CtsY is a bacterial 3-alpha-isomaltosyltransferase, first identified in Arthrobacter globiformis, that produces cyclic tetrasaccharides together with a closely related enzyme CtsZ. CtsY and CtsZ both have a glycosyl hydrolase family 31 (GH31) catalytic domain; CtsZ belongs to a different subfamily. All GH31 enzymes cleave a terminal carbohydrate moiety from a substrate that varies considerably in size, depending on the enzyme, and may be either a starch or a glycoprotein.
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