Question

Discuss the role of carbohydrates, lipids and proteins in maintaining the structure of the cell membrane and explain the effect of denaturation agents on the structure of proteins.

Answer 1

Carbohydrates: Carbohydrates are present in plasma membrane as short chains linked to peripheral proteins. Such proteins are called glycoproteins. `They also attach to polar ends of lipids to form glycolipids.

D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (or sialic acid), N-acetyl-D-glucosamine, and N-acetyl-D- galactosamine are the main carbohydrates in cell membrane. These sugars are hydrophillic. Thus, they help to orient the glycoproteins and glycolipids in the membrane. As they are exposed to the external environment, they do not rotate towards the hydrophobic interior of the cell membrane.

The glycoproteins are involved in plasma membrane transport, act as hormones and enzymes. Blood types are due to presence of glycoproteins on surface of cell membrane. Carbohydrates act as antigens on cell membrane. They can trigger immune response if identified as foreign by immune system. Carbohydrates also are involved in cell-cell adhesion. Contact inhibition is a response of cell to cell interaction via carbohydrates on cell membrane. Glycosphingolipids are involved in cell-cell recognition.

Proteins: The proteins in cell membrane are: peripheral proteins and glycoproteins. Peripheral proteins are exterior to cell membrane with one end embedded in cell membrane by interactions with other proteins. The integral proteins are embedded in cell membrane with both sides exposed. Glycoproteins are proteins associated with carbohydrates. They are involved in cell-cell communication and membrane transport. Membrane proteins are involved in maintaining the structure of membrane. They are transport/carrier proteins that allow transport of selective molecules in and out of the cell membrane. Mitochondrial and chloroplast membranes have proteins associated with them that are involved in ATP synthesis and photosynthesis. ER membranes are involved in protein synthesis and vesicle transport. Proteins are involved in attaching the membrane to the extracellular matrix or to cytoskeletal proteins in inside of the cell. Integral proteins may act as receptors to hormones and other ligands. Integral membrane protein have nonpolar amino acids that interact with the hydrophobic lipids in the interior of cell membrane.

Lipids/Fats: The phospholipid bilayer is composed of a hydrophobic core in the center and hydrophillic nonpolar heads exposed to the external environment. The hydrophobic core prevents the movement o hydrophillic molecules across the membrane. They allow the passage of hydrophobic molecules. Hence, the membrane is semi permeable. In addition, cholesterol is present which determines the fluidity of the membrane. It limits lateral movement of the phospholipid bilayer. Cholesterol is only present in animal cells. Cholesterol also helps maintain fluidity at different temperatures. It is also involved in cell signaling.

Denaturing agent: Denaturation alters the shape of the protein leading to loss of solubility or aggregation. Denaturing agents causes loss of secondary, tertiary and quaternary structure of the proteins. Thus, the native configuration of the protein is destroyed. This will lead to loss of solubil­ity, viscosity, optical rotation, sedimentation rate, electrophoretic mobility etc. Such denatured proteins are susceptible to Proteolytic cleavage by enzymes in the cell. Denaturing agents cause precipitation of proteins due to aggregation. Denaturation is a reversible process except when performed for a longer time. However, denaturation does not normally affect primary structure of protein. Urea and guanidine HCl breaks hydrogen bonds between the polypeptides. SDS interferes with hydrophobicity by associating with non polar groups of proteins. Organic solvents disrupt hydrophobic interactions. Example of heat denaturation is frying of eggs in oil. Agitation can also denature proteins.