“Real time” molecular dynamics simulations of water permeation through human aquaporin-1 (AQP1) and the bacterial glycerol facilitator GlpF are presented. We obtained time-resolved, atomic-resolution models of the permeation mechanism across these highly selective membrane channels. Both proteins act as two-stage filters: Conserved fingerprint [asparagine-proline-alanine (NPA)] motifs form a selectivity-determining region; a second (aromatic/arginine) region is proposed to function as a proton filter. Hydrophobic regions near the NPA motifs are rate-limiting water barriers. In AQP1, a fine-tuned water dipole rotation during passage is essential for water selectivity. In GlpF, a glycerol-mediated “induced fit” gating motion is proposed to generate selectivity for glycerol over water.

/pdf/water-permeation-across-biological-membranes-mechanism-and-3ca6t05uda.pdf

Water Permeation Across Biological Membranes: Mechanism and Dynamics of Aquaporin-1 and GlpF

Water channels facilitate the rapid transport of water across cell membranes in response to osmotic gradients. These channels are believed to be involved in many physiological processes that include renal water conservation, neuro-homeostasis, digestion, regulation of body temperature and reproduction. Members of the water channel superfamily have been found in a range of cell types from bacteria to human. In mammals, there are currently 10 families of water channels, referred to as aquaporins (AQP): AQP0–AQP9. Here we report the structure of the aquaporin 1 (AQP1) water channel to 2.2 A resolution. The channel consists of three topological elements, an extracellular and a cytoplasmic vestibule connected by an extended narrow pore or selectivity filter. Within the selectivity filter, four bound waters are localized along three hydrophilic nodes, which punctuate an otherwise extremely hydrophobic pore segment. This unusual combination of a long hydrophobic pore and a minimal number of solute binding sites facilitates rapid water transport. Residues of the constriction region, in particular histidine 182, which is conserved among all known water-specific channels, are critical in establishing water specificity. Our analysis of the AQP1 pore also indicates that the transport of protons through this channel is highly energetically unfavourable.

/pdf/structural-basis-of-water-specific-transport-through-the-3l9yu3ibr9.pdf

Structural basis of water-specific transport through the AQP1 water channel

The water permeability of biological membranes has been a longstanding problem in physiology, but the proteins responsible for this remained unknown until discovery of the aquaporin 1 (AQP1) water channel protein. AQP1 is selectively permeated by water driven by osmotic gradients. The atomic structure of human AQP1 has recently been defined. Each subunit of the tetramer contains an individual aqueous pore that permits single-file passage of water molecules but interrupts the hydrogen bonding needed for passage of protons. At least 10 mammalian aquaporins have been identified, and these are selectively permeated by water (aquaporins) or water plus glycerol (aquaglyceroporins). The sites of expression coincide closely with the clinical phenotypes--ranging from congenital cataracts to nephrogenic diabetes insipidus. More than 200 members of the aquaporin family have been found in plants, microbials, invertebrates and vertebrates, and their importance to the physiology of these organisms is being uncovered.

Aquaporin water channels – from atomic structure to clinical medicine

Our understanding of the movement of water through cell membranes has been greatly advanced by the discovery of a family of water-specific, membrane-channel proteins — the aquaporins. These proteins are present in organisms at all levels of life, and their unique permeability characteristics and distribution in numerous tissues indicate diverse roles in the regulation of water homeostasis. The recognition of aquaporins has stimulated a reconsideration of membrane water permeability by investigators across a wide range of disciplines.

/pdf/from-structure-to-disease-the-evolving-tale-of-aquaporin-162r5vifxy.pdf

From structure to disease: the evolving tale of aquaporin biology

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H+ channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H+ch...

https://www.physiology.org/doi/pdf/10.1152/physrev.00028.2002

Voltage-Gated Proton Channels and Other Proton Transfer Pathways

The water-selective pathway through the aquaporin-1 membrane channel has been visualized by fitting an atomic model to a 3.7-A resolution three-dimensional density map. This map was determined by analyzing images and electron diffraction patterns of lipid-reconstituted two-dimensional crystals of aquaporin-1 preserved in vitrified buffer in the absence of any additive. The aqueous pathway is characterized by a size-selective pore that is approximately 4.0 +/- 0.5A in diameter, spans a length of approximately 18A, and bends by approximately 25 degrees as it traverses the bilayer. This narrow pore is connected by wide, funnel-shaped openings at the extracellular and cytoplasmic faces. The size-selective pore is outlined mostly by hydrophobic residues, resulting in a relatively inert pathway conducive to diffusion-limited water flow. The apex of the curved pore is close to the locations of the in-plane pseudo-2-fold symmetry axis that relates the N- and C-terminal halves and the conserved, functionally important N76 and N192 residues.

https://www.pnas.org/content/pnas/98/4/1398.full.pdf

A. Cheng

Papers

Visualization of a water-selective pore by electron crystallography in vitreous ice