Biological Membranes

• Histidyl residues, with pK ~ 7, can be buried with less cost in free energy than other ionic amino acid residue
• If Fluid Mosaic Model is right and proteins freely diffuse - expect homogenous distribution
• The introduction of unsaturated lipids; adds more fluidity the membrane bilayer, reduces the packing of the acyl chains, reduces the phase transition temperature, unsaturated fatty acids re usually attached to C2 of glycerol
• True or False: In all biological membranes lipids are found in a liquid crystalline phase (liquid disordered).

  False
• The organization of lipids within the bilayer can exist in different phases, and two common phases are the liquid crystalline phase (fluid or liquid disordered) and the gel phase (solid or liquid ordered).
• The addition of which lipid usually induces the formation of the liquid ordered phase?

  Cholesterol
• What is a phase diagram for a biological membrane?

  A plot correlating the phase properties of a membrane in relation to the variation of one or more parameters
• Do lipid molecules flip spontaneously from one leaflet to the other, and are there specialised enzymes to prevent this from happening?

  No. Specialised enzymes: Flippases, Floppases, Scramblases
• The plasma membrane contains the highest abundance of lipids, followed by the endoplasmic reticulum, mitochondria, nuclei and microsomes, while the cytoplasm accounts for the lowest levels of lipid molecules
• Unsaturated fatty acids in lipids

  Unsaturated fatty acids are usually attached to the C2 position of glycerol in lipids, meaning that the double bond in the unsaturated fatty acid chain is located near the C2 carbon atom of the glycerol molecule.
• Which of the following statements best describes the role of re-entrant loops in channels and transporters? i. They serve as structural support for the membrane protein. ii. They regulate the flow of ions across the cell membrane. iii. They are involved in the formation of disulfide bonds. iv. They act as gatekeepers for substrate entry and exit.

  iv. They act as gatekeepers for substrate entry and exit.
• Amino acid residues within the re-entrant loops contribute to the overall structural integrity of the channel or transporter protein. These residues may interact with adjacent protein domains or lipid molecules in the membrane, providing stability to the protein structure.
• Ions such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), or chloride (Cl⁻) are typically transported through channels or transporters with re-entrant loops. The amino acid residues within these loops contribute to the selective permeability of the channel or transporter, allowing specific ions to pass while excluding others.
• The alternating access mechanism in transporters involves:

  Conformational changes that allow substrate binding on one side and release on the other.
• Which of the following is an example of a primary transporter? Glucose transporter (GLUT). Aquaporin. Na⁺K⁺-ATPase. Aquaglyceroporin

  Na+K+-ATPase
• GLUT proteins facilitate the facilitated diffusion of glucose across cell membranes, driven by the concentration gradient of glucose. However, some GLUT isoforms may also function as secondary active transporters depending on the cellular context.
• Aquaglyceroporins a channel protein, similar to aquaporin. Aquaglyceroporins facilitate the transport of water, glycerol, and other small solutes across cell membranes.
• Channels: Aquaporins, K+ channels, Gap junctions
• Primary transporters examples: K+/Na+-ATPase, ABC transporters