Читать книгу Drug Transporters - Группа авторов - Страница 16

Because of the involvement of transporters in all facets of drug absorption, tissue distribution, excretion, and efficacy/toxicity, characterization of transporter structure can provide a scientific basis for understanding drug delivery and disposition, as well as the molecular mechanisms of drug interaction and inter‐individual/inter‐species differences. However, compared to soluble proteins, the atomic resolution crystal structures of membrane transporters have been extremely difficult to obtain for several reasons: first is the amphipathic nature of the surface of the transporters, with a hydrophobic area in contact with membrane phospholipids and polar surface areas in contact with the aqueous phases on both sides of the membrane; second is the low abundance of many transporters in the membrane, making it impossible to overexpress them, a prerequisite for structural studies; and third is the inherent conformational flexibility of the transporters, making it difficult to obtain stable crystals.

Due to these difficulties, high‐resolution three‐dimensional structures have been obtained for only limited number of transporters. For other transporters, the three‐dimensional structures have been achieved through homology modeling. In this approach, similar folding patterns between any protein and one for which the crystal structure is known enable the construction of a fairly accurate three‐dimensional protein model of the unknown structure using the related crystal structure as a template and modern computational techniques. Three‐dimensional structures have revealed that transporters have alpha‐helical structures of the membrane‐spanning domains, and some of the helices have irregular shapes with kinks and bends. Certain transporters undergo substantial movements during the substrate translocation process. Recent advances in cryo‐electron microscopy (cryo‐EM) have allowed high‐resolution determination of the structure of macromolecules without the need for crystallization and therefore viewed as an alternative to X‐ray crystallography. Construction of three‐dimensional transporter models have provided insight into functional mechanisms and molecular structures and enabled formulation of new hypotheses regarding transporter structure and function, which may be experimentally validated.