The isoelectric point is just the pH that something is neutral. To determine the charge of a molecule from the isoelectric point you will also have to be given the pH of the medium that it's in. When talking about amino acids most of the time they are referring to the physiological pH (~7). It's pretty easy to remember the charges of all the amino acids though. They all have one amino group and one carboxy group - with pKa's of ~9.5 and ~2.3 respectively. When the pH is higher than the pKa the molecule will be deprotonated and vice versa (remember that rule). With that being said, all amino acids that do not have ionizable side chains will be neutral at physiological pH (the carboxy will be deprotonated (-) and the amino will be protonated (+)). The only ones you have to actually think about are: aspartic acid, glutamic acid, histidine, cysteine, tyrosine, lysine, arginine. There are plenty of tricks you can think of to remember these too, look at a list and find a way to commit them to memory. The way I've remembered them is that aspartic acid and glutamic acid are the only ones that are negative at physiological pH, in my eyes this is easy to remember because they are acids (also called aspartate and glutamate). Histidine does have a pKa lower than physiological pH (6) but it is actually neutral there because the deprotonated version is neutral. That's another thing you're gonna have to remember is that just because it's deprotonated does not mean it's negative. Arginine, lysine and and histidine are all neutral when deprotonated - arginine and lysine have a much larger pKa of 12 and 10. The acids, tyrosine and cysteine are all negative once deprotonated. Cysteine and and tyrosine pKa ~8 and 10. So for example, at physiological pH tyrosine and cysteine are neutral because pH<pKa ->protonated -> neutral (for cysteine and tyrosine).
If you remember all those rules not only will it make it easier to keep the amino acids in check but you will be able to figure out any of the acid and charge **** for the exam.
