Amylolytic Enzymes: Types, Structures and Specificities

Abstract

Cellulose and starch are the most abundant polymers on Earth. They both consist of glucose monomer units which are, however, differently bound to form polymer chains: starch contains the glucose linked up by the -glucosidic bonds, while the glucose in cellulose is bound by the -glucosidic linkages. Therefore these two important sources of energy for animals, plants and micro-organisms are biochemically hydrolysed by two different groups of enzymes: starch by -glycoside hydrolases, and cellulose by -glycoside hydrolases. Starch (amylon in Greek) consists of two distinct fractions: amylose – linear -1,4-linked glucans, and amylopectin – linear -1,4-linked glucans branched with -1,6 linkages (Ball et al., 1996; Mouille et al., 1996), therefore the enzymes responsible for its hydrolysis are called amylolytic enzymes or – simply – amylases. Amylolytic enzymes form a large group of enzymes among which the most common and best known are -amylases, -amylases and glucoamylases. Since starch (like the structurally related glycogen) is an essential source of energy, amylolytic enzymes are produced by a great variety of living organisms (Vihinen and Mantsala, 1989). Although the different amylases mediate the same reaction – they all catalyse the cleavage of the -glucosidic bonds in the same substrate – structurally and mechanistically they are quite different (MacGregor et al., 2001). Both -amylase and -amylase adopt the structure of a TIM-barrel fold (for a review see Pujadas and Palau, 1999), i.e. their catalytic domain consists of a ( / 8-barrel formed by 8 parallel -strands surrounded by 8 -helices