Key Concepts of Section 7.2

• Biological membranes usually contain integral (transmembrane) proteins as well as lipid-anchored proteins and peripheral membrane proteins, which do not enter the hydrophobic core of the bilayer (see Figure 7-1).

• Most integral membrane proteins contain one or more membrane-spanning hydrophobic α helices bracketed by hydrophilic domains that extend into the aqueous environment surrounding the cytosolic and exoplasmic faces of the membrane (see Figures 7-14, 7-15, and 7-17).

• Fatty acyl side chains as well as the polar head groups of membrane lipids pack tightly and irregularly around the hydrophobic segments of integral membrane proteins (see Figure 7-16).

• The porins, unlike other integral membrane proteins, contain membrane-spanning β sheets that form a barrel-like channel through the bilayer (see Figure 7-18).

• Lipids attached to certain amino acids anchor some proteins to one or the other membrane leaflet (see Figure 7-19).

• All transmembrane proteins and glycolipids are asymmetrically oriented in the bilayer. Invariably, carbohydrate chains are present only on the exoplasmic surface of a glycoprotein or glycolipid.

• Many water-soluble enzymes (e.g., phospholipases) whose substrates are phospholipids must bind to the membrane surface to carry out their function. Such binding is often due to the attraction between positive charges on lysine and arginine residues in the protein and negative charges on phospholipid head groups in the bilayer.

• Transmembrane proteins can be selectively extracted from membranes with the use of non-ionic detergents.