11.2 Membrane Dynamics

The structure and the flexibility of the lipid bilayer depends on three factors - the:

1. temperature
2. types of lipids
3. presence of sterols

11.2.1 Flippases and floppases

Figure 11.8: Flippases and Floppases in Action

Flippases are proteins that move phospholipids from the outer leaflet to the inner leaflet.

Floppases are proteins that move phospholipids (in particular, phospatidylcholine and sphingomyelin) in the opposite direction.

Scramblases are another class of proteins that exchange phospholipids between two leaflets in a Ca²⁺ activated, ATP-independent process.

11.2.2 Functions of membrane proteins

Figure 11.9: Some Membrane Proteins

All biological membranes contain integral and peripheral membrane proteins.

11.2.2.1 Integral membrane proteins

Figure 11.10: Labelled Transmembrane Domain

Integral membrane proteins include transmembrane proteins and lipid-anchored proteins. Integral membrane proteins that span the entire membrane’s hydrophobic region have one or more stretches of nonpolar amino acids that are coiled into \(\alpha\)-helices. Note that the spanning region of an integral membrane protein is called the transmembrane domain (see figure 11.10).

Furthermore, some integral membrane proteins have extensive nonpolar regions that form \(\beta\)-barrels instead of \(\alpha\)-helices.

Figure 11.11: Beta Sheet in a Membrane Protein

The \(\beta\)-sheets form a characteristic motif: one that is arranged like a pipe through the membrane. This is also called a \(\beta\)-barrel and is a common feature of porin proteins.

11.2.2.2 Peripheral membrane proteins

Peripheral membrane proteins interact with integral membrane proteins or with the polar head groups of membrane phospholipids.

Membrane proteins play roles in…

1. Cell-to-cell signalling and interactions
2. Movement of molecules across the membrane
3. Triggering intracellular pathways