Identification of Isoenzymes

Isoenzymes are enzymes from different species, or produced by different mechanisms within the same species, that catalyze identical reactions. Isoenzymes often differ from each other by only a few amino acid residues, and may have very similar molecular weights. In some cases, charged amino acid residues in one isoenzyme (e.g., lysine, arginine, aspartate, glutamate) may be substituted by uncharged amino acid residues in another, and this type of substitution will result in different net charges at a given pH, and different pi values for the different isoenzymes. 

For example, human alcohol dehydrogenase exists in more than twenty forms.6 These isoenzymes are all dimers, and all have molecular weights of 80 kDa, or 40 kDa per subunit. The different subunits have been called the a, p, yl, y2, n, and X subunits. All of the isoenzymes catalyze the conversion of ethanol to acetaldehyde according to Eq. 10.1  

Human alcohol dehydrogenase isoenzymes have been divided into three classes. Class I isoenzymes contain a, p, yl, and y2 subunit isoenzymes, are cationic at neutral pH, and are strongly inhibited by substituted pyrazole inhibitors. Class II isoenzymes contain the 7i isoenzyme subunit, are cationic at neutral pH, and are relatively insensitive to pyrazole inhibition. Class III isoenzymes contain the X subunit, are anionic at neutral pH and are insensitive to pyrazole inhibitors. 

---

Dobbs PC, Epstein DL, Anderson PJ. Identification of isoenzyme C as the principal carbonic anhydrase in human ciliary processes. Invest Ophthalmol Vis Sci 1979 18 867-870. 

---