In this video, we're going to talk about isoelectric focusing. Isoelectric focusing can be abbreviated as IEF, which stands for isoelectric focusing. Isoelectric focusing is a type of electrophoresis technique, which uses an electric field to separate proteins. As indicated by the word isoelectric, proteins are separated based on their isoelectric points. Recall from our previous lesson videos that the isoelectric point can be abbreviated as pI, and the pI (isoelectric point) is a specific pH where the net charge of the protein is equal to zero. This means that the protein will have an overall neutral net charge when the pH is equal to the isoelectric point.
How isoelectric focusing works: A stable or immobile pH gradient is established in the gel. In our example, notice to the far left, there is a container that contains our isoelectric focusing gel. It's important to note that within this gel, there is a linearly decreasing pH gradient, where the pH starts high at the top of the gel, and as you move down, the pH decreases linearly until it reaches the pH of 3 at the very bottom. By the term immobile, we mean that the pH does not move throughout the entire process. Thus, at different points in the gel, the pH is constant throughout the process: pH 9, pH 5, and pH 3, respectively, from top to bottom.
During the process, proteins alter their charges as they migrate through regions of the gel with different pH values. This is because the ionization states of ionizable groups change with pH, while the pKa values remain constant. Proteins will continue to migrate until reaching the specific portion of the gel where the pH equals their pI. At this point, when a protein's pH matches its pI, it will have a neutral net charge of zero and, consequently, will stop moving in the electric field.
Let's look at how this works in practice. We load our protein sample at the top of our isoelectric focusing gel and apply an electric field. The proteins begin to move through the gel, navigating through regions with varying pH levels until each protein reaches the region where the pH matches its pI. The protein then stops moving because it possesses a neutral net charge. As a result, we observe distinct protein bands, each stopping exactly where the pH equals the pI. By knowing that the pH gradient decreases linearly, we can determine the isoelectric points of all the proteins based on where they stop moving in the gel. Towards the top, we have proteins with high pIs corresponding to higher pH values, and towards the bottom, we have proteins with low pIs stopping at regions with lower pH.
The key takeaway from isoelectric focusing is that proteins migrate until they reach the region of the gel where the pH matches their pI, effectively separating them. We'll have an opportunity for practice in our next few videos. I'll see you guys there.