Question

1. What family of proteins facilitates the movement of water across the cell plasma membrane? Describe the structure of these proteins, and relate this structure to their function.

Answer 1

Aquaporins are a family of channel proteins that are embedded in cell lipid membranes and facilitates the movement of water across the cell plasma menbrane.

The structure of aquaporins unique because of  their hourglass pseudosymmetric structure (shown in attached Figure ). All aquaporins are made with six transmembrane α-helices arranged in right-handed bundle and the amino terminal and the carboxyl terminal area situated on the cytoplasmic surface of cell membrane. The amino terminal and carboxyl terminal having sequence possess similarity to each other, which appears like a tandem repeat. There is also five interhelical loop regions A to E which creates the cytoplasmic and extracellular vestibules . The conserved three amino acid motif, Aspargine-Proline-Alanine i.e. NPA is present in the B & E loops of almost all the Aquaporins. The NPA motifs located in the non-membrane spanning helices are inserted into the membrane and the Intracellular loop B and extracellular loop E fold into the membrane, connect with each another and form a 3D hourglass model which characterized by the wide external openings to the channel with narrow central channel where the NPA motifs interact and forming a functional water pore or channel, Each individual aquaporin is a functional water pore and assemble in groups of four identical protein channels to form tetramers in the cell membrane. Water movement through the pores can be unidirectional to bidirectional.
In liquid state, the water the molecules are loosely bound by chemical bonding between the hydrogen and oxygen atoms. When water molecules come close to a pore of aquaporins, it enters inside a channel with the oxygen atom head first towards the pore. when they reached the channels center, they are snatched aside via attraction of hydrogen - oxygen bond of the water molecule and the amino acid asparagines in the two NPA motifs that locate the canal which causes the molecules to perform a pirouette (whirl), the single file got broken and the molecules comes to the opposite end of the pore with hydrogen atoms head first. Normally, protons use water molecules for the movement from one place to another place. A file of water molecules where the protons pushed back from one molecule to another. This is exactly what happens inside an aquaporin.