Question

Discuss the biological efficiencies gained by organizing protein function on a membrane (hint: one critical point is the membrane serving as an interface between environments). Provide an example for each.

Answer 1

Eukaryotic cells have groups  of proteins that carry out their function as a unit called organelles. Some of these organelles are embedded by a layer or a membrane similar in structure to the cell membrane. The membrane around the cell is different in composition of proteins and lipids to the membrane surrounding the organelle.

Membranes serve as an interface between the cell and its environment. One class of membrane proteins known as cell adhesion molecules play an essential role in coupling the cell to its extracellular environment, which includes neighbouring cells as well as the extracellular matrix.

Membrane-bound organelles provide  several merits  to eukaryotic cells. First, cells can concentrate and isolate enzymes and reactants in a smaller volume, thereby increasing the rate and efficiency of chemical reactions.

Second, cells can confine potentially harmful proteins and molecules in membrane-bound organelles, protecting the rest of the cells from their harmful effects.

Lysosome is one of the organelle with a membrane bound, containing many enzymes that help in the digestion of proteins,fats and nucleic acids. If these enzymes were released in the cytosol, they could chew up the cell's proteins, nucleic acids and lipids, leading to cell death. The membrane surrounding the lysosome keeps those digestive enzymes away from the rest of the cell.

Organelles and proteins are organised in the areas where they are necesssary but usually not randomly distributed throughout the cell. Microtubules are long filaments that extend throughout the cytosol.These microtubules help the cell in generating force to pull organelles through the cytoplasm.Two types of motor proteins, kinesins and dyneins, walk along microtubules and generate force to pull organelles through the cytoplasm.

To transport and position organelles, cells often use both microtubules and actin filaments. Microtubules, kinesins and dyneins are used to move organelles over long distances whereas actin filaments transport organelles over short distances. Often an organelle will contain more than one type of motor protein to allow cells to utilize both sets of filaments to position the organelle.

Lipid molecules bound to membrane proteins are resolved in some high-resolution structures of membrane proteins. An analysis of these structures provides a framework within which to analyse the nature of lipid–protein interactions within membranes.

Membrane proteins are surrounded by a shell or annulus of lipid molecules, equivalent to the solvent layer surrounding a water-soluble protein. The lipid bilayer extends right up to the membrane protein, with a uniform thickness around the protein. The surface of a membrane protein contains many shallow grooves and protrusions to which the fatty acyl chains of the surrounding lipids conform to provide tight packing into the membrane.

An individual lipid molecule will remain in the annular shell around a protein for only a short period of time. Binding to the annular shell shows relatively little structural specificity.

As well as the annular lipid, there is evidence for other lipid molecules bound between the transmembrane α-helices of the protein; these lipids are referred to as non-annular lipids. The average thickness of the hydrophobic domain of a membrane protein is about 29 Å, with a few proteins having significantly smaller or greater thicknesses than the average

Hydrophobic mismatch between a membrane protein and the surrounding lipid bilayer generally leads to only small changes in membrane thickness. Possible adaptations in the protein to minimise mismatch include tilting of the helices and rotation of side chains at the ends of the helices. Packing of transmembrane α-helices is dependent on the chain length of the surrounding phospholipids.

The function of membrane proteins is dependent on the thickness of the surrounding lipid bilayer, sometimes on the presence of specific, usually anionic, phospholipids, and sometimes on the phase of the phospholipid.