Enzyme-catalyzed Modifications

Currently, only hydrolases (amylases) are used to modify starch. The use of amylases to produce products derived from hydrolysis of starch is described in Chapters 7,20, 21 and 22. Starch hydrolyzates with good adhesion property that can be applied at high solids to minimize the energy required to remove moisture after application are very desirable for coating food items with seasonings, flavors and colorants. This property can be achieved by treating starch with an amylase or amylases to a dextrose equivalency (DE) (see Chapter 21) of 2-40.228 Waxy maize is the preferred starch. 

A low DE starch hydrolyzate with improved sweetness and browning capacity is prepared by treating starch with a combination of a- and (3-amylase or a-amylase and glucoamylase (amyloglucosidase).229 A further improved process employs a heat-stable a-amylase to convert the starch, with recovery of the products at high temperature.230 

A starch hydrolyzate with a peak average molecular weight of 10000 that is capable of forming a gel was prepared by treating high-amylose starch with an a-amylase to a DE between 5 and 15.231 The hydrolyzate is useful for preparing a foodstuff with reduced fat. Hydroxypropyl high-amylose starch hydrolyzate functions as a fat replacer.232 

A hydroxypropylated starch hydrolyzate with a DP 2-6 (DE 20-45) functions as a bulking agent. When combined with a high-intensity sweetener, it is useful as a reduced calorie replacement for sucrose.233 

A low-viscosity granular starch can be produced by contacting raw com starch granules with an a-amylase in water.234 The preferred degree of hydrolysis is 0.1 to 1.0%. 

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For the purpose of this chapter, enzyme-catalyzed modifications of proteins will be divided into two groups hydrolytic and nonhydrolytic reactions. Generally speaking, post-translational reactions occurring in vivo are catalyzed by highly specific enzymes under rather restricted conditions in contrast with in vitro modifications which are carried out under less specific conditions. 

Hayes, D.G. Enzyme-catalyzed modification of oilseed materials to produce eco-friendly... 

Recent studies also supported covalent amino acid incorporation during the enzyme-catalyzed modification reaction [105,106], and proteolysis in organic solvents is also mentioned as a particular way of amino acid incorporation involved in aminolysis [107]. 

This chapter describes recent developments in carbohydrate synthesis using microbial aldolases. Recombinant type II fructose-1,6> diphosphate aldolase and type I 2-deoxyribose-5-phosphate aldolase have been exploited for the synthesis of monosaccharides and analogs. A new procedure for the synthesis of dihydroxy acetone phosphate has been developed. Sialic acid aldolase has been used in conjunction with other enzyme-catalyzed modifications of monosaccharides for the synthesis of sialic acid-related sugars. 

Another field of enzymatic polymer synthesis is the enzyme-catalyzed modification of preformed polymers by esterification or transesterification. Thereby, it is possible to either introduce functional side groups into an existing polymer with a stable backbone (no polyester) to synthesize functional homopolymers as well as random copolymers or to generate multiblock copolymers by enzymatic transesterification between two different homopolymers. 

This enzyme catalyzes the covalent insertion of a tyrosine at the C-terminal glutamate of the tubulin a subunit to effect the posttranslational synthesis of a peptide bond (Flavin et al., 1982 Thompson, 1982). The role of this modification reaction remains to be established... 

A true appreciation of the subtle and complex ways in which the nucleosome can influence gene expression, has come only recently, largely through studies of the post-translational modifications to which all histones are subject and of the enzymes that add and remove these modifications. It has been known for many years that the histone N-terminal tails are exposed on the surface of the nucleosome and that selected amino acid residues are subject to a variety of enzyme-catalyzed, post-translational modifications. These include acetylation of lysines, phosphorylation of serines, and methylation of lysines and arginines ([6,7], see also chapters by Davie, and Ausio and Abbott, this volume). The locations of the histone N-terminal tails in the nucleosome and the residues that can be modified are shown in Fig. 1. 

Figure 2. A multistep enzyme interconversion cascade illustrating the sequential modification and conversion of target enzymes from one state of activity to another. Although written here as a cascade of increasing catalytic activity, enzyme-catalyzed covalent modification can either activate or inhibit target enzymes, depending on the particular system under study.

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