Aquaporin in Mammalian Membranes

Introduction

Aquaporins (AQP)  form the essential tissue proteins that act as channels responsible for transporting water molecules, other solutes across the mammalian cells. These channels are found in bacteria, animals and plants (Agre & Kozono, 2003, pg. 14). Research has identified that there exist some pores in the middle of each aquaporin molecules. Mammalian chambers are determined to have over ten isoforms which are represented in many different cells and tissue elements in the bodies. Agre & Kozono (2003, pg. 23) identify that aquaporin tissues are found in the blood cells, kidney tubules, eyes and ears. AQP2 is located in the kidney collection duct where they shuttle among the intracellular stowage elements plus the plasma membranes controlled by the antidiuretic hormone (ADH).  However, it is essential to note that any form of mutations done on the AQP2 might lead to inspidus diabetes.  The condition is caused by the blockage of the water supply and flows from the respective cell elements. AQP3 is found in the kidney collection channels, urinary, integument and the digestive system. In all these organs, AQP3 is involved in the transportation of water molecules. AQP4 is found in the brain cells, eyes, skeletal muscles, stomach cells and the kidney cells. AQP5 forms the primary secretory membranes like lacrimal, salivary glands and the sweat gland. It is also found in the eyes, ears where it performs the role of eliminating the waste matter. AQP6 is also found in the kidney secretory channels. AQP7 and AQP8 are present in the testis, kidney and adipocytes. AQP 9 and AQP10 are found liver, intestine and leukocyte. The difference in the characteristics and location of the aquaporin elements imply that they have specific roles and essential to the body organs. This paper discusses the structure of aquaporin channels and water transport function of aquaporins.

Structure of the aquaporin

According to Hacke & Laur (2016), aquaporins form a family of membrane proteins with a size of 30 kDa found in the plasma membranes. They are found in many transporting cells. The mutagenesis use has demonstrated that aquaporin structures are spectroscopic, freeze-fracture and epitope classifications  (Nesverova & Törnroth-Horsefield, 2019, pg. 102). Aquaporins are seen to converge in the tissues as homotetramers where every monomer, comprising of six tissue-rotating α-helical areas with sloping cytoplasmic amino and carboxy depots, covers a separate water stoma (Agre & Kozono, 2003, pg. 20).  Medium-solution structural review via the cryocrystallography electron revealed that about six slanted helical sections make a butt adjacent a middle pore-like area that comprises extra protein thickness. Many of mammalian’s aquaporins, for instance, the “AQP1, AQP2, AQP4, and AQP9 and AQP9”  seem to be extremely selective for the passing of the water molecules. At the same time, some called aquaglyceroporins majorly convey glycerol like the AQP8 and AQP3. The AQP9 transports other higher solutes). Proof for potential movements of body solutes like the ions and the respiratory gases are provided at this level (Papadopoulos & Saadoun, 2015, pg. 65). In this regard, the molecular pathways of the aquaporin elements are shown to be based on the flow of the physiological process.

The aquaporin elements form the tiny hydrophobic, essential membranes.  The aquaporin Hydropath schemes form amino-acid like arrangements with a minimum of six nonpolar areas of appropriate lengths. This is shown by F1 below. Structure arrangement of the aquaporins display numerous highly preserved designs, containing two “NPA” orders, and solo “AEFL” and the “HW [V/I][F/Y] WXGP” structures (Soveral, Nielsen & Casini, eds., 2018 pg. 14). General amino acids characteristics amongst the mammals’ aquaporins variety from 19 to 52% are present in various organs. Inside the aquaporin kinfolk of mammalian organs, two subcategories are distinct: “the aquaporin” and “aquaglyceroporin,” which form the “HW[V/I][F/Y]WXGP” structures.  The elements carry different matter based on their adaptations.

F1. Cartoon structure of aquaporin

The aquaporins range from AQP1 to AQP8 except for AQP3 and AQP7; aquaglyceroporins entail aquaporin 3, 7 and 9 that have two extra peptide extents as shown by F1 above (Nesverova & Törnroth-Horsefield, 2019, pg. 99). Additional examinations of aquaporin arrangements indicate homology of the two halves of this molecule. This signifies the development of the aquaporin protein sequence from the intragenic doubling occurrence. The cycle repeat design has insinuations about aquaporin construction as defined by the figure above.

