Glucose isomerase GI, also known as xylose isomerase, refers to the isomerase that can isomerize D-xylose, D-glucose, D-ribose and other aldoses into corresponding ketoses. GI has a wide range of sources, including bacteria, fungi, actinomycetes and other microorganisms, as well as plant and animal cells. It is a key enzyme in the industrial production of high fructose syrup and plays a key role in the industrial production of high fructose syrup and fuel ethanol.
Its activity was first discovered in Pseudomonas hydrophila in 1957. Later, nearly 100 bacteria and actinomycetes were identified as GI-producing strains [1]. Its sources are very wide, including bacteria, fungi, actinomycetes, and other microorganisms, as well as plant and animal cells.
GI is an intracellular enzyme that participates in the utilization of xylose entering the body, and its most suitable natural substrate is D-xylose [2]. Later, it was discovered that it can convert D-glucose into D-fructose outside the cell. The application of this enzyme can convert more than 90% of the sugar in glucose syrup into fruit essence, greatly improving the sweetness. Since then, GI has been widely used in the industrial production of high fructose syrup (HFS).
Mechanism of action
The catalytic process of GI is mainly divided into four steps: substrate binding, substrate ring opening, hydrogen transfer reaction (isomerization) and ring closure of product molecules, among which the hydrogen transfer reaction is considered to be the rate-limiting step of the entire reaction process.
Catalytic mechanism of enediol intermediate
This catalytic mechanism first proposed that the substrate binds to the enzyme in a ring-opening manner. H54 interacts with substrate C1 as an alkaline catalyst. The water molecules near substrates O1 and O2 may be catalytic acids, which polarize the substrate carbonyl group and promote the formation of enediol intermediates.
Negative hydrogen ion transfer mechanism
Evidence from crystallography and enzyme kinetics shows that GI adopts a metal ion-mediated negative hydrogen ion transfer mechanism. There are two views on the form of negative hydrogen ion transfer intermediates. One is the cationic form. During the isomerization process, Mg-2 polarizes the carbonyl group of substrate C1 to produce a carbon cation. Mg-1 and K183 act as Lewis acids to stabilize the carbon cation, while the two metal ions stabilize the negative charge of O2. The other is the anionic form, which proposes that the water molecule forms hydrogen bonds with the O1, O2 and carboxyl groups of the substrate and coordinates with the catalytic ion Mn2+, transfers the proton to the CO1 of D257 and forms an OH- ion itself. This OH- ion captures the proton of the substrate and makes it negatively charged. The proton transfer is completed by the water molecule/hydroxyl ion
