Aquaporin

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Aquaporins are water channels which present in the membrane of cells (prokaryotic and eukaryotic). An aquaporin comprises of a crystal structure with a central pore which is lined on one side by carbonyl oxygen atoms, and on the other side by hydrophobic amino acids. Aquaporins in the lipid bilayer are arranged as tetramers. Within this structure, each monomer is composed of 6 transmembrane helices. [1] The water molecules can flow through the pore in a single fashion [2]. Another important point to note about aquaporin structure is that it contains 2 essential asparagine molecules, which bind to the oxygen atoms of the water molecule [3], along with an important alanine and proline molecule (NPA). This is important in explaining why the aquaporin is impermeable to ions and protons. The oxygen molecules become unavailable for hydrogen bonding to occur so the water molecule cannot undergo a H+ relay and therefore explains the reasoning behind the impermeability to protons [4].

Aquaporins are also essential in the kidneys. When the body has a low blood pressure, the Renin-Angiotensin system is stimulated. A low blood pressure is detected by the Autonomic Nervous System (i.e. baroreceptors in the aortic arch and the carotid sinus), which increases stimulation of the sympathetic nervous system, increasing contractility of the heart and simultaneously, blood pressure. Blood pressure is also regulated by hormonal control. A low blood pressure stimulates the release of renin which converts the precursor Angiotensinogen into Angiotensin 1. An angiotensin converting enzymes cleaves angiotensin I into angiotensin II. This stimulates the posterior pituitary gland where ADH (anti-diuretic hormone/vasopressin) is stored in vesicles. Upon its release into the bloodstream, Aquaporin 2 channels migrate towards the apical membrane of the collecting duct, where an increase in their insertion increases water retention. Water can now be reasborbed by the body to allow for rehydration.

References:

  1. Boron W and Boulpaep E(2005) Medical Physiology,updated ed, Philadelphia: Elsevier Inc, Chapter 3 page 58
  2. Molecular Biology of the Cell, 5th Edition: Alberts, Johnson, Lewis, Raff, Roberts, Walter; Chapter 11: (673-5)
  3. Molecular Biology of the Cell, 5th Edition: Alberts, Johnson, Lewis, Raff, Roberts, Walter; Chapter 11: (673-5)
  4. Molecular Biology of the Cell, 5th Edition: Alberts, Johnson, Lewis, Raff, Roberts, Walter; Chapter 11: (673-5)

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