so as to provide optimal signal-to-noise ratios for both the intraand extra-cellular resonances and (b) to minimize relaxation during the pulse sequence. The equilibrium intensities of the intra-and extra-cellular resonances in each sample (and the anomeric composition of 3FG) were determined from a 'fully-relaxed' spectrum acquired with the same n.m.r.…”

Estimation of Glucose Uptake by Ovarian Follicular Cells

“…The Michaelis-Menten function has proved to be an adequate description of carrier facilitated transport uptake in several cell types, 20 and glucose carrier proteins are often characterized by a Michaelis-Menten parameter (their K M value). 17 Therefore, a Michaelis-Menten form for glucose uptake in COCs or granulosa cells is assumed appropriate.…”

Channelling free energy into work in biological processes

“…observations on a variety of systems (Krupka, 1989b(Krupka, , 1990(Krupka, , 1994a Honig & Stein (1978) and by Jencks (1980Jencks ( , 1989. Very Cs C's Fig.…”

“…The initial state is in general an intermediate in the uncoupled pathway, the altered state an intermediate in the coupled pathway; the abruptness of the change in state, and therefore the importance of uncoupled reactions, depends on the increment in binding energy. On this basis the following expression for the tightness of coupling is found (Krupka, 1989a(Krupka, , 1990(Krupka, , 1993a Fig. 2), Kfinal state is the virtual dissociation constant in the transition state; here, the substrate catalyses the transformation by becoming strongly bound in, and so stabilizing, the transition state.…”