Question 22.5: Enzyme Regulation An enzyme called Mythical Dehydrogenase is...
Enzyme Regulation
An enzyme called Mythical Dehydrogenase is made up of three subunits, but there are two types of subunits – A and B. The isoelectric point for subunit A is 7.0, and the isoelectric point of subunit B is 8.5. All permutations of subunits are found in the body.
(a) How many isozymes exist for Mythical Dehydrogenase?
(b) How would they separate using electrophoresis in a buffer at pH 7.8 (assume the molecules run towards the + electrode)?
Strategy
(a) If the native molecule has three subunits, and there are two types, you have to figure out all the possible combinations. An easy way is to write them out starting with the simplest and then change one at a time:
AAA
AAB
ABB
BBB
(b) To figure out how they would separate with electrophoresis, we need to know what the charge would be on the molecule. If the separation is done at pH 7.8, then this is lower than the pI for subunit B. This means subunit B is (+) charged at pH 7.8. This same pH is more basic than the pI for subunit A, so subunit A is (-) charged at pH 7.8. Since we are running the molecules from the (-) pole to the (+) pole of the electrophoresis, the isozymes with the most subunit A would move the fastest.
(a) There are 4 isozymes of Mythical Dehydrogenase
(b) The isozymes would separate as:
Fastest – AAA > AAB > ABB > BBB – Slowest
■ Quick Check 22.5
Match the description of the enzyme process with its type of regulation.
(a) The product of a series of 10 reactions inhibits the third reaction in the series.
(b) An enzyme is produced in an inactive form that becomes active only after several amino acids are removed.
(c) A small molecule binds to enzyme A at a site other than the active site. When this molecule is bound, the activity of the enzyme doubles.
(d) There are two forms of an enzyme. One is more active than the other. The most active form becomes less active when a particular amino acid on its surface is methylated.
(e) An enzyme is made up of four subunits. There are two types of subunits with slightly different isoelectric points, and different permutations are found in different tissues of the body.
1. Covalent modification
2. Isozymes
3. Allosterism
4. Feedback Inhibition
5. Zymogens