Why is the potassium sodium ratio so important in food? The reason is because it determines the size of the charge differential across cell membranes. And it is the charge differential across cell membranes that determine how well proteins in the membranes, and in the rest of the cell work. If there is too big of a differential, the proteins need a bigger change in charge to change shape and do work in the cell. If there is too little of a difference, the proteins may change shape too quickly or easily.
The last post discussed how changes in electric charges in the cell have optimal rates and an optimal voltage (optimal difference in charge across the membrane) to perform cell functions. So how do these charge changes occur? The charges change by moving ions across the cell membranes. A whole bunch of ions move from inside the cell to outside. And other ions move from outside to inside.
Ion Channels
The ions do this movement through channels, which are passive, and pumps, which are active and require energy. Some of the channels only need a change in the electrical field they sit in to open and close. Others need something, such as a protein or calcium, to attach to them in order to change in shape.
These channels that only need a change in charge have an area on them that senses when the electric field changes. This area is called a voltage sensing domain (VSD). The voltage changes when the electric charge inside or outside the cell changes.
The most common example is when a nerve fires. But almost all, if not all, of our cells perform their functions in a similar manner. When the electric charge changes in the cell, events in the cell occur that result in the cell performing its functions.
Each of these VSDs is similar in structure. They are present and retain the same basic structure from the most primitive organism to the most complex beings. When a cell structure is shared so extensively throughout living organisms, it performs an extremely important function. The electric field in the cell is the basis of all the functions in the cell.
Potassium And Sodium Make The Difference
Almost all of the difference in charge across the cell membrane is determined by the potassium and sodium inside and outside the cell. When this charge difference changes, potassium and sodium move. For a nerve cell depolarization, sodium moves inside the cell and potassium outside. When the nerve cell re-polarizes, sodium moves back outside the cell and potassium back inside.
The gate on a channel must open to allow the passage of potassium and sodium. And the gate must close to stop them from passing. The VSD on the channel initiates the opening and closing of the gate. Similar processes in muscle cells, heart cells, hormone cells and other functioning cells are initiated by the movement of potassium and sodium.
How Channels Open And Close
A recent article (1) discusses how these gates open and close. A very important part of how they open and close is based on the electric charge inside and outside the cell. This electric charge difference is known as a voltage potential. It crosses the membrane that these channels sit in. In addition to this voltage potential across the membrane, there are smaller local electric fields determined in part by the voltage potential across the membrane, and in part by the amino acids in the protein which have charges with effects over shorter distances.
Changing the large electric field that results from this voltage potential affects all the protein molecules sitting in the field. One way the large electric field affects the protein molecule is by affecting the smaller local electric fields in the various parts of the protein. When the local fields change, the protein changes shape.
This article looked at the specific local fields in the VSD to determine how this VSD protein changed shape to open and close the channel gate. There is presently no way to directly measure the inter-atomic forces between the atoms in the gate. Instead, the researchers used a computer simulation based upon many known forces acting at the atomic level. The values of these known atomic forces are collected in multiple databases available to researchers.
The researchers found that the change in electric charge caused a change in the local electric field in salt bridges in the VSD. The change in the local electric field caused salt bridges in the protein in the VSD to rearrange. The rearrangement of the salt bridges caused the channel to close. Reversal of the field resulted in them opening.
Salt Bridge
The salt in a salt bridge does not refer to table salt. It is called a salt bridge because the interaction between 2 ionized sites that sit close to each other in a protein is similar to the interaction between 2 ions of a salt. However the salt bridge has a hydrogen bond as well as an electrostatic interaction.
When the salt bridges rearrange, the protein changes shape, thus opening or closing the gate. This all happens in about 10 microseconds. If the local electric field is not quite right, the timing of the gate opening and closing is not quite right. In this particular study, the researchers found that the depolarization potential had to be complete before a repolarization could occur. If these channels do not function properly, the cell will not function properly.
The post last week showed that there is an optimal voltage and timing for proper function of the sodium potassium pump. The study discussed this week showed that there also is an optimal voltage and timing for proper function of these voltage dependent channels.
You might ask what all this molecular biology has to do with the food I should eat. If your goal is to eat food that will allow the cells in your body to function optimally, you will need to obtain food with enough potassium, and not too much sodium, so that the charge inside and outside your cells is at the proper level. If these charges are not at the proper level, the channels and the pumps inside your cells will work differently. And usually this means that your body will not function as well as it could.
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1. Microscopic origin of gating current fluctuations in a potassium channel voltage sensor. Freites JA, Schow EV, White SH, Tobias DJ. Biophys J. 2012 Jun 6;102(11):L44-6. doi: 10.1016/j.bpj.2012.04.021. Epub 2012 Jun 5.
