Isomerase is the name given to a class of enzymes which are able to convert molecules from one isomer to another. These isomerases are a key component in the formation and deterioration of intramolecular rearrangements.
When this occurs, only one product is yielded by a single substrate. While sharing a common molecular formula with the substrate, the product will differ in bond connectivity or spatial arrangement.
Isomerases are the key to catalyzation in various reactions across many biological processes. Common examples might be the metabolism of carbohydrates and glycolysis.
Isomerases have a range of mechanisms.
Epimerization, In the Calvin cycle, for example, when D-ribulose-5-phosphate is converted into D-xylulose-5-phosphate by ribulose-phosphate 3-epimerase. The only difference in stereochemistry between the substrate and product is at the third carbon in the chain. The underlying mechanism requires third carbon to become deprotonated to form a reactive enolate intermediate.
Intramolecular transfer, Chorismate mutase is an intramolecular transferase which catalyzes the conversion of chorismate to prephenate. This is sometimes used as a precursor for L-tyrosine and L-phenylalanine in certain bacteria and flora. This reaction is a Claisen rearrangement that can proceed with or without the isomerase. That said, the rate increases exponentially in the presence of chorismate mutase. The reaction goes through a chair transition state with the substrate in a trans-diaxial position with the isomerase selectively binding the chair transition state. The exact mechanism of catalysis is not yet known.
Intramolecular oxidoreduction, Isopentenyl diphosphate (IPP) isomerase is seen in the synthesis of cholesterol synthesis, especially when it catalyzes the conversion of isopentenyl diphosphate (IPP) to dimethylallyl diphosphate (DMAPP). Here, a stable carbon-carbon double bond is rearranged to create a highly electrophilic allylic isomer.
Ring expansion and contraction, A highly illustrative example of ring opening and contraction is the isomerization of glucose to fructose. In other words, the isomerization of an aldehyde with a six-membered ring to a ketone with a five-membered ring. The conversion of D-glucose-6-phosphate to D-fructose-6-phosphate is catalyzed by glucose-6-phosphate isomerase, an intramolecular oxidoreductase. The reaction requires the opening of the ring to form an aldose via acid/base catalysis. This leads to the formation of a subsequent cis-endiol intermediate. Thus, a ketose is formed and then the ring is closed again.
Isomerase has a broad array of functions in everything from medicine to food manufacture. Deficiencies in isomerases have been known to lead to diseases in humans such as Phosphohexose isomerase deficiency and Triosephosphate isomerase deficiency.
However, perhaps the most common application of isomerase is in the manufacture of table sugar where glucose isomerase catalyzes the conversion of D-xylose and D-glucose to D-xylulose and D-fructose.
Structural isomers are differently ordered in terms of a different ordering of bonds and/or different bond connectivity from one another. Take, for example, hexane and its four other isomeric forms; 2,2-dimethylbutane, and 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane.
Stereoisomers are isomers that have the same ordering of individual bonds and the same connectivity. However, the three-dimensional arrangement of bonded atoms differs. 2-butene, for example, exists in two isomeric forms: cis-2-butene and trans-2-butene.
The sub-categories of isomerases are examples of enzymes catalyzing the interconversion of stereoisomers. Intramolecular lyases, oxidoreductases and transferases, on the other hand, catalyze the interconversion of structural isomers.