1 - Function and Structure of Membrane Transport Proteins

Abstract

This chapter presents a general introduction to functions and structures of membrane transport proteins, which catalyze the translocation of substrates through the hydrophobic bilayer membrane of cells. It defines some terms used in the characterization of transport processes, such as passive diffusion, facilitated diffusion, active transport, and group translocation. Thousands of transport processes, each catalyzed by its own protein, have been identified; however, there are relatively few strategies found in coupling metabolic energy to the translocation process. The chapter describes these strategies to provide a formal basis for a preliminary classification of all the processes. It describes the chemiosmotic theory of oxidative and photosynthetic phosphorylation and applies it to explain substrate transport mechanism. It tabulates the names and examples of several families of transport proteins. Facilitated diffusion transport systems usually contain a single protein. Secondary active transport systems usually contain one protein, if those that generate the driving ion gradient are not counted. Primary active transport systems may occasionally contain one protein, for example, bacteriorhodopsin; however, most appear to comprise a protein complex, involving from as few as 2–20 or more domains.