Elsevier

Journal of Biotechnology

Volume 94, Issue 2, 28 March 2002, Pages 137-155
Journal of Biotechnology

Review article
Properties and applications of starch-converting enzymes of the α-amylase family

https://doi.org/10.1016/S0168-1656(01)00407-2Get rights and content

Abstract

Starch is a major storage product of many economically important crops such as wheat, rice, maize, tapioca, and potato. A large-scale starch processing industry has emerged in the last century. In the past decades, we have seen a shift from the acid hydrolysis of starch to the use of starch-converting enzymes in the production of maltodextrin, modified starches, or glucose and fructose syrups. Currently, these enzymes comprise about 30% of the world's enzyme production. Besides the use in starch hydrolysis, starch-converting enzymes are also used in a number of other industrial applications, such as laundry and porcelain detergents or as anti-staling agents in baking. A number of these starch-converting enzymes belong to a single family: the α-amylase family or family13 glycosyl hydrolases. This group of enzymes share a number of common characteristics such as a (β/α)8 barrel structure, the hydrolysis or formation of glycosidic bonds in the α conformation, and a number of conserved amino acid residues in the active site. As many as 21 different reaction and product specificities are found in this family. Currently, 25 three-dimensional (3D) structures of a few members of the α-amylase family have been determined using protein crystallization and X-ray crystallography. These data in combination with site-directed mutagenesis studies have helped to better understand the interactions between the substrate or product molecule and the different amino acids found in and around the active site. This review illustrates the reaction and product diversity found within the α-amylase family, the mechanistic principles deduced from structure–function relationship structures, and the use of the enzymes of this family in industrial applications.

Introduction

Starch-containing crops form an important constituent of the human diet and a large proportion of the food consumed by the world's population originates from them. Besides the use of the starch-containing plant parts directly as a food source, starch is harvested and used as such or chemically or enzymatically processed into a variety of different products such as starch hydrolysates, glucose syrups, fructose, starch or maltodextrin derivatives, or cyclodextrins. In spite of the large number of plants able to produce starch, only a few plants are important for industrial starch processing. The major industrial sources are maize, tapioca, potato, and wheat. In the European Union, 3.6 million tons of maize starch, 2 million tons of wheat starch, and 1.8 millions tons of potato starch were produced in 1998 (DeBaere, 1999).

Section snippets

Starch

Plants synthesize starch as a result of photosynthesis, the process during which energy from the sunlight is converted into chemical energy. Starch is synthesized in plastids founds in leaves as a storage compound for respiration during dark periods. It is also synthesized in amyloplasts found in tubers, seeds, and roots as a long-term storage compound. In these latter organelles, large amounts of starch accumulate as water-insoluble granules. The shape and diameter of these granules depend on

Starch-converting enzymes

A variety of different enzymes are involved in the synthesis of starch. Sucrose is the starting point of starch synthesis. It is converted into the nucleotide sugar ADP-glucose that forms the actual starter molecule for starch formation. Subsequently, enzymes such as soluble starch synthase and branching enzyme synthesize the amylopectin and amylose molecules (Smith, 1999). These enzymes will not be discussed in this review. In bacteria, an equivalent of amylopectin is found in the form of

The α-amylase family: characteristics and reaction mechanism

Most of the enzymes that convert starch belong to one family based on the amino acid sequence homology: the α-amylase family or family 13 glycosyl hydrolases according to the classification of Henrissat (1991). This group comprises those enzymes that have the following features: (i) they act on α-glycosidic bonds and hydrolyze this bond to produce α-anomeric mono- or oligosaccharides (hydrolysis), form α,1-4 or 1-6 glycosidic linkages (transglycosylation), or a combination of both activities;

Industrial production of glucose and fructose from starch

A large-scale starch processing industry has emerged since the mid-1900s. Before further processing can take place, the starch-containing part of the plants have to be processed and the starch harvested (see Bergthaller et al., 1999). Besides starch, sugars, pentosans, fibres, proteins, amino acids, and lipids are also present in the starch-containing part of the plant. A typical composition of a potato is as follows: 78% water; 3% protein and amino acids; 0.1% lipids; 1% fibers; and 17%

Engineering of commercial enzymes for improved stability

The conditions prevailing in the industrial applications in which enzymes are used are rather extreme, especially with respect to temperature and pH. Therefore, there is a continuing demand to improve the stability of the enzymes and thus meet the requirements set by specific applications. One approach would be to screen for novel microbial strains from extreme environments such as hydrothermal vents, salt and soda lakes, and brine pools (Sunna et al., 1997, Niehaus et al., 1999, Veille and

Conclusions

The α-amylase family comprises a group of enzymes with a variety of different specificities that all act on one type of substrate, being glucose residues linked through an α,1-1, α,1-4, or α,1-6 glycosidic bond. Members of this family share a number of common characteristics but at least 21 different enzyme specificities are found within the family. These differences in specificities are based not only on subtle differences within the active site of the enzyme but also on the differences within

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