8.12 Membrane Proteins for Secondary Active Transport and their Molecular Mechanisms

Membrane proteins responsible for transport of substrates by facilitated diffusion and secondary active transport are found in all organisms from microbes to man. Their bioenergetics and general properties are compared in this chapter. Very many are related by similarities in amino acid sequence that trace the evolution of sub-families in a widespread group called the major facilitator superfamily. Many others, previously thought to be unrelated from amino acid sequence comparisons, are in fact similar in their three-dimensional structures, forming a newly recognized superfamily. The distinguishing features of these two types of transport proteins are discussed. Recent successes in determining crystal structures of different conformations of some examples of each type have yielded insights into their molecular mechanism and the structural basis of the alternating access mechanism of membrane transport.