Fascinating Proteins #Aquaporins

The water pump of our cells.

Greetings Steemit, Time for another wonder of nature!

Introduction

I have something very interesting to present today, and i am sure you already know that all life needs water to stay alive. This is due to the simple fact that cells use water in many processes.

This is true for the cells in our body as well and it is really important that water can flow in and out of the cells in order to keep them healthy.
It is equally important that the brain can signal, that more water is needed in the organism, or in our case, the body. This will make us thirsty and force our body to drink, while our cells will take up more water.

Today we will look at how the cell's demand water and how water move in and out of cells.
Aquaporins is a category of channel molecules, making tunnels for water transport.
Prior to the knowledge of Aquaporins we thought that simple diffusion was enough to explain the water in and outputs of water over cell membranes.

Discovery

In 1992 thou, we discovered or rather stumbled upon the Aquaporins, proteins able to facilitate water transport over the cell membrane at much higher rates than simple diffusion could explain. Peter Agre was working on a slightly larger protein in blood cells, a 32 kilo Dalton heavy protein, but kept getting a signal for a 28 kilo Dalton large protein while purifying.

They called it CHIP, channel protein 28, thanks to it´s weight and this resulted in a Nobel prize during 2003´s ceremony, congratulations Peter Agre
The Aquaporin channels do not use ATP for energy, only Ion channels need energy. The Aquaporin channel only allow transport of uncharged particles via gradient pressure, making it highly effective with no stored energy. Water will move from high to low concentration, facilitated both by Aquaporins and diffusion!
Some cells must stay at a constant water level to be healthy, otherwise pH and Ion levels will be affected and make biological processes hard to carry out.

I decided to make a little simplified graphical summary of why we need water

The pressure of some cells are very important as well, especially for the red blood cells, which must be able to change volume, and thereby pressure at fast rate. The Aquaporins are amazingly fast, a single pore can move 1,000,000 H20 molecules per second, pewh!

Structure

Aquaporin 1 and 2 have high permeability for water thanks to a much hydrophilic center of the pore. The proteins is embedded in the membranes of our cells with 6 helical regions, as seen in the picture. The helices come together when activated to form a central pore.

The hydrophilic character come from the amino acids in the helices, they must have hydrophilic character as well, making them very sensitive to mutations. Changing one amino acid in the pore to a less hydrophilic will lead to problems, resulting in similar structures in many different cells types.
In the lower part, we can study two different Aquaporins, Aq1, permeable for water and Aq3 also permeable for glycerol and urea, other molecules that must get in and out of cells.

The 6 helices form a central pores straight through the lipid layer of our cell and works just like water channels activated when needed. From a Top down view it would look like this:

This is actually 4 sub units each having its own pore and a weight of 28 kilo Dalton, not like other ion channels which form one central pore.
Now we know how water can flow in and out of a cell but how does the cell tell our brain that it needs water?

Signaling for water

This is done via the hormonal system, which is responsible for most of the homeostatic functions such as keeping salt levels, pH and the body temperature constant. When the Posterior pituitary gland senses higher tonicity outside its cell membranes, meaning water levels have dropped, it will release a signal hormone to increase water uptake.

The particular hormone secreted to generate thirst in response as well as cellular water up take, is called vasopressin or called antidiuretic hormone ADH.

ADH induces cell signals that bring water into the cirkulation:

Metabotropic receptor binds to ADH on the cell surface, signaling via the G-protein complex for more Aquaporines.
Nephrons in the kidneys will be affected the most since Aquaporine 1 and 2 are most common here. Water uptake increases as absorption from e.g urine in collection tubes goes up, and we get more and more thirsty.
Pressure sensors in the Aorta helps with keeping the blood volume on track, Porines are very important to blood cell function.

A related disease

Genetic mutations in the sequence of the pores often results in lowered permeability and leads to different diseases depending on which type of organ and Porine it affects. As i mentioned, the effectivity of the porines are very sensitive to mutations and Diabetes insipidus is a disease related to mutation in the gene coding for Apuaporine 2, a water specialized poirune like Aquapourine 1.

The disease can also be caused by lack of Vasopressin or damages in the hypothalamus & pituitary glands since this would damage the signaling for water uptake and secretion.

Diabetes insipidus is characterized by massive production urine, 10-20 L/day compared to the regular 1,5 L/day, this off course, leads to immense thirst, luckily it´s a rare condition.

Ever thought about drinking water?

The thought or feeling of drinking or wanting water is simply conjured by your brain as a response to cells signal and receptor signals for water reaches a certain level.

This could be applied to all our thoughts i guess, since thinking simply is the collective will(signaling) from trillions of cells trying to stay alive no matter what.
No, just kidding..... or am I? :)

Figure 1 - The dry pharmacist

Figure 2 - Delving into water 2016
Figure 3 - Wikipedia, Peter Agre
[Figure 4 & 5 - Educational use]
Figure 6 - Wikipedia, Vassopresin
[Poster about water @Clausewitz - The Biochemist]