The flux of a substrate (rate of transport) across the plasma membrane via transporter-mediated processes is characterized by saturability. The relationship between the flux v and substrate concentration C in a transporter-mediated process is analogous to the rate of product formed by an enzyme and the concentration of substrate. The maximum transport rate (Vmax) is proportional to the density of transporters on the plasma membrane, and the Km represents the substrate concentration at which the flux is half maximal. When C is small compared with the Km, the flux is increased in proportion to the substrate concentration (roughly linearly). If C is large compared with the Km value, the flux approaches the maximal value (Vmax). The Km and Vmax values can be determined by examining the flux at different substrate concentrations.
Transporter-mediated membrane transport of a substrate is also characterized by inhibition by other compounds. As with enzyme or receptor inhibition, this inhibition can be categorized as one of three types: competitive, noncompetitive, and uncompetitive.
Competitive inhibition occurs when substrates and inhibitors share a common binding site on the transporter, resulting in an increase in the apparent Km value. Noncompetitive inhibition occurs when the inhibitor allosterically affects the transporter in a manner that does not inhibit the formation of an intermediate complex of substrate and transporter but does inhibit the subsequent translocation process. Uncompetitive inhibition assumes that inhibitors form a complex only with an intermediate substrate-transporter complex and inhibit subsequent translocation.
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