Introduction

Membrane proteins play a significant role in biological processes, including biological energy conversion and photosynthesis. They also can act as signal receptors and transmitters of hormones, light, or chemicals. However, there is far less structural information about these proteins than about globular proteins. A major factor is their insolubility in aqueous solutions. As a consequence, far fewer membrane protein structures are known than for globular proteins; the number of successfully crystallized membrane proteins is well below 100.

Membrane proteins are different from globular proteins in one main respect: the former are usually “anchored” to a membrane lipid layer. Two predominant types of interactions are responsible for the overall structure observed in a membrane: hydrophobic and hydrophilic interactions. Other non-covalent interactions are present as well, such as hydrogen bonding and electrostatic interactions. However, hydrophobic and hydrophilic interactions are the main forces responsible for the overall structure of a membrane and its attached proteins [350]. Hydrophobic interactions are responsible for keeping a non-polar object away from polar groups such as water. The well known example of the immiscibility of hydrocarbons (i.e. oil) and water applies. An object is termed hydrophobic if it prefers to be oriented away from water. More specifically, it would require a significant expenditure of free energy to transfer a hydrophobic particle from a non-polar environment to a polar one.