Thermoinactivation mechanism of glucose isomerase.

In this article, the mechanisms of thermoinactivation of glucose isomerase (GI) from Streptomyces rubiginosus (in soluble and immobilized forms) were investigated, particularly the contributions of thiol oxidation of the enzyme's cysteine residue and a "Maillard-like" reaction between the enzyme and sugars in high fructose corn syrup (HFCS). Soluble GI (SGI) was successfully immobilized on silica gel (13.5 microm particle size), with an activity yield between 20 and 40%. The immobilized GI (IGI) has high enzyme retention on the support during the glucose isomerization process. In batch reactors, SGI (half-life =145 h) was more stable than IGI (half-life =27 h) at 60 degrees C in HFCS, whereas at 80 degrees C, IGI (half-life =12 h) was more stable than SGI (half-life =5.2 h). IGI was subject to thiol oxidation at 60 degrees C, which contributed to the enzyme's deactivation. IGI was subject to thiol oxidation at 80 degrees C, but this did not contribute to the deactivation of the enzyme. SGI did not undergo thiol oxidation at 60 degrees C, but at 80 degrees C SGI underwent severe precipitation and thiol oxidation, which caused the enzyme to deactivate. Experimental results show that immobilization suppresses the destabilizing effect of thiol oxidation on GI. A "Maillard-like" reaction between SGI and the sugars also caused SGI thermoinactivation at 60, 70, and 80 degrees C, but had minimal effect on IGI. At 60 and 80 degrees C, IGI had higher thermostability in continuous reactors than in batch reactors, possibly because of reduced contact with deleterious compounds in HFCS.