by / Tuesday, 22 February 2011 / Published in Biochemistry

There are a few different ways to solve the iso-electric point in biochemistry questions on the USMLE exams… BUT, there is a very easy way that I guarantee you are not being taught in medical school – and I’m going to give it to you right now.

Assuming we have a simple iso-electric graph (like the one pictured below), we have 2 COO- groups and 1 NH3+ group, in this case all we do is go to the side of the NH3+ group with the most COO- groups and add the two values and divide by 2…. like this:


Now, we are obviously going to be given a much tougher question, so the way to solve it is as follows:

Step 1 – Start on the side with the most COO- groups

Step 2 – In this example, there are three acids on the left side, so start as far away from the NH3+ as possilbe

Step 3 – Look at the # of bases in the graph, in this example we have 2

Step 4 – Starting from the last COO-, titrate the number of bases (2) from left to right

This brings you to your isoelectric point in a more complicated situation… See the picture below for better understanding!




Hope that helps clarify an otherwise complicated process!

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