aquaporins

• in certain aquaporins like AQP1, Arg216 has been implicated in gating, often interacting with nearby histidine residues. However, in other aquaporins, such as AQP4, different residues like Arg189 may be more involved in gating.
• aquaporins are dedicated water channels. Water can flow through passive diffusion but it takes more time
• functional characteristics of aquaporins: bidirectional, fast, transports not just water but sometimes glycerol
• Water pores in AQPs strictly exclude protons, ions, and charged solutes while conducting water effectively
• GlpF (glycerol channel) is less narrow and doesn't have a periplasmic vestibule. glycerol uses hydroxyl group to form h bonds. glycerol have a less pronounced orientational preference, more restricted
• some uncharged solutes/gases (urea, ammonia, CO2, H2O2, NO) pass cell membrane via aquaporins
• Aquaporins are predicted to have 6 tmh with 2 repeats

  
• The structure of aquaporins consists of six alpha-helical transmembrane domains connected by loops that form an internal pore with a diameter of approximately 3 Å.
• aquaporins have a general tetramer structure, with each protomer being a channel itself
• tetrameric organisation is needed for function, suggests synergistic benefit of oligomerisation
• each protomer has its own pore, plus a central pore
• pore has 2 constriction sites: aromatic region made of conserved Arg195 forms narrowest part + NPA motifs form a second filter where a single molecule interacts with 2 Asn side chains
• each protomer has extracellular and cytoplasmic vestibule. ampipathic pore region (positive charged amino acids in center of passage way)

  • water moves in single file movement, bump into each other
  • protons can't make their way across channels (positive charges in center will repel)
• 6 transmembrane helices, hair pin loops (small proteins, doesn't go fully through membrane), highly conserved Asp, Pro, Ala motif, carbonyl groups 59-62 and 182-185 on half loops serve as h bond collectors

  
• one of the cytoplasmic loops (Arg-rich loop D), may play a role in cGMP-mediated activation of central pore; However, if this loop is lost or altered, the ion channel will no longer function properly; loss of ion conductivity
• AQP4 is expressed at high levels in brain endothelial cells lining blood vessels that supply the CNS
• the conserved NPA motif forms barrel around central pore
• transport of water molecules from one Asn to another causes a release of molecules from a continuous hydrogen bond system formed due to the water movement along the water pore
• The carbonyl groups can form hydrogen bonds with water molecules, facilitating their movement through the channel while excluding larger solutes.
• The carbonyl groups on the half loops may be involved in these gating mechanisms, helping to control the opening and closing of the channel in response to various stimuli, such as changes in osmotic pressure or pH.
• carbonyl groups participate in hydrogen bonding interactions with neighboring amino acid residues and water molecules, helping to maintain the structural integrity of the channel and ensure its proper functioning.
• Mutations in carbonyl groups 59-62 and 182-185 could disrupt aquaporin function, impairing water transport and cellular balance.
• 2 half helices, 1 comes from the top the other from the bottom, they meet in the middle, NPA motif in the middle. Cys182 at the top, plugs hole
• Cys182 is involved in the formation of a disulfide bond with another cysteine residue, typically found at position 109. This disulfide bond stabilizes the channel structure and can modulate the permeability of the aquaporin channel.
• AQP5 expression has been reported to be regulated by osmolality. It was suggested that an osmotic gradient between a cell and its environment is involved in regulating AQP5 expression. AQP5 expression is reported to be regulated by a cyclic AMP/protein kinase A (cAMP/PKA)-dependent pathway
• water adopts a bipolar orientation, oxygens always face centre of the channel from both entrances, disrupts potential proton conduction
• water molecule in the middle forms 2 hydrogen bonds with conserved NPA motif (NH bond to Asparagine), offers 2 hydrogen bonds to oxygens of neighbouring waters
• amino and carboxy-terminals responsible for regulation of aquaporin activity
• Cytoplasmic

  Refers specifically to the material inside a cell, excluding the nucleus. It includes all the organelles, cytosol, and other structures contained within the cell membrane.
• Intracellular


  Simply means "within the cell" and can refer to any component or location within the cell, including the cytoplasm, nucleus, or any organelles.
• Arg-rich loop D


  A specific cytoplasmic loop that is rich in the amino acid arginine. It is also known as the "disheveled binding domain" and is found in certain types of proteins called ion channels.
• arginine189 acts as gate; when in up position gate is open, single file can pass through. when in DOWN position gate is closed, inflow stops
• extracellular Arg and His region at the top thought to be involved in size contriction
• narrowest point of the channel excludes anything larger than water
• pore is wider in the outer vestibule, as we get to the selectivity filter, pore reaches the narrowest part, then gets wider towards the intracellular vestibule
• water wires: waters in a row are able to transport protons between them using Grotthus mechanism, NPA important for preventing water wires
• Theory 1 to break water chain: Precise positioning of 2 Asn in NPA motif allows for bonding with the central H2O molecule to prevent central water rotation and proton hopping
• if central water can't rotate can't accept proton from water before and can't continue the wire
• multiple crystal structures show both Asn can interact with individual water molecules
• helical dipole (has its own polarity - electronegativity differences) forces the water molecule to reorient, both positively charged end towards the middle