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The alpha glucan lyases of Glycoside Hydrolase Family 31 degrade starch via an elimination mechanism, rather than via hydrolysis, forming an unsaturated (enol) product that quickly tautomerises to its keto form, 1,5-anhydro fructose.
The first half of their mechanism resembles that of Glycoside Hydrolase Family 31 retaining alpha-glycosidases, and involves the formation of a covalent glycosyl-enzyme intermediate. Instead of hydrolyzing, this intermediate then decomposes through a syn-elimination mechanism, again via oxocarbenium ion-like transition states, as shown below. This mechanism was demonstrated through a combination of studies involving intermediate trapping, kinetic isotope effects and linear free energy relationships [1, 2].
- Lee SS, Yu S, and Withers SG. (2003). Detailed dissection of a new mechanism for glycoside cleavage: alpha-1,4-glucan lyase. Biochemistry. 2003;42(44):13081-90. DOI:10.1021/bi035189g |
- Yip VL, Varrot A, Davies GJ, Rajan SS, Yang X, Thompson J, Anderson WF, and Withers SG. (2004). An unusual mechanism of glycoside hydrolysis involving redox and elimination steps by a family 4 beta-glycosidase from Thermotoga maritima. J Am Chem Soc. 2004;126(27):8354-5. DOI:10.1021/ja047632w |