Water Transport Function of Aquaporins

Water transport in the mammalian cells often follows an osmotic swelling mechanism that entails a transfer of water through the porous membranes. Xenopus oocytes form the significant elements of the aquaporin transport systems (Agre & Kozono, 2003, pg. 200). The process involves the permeability of water solutions. The inherent “single-channel” aquaporin H2O penetrability is calculated from proteoliposome sizes, the process of capacity change succeeding an osmotic task and the quantity of the aquaporin proteins in proteoliposome. This single-channelled water penetrability of AQP1 is demonstrated as ∼6 × 10⁻¹⁴cm³/s (Nesverova & Törnroth-Horsefield, 2019, pg. 104). The aquaporin water passage forms a thin cylindrical stoma that allows single-line water flow, and then archetypal model forecasts a channel width of 3.8 Å. Furthermore, the noticeable osmotic water penetrability constant (P_f) of the membranes is the creation of single-canal water penetrability that allows water to flow in the form of molecules.

Hydrostatic gravity is particularly significant in leading the flow of water in all the nephrons of mammalian kidneys because is enhances proper sieving of water in the plasma to create urine (Agre & Kozono, 2003 pg. 204).  While hydrostatic gravity inside organs rises, the quantity of water exiting the tubes also upsurges, and extra urine deposit is made. When hydrostatic density inside the kidneys dribs excessively low, especially during dehydrations, the activities by the kidney cells might be reduced, and fewer nitrogenous excretions will be eliminated from capillaries. Dangerous dehydration can lead to the failure of the kidneys.

Liquid also transports between sections alongside an osmotic ascent. Remember that the alteration makes the osmotic rise in concentrations of the solutes on whichever adjacent of the semi-permeable membranes (Nesverova & Törnroth-Horsefield, 2019 pg. 104). The degree of an osmotic slope is relative to the change in the concentrations of fluids in one part of the tissue membranes to the other part. Water then moves through osmosis inside the highly concentrated cells to the lowly concentrated ones.

Consequently, the little inherent water penetrability of aquaporins, which is restricted by the biophysical restraints on fluid movements, entails the manifestation of numerous aquaporins for practical importance (Agre & Kozono, 2003, pg. 208). A result of the high-pressure aquaporin appearance is that outflows to solute elements and some ions, even though small, can have harmful properties inside the cell membranes. For instance, when kidney AQP2 are porous to protons, the acidic urine cleaning the accumulating channel lumen can produce intense acidification in the intracellular elements (Nesverova & Törnroth-Horsefield, 2019, pg. 200).

Agre & Kozono, (2003 pg. 211) denotes that 80% of mammalian brain contains water. This water is transported by channels proteins found in mind, including aquaporins 1, 4, and 9. AQP1 is stated inside choroid plexus and contributes to the formation of cerebrospinal liquid. AQP4, create foot routes inside astrocyte, ependyma, and glia limitans enables water flow inside and outside the brain, hurries astrocyte movement and changes neuronal action. Lately, AQP4 autoantibodies were exposed in victims with neuromyelitis optical, a demyelinating illness, and can be used in the diagnosis process. This process is shown below by F2.

Figure 2. Water movement in the brain

Reference List

Agre, P. and Kozono, D., 2003. Aquaporin water channels: molecular mechanisms for human diseases1. FEBS letters, 555(1), pp.72-78.

Hacke, U.G., and Laur, J. (2016). Aquaporins: Channels for the Molecule of life.elS,1-6

Nesverova, V. and Törnroth-Horsefield, S., 2019. Phosphorylation-dependent regulation of mammalian aquaporins. Cells, 8(2), p.82.

Papadopoulos, M.C. and Saadoun, S., 2015. Key roles of aquaporins in tumor biology. Biochimica et Biophysica Acta (BBA)-Biomembranes, 1848(10), pp.2576-2583.

Soveral, G., Nielsen, S. and Casini, A. eds., 2018. Aquaporins in health and disease: new molecular targets for drug discovery. CRC Press.