Nonhydrolytic enzyme-catalyzed

Table I. In Vivo Nonhydrolytic Enzyme-Catalyzed Post-Translational Modifications of Amino Acid Residues of Proteins0...

A few nonhydrolytic enzymes are able to catalyze the hydrolysis of some xenobiotics. These are briefly mentioned in the present context, and will be given greater attention in Chapt. 3, in regard to catalytic mechanism. 

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. 

Table II. Enzyme-Catalyzed Nonhydrolytic Modifications of Proteins...

In this manner, both /3- and a-D-xylosyl fluoride generate a-D-xylose, in the former instance by stereochemical inversion and in the latter by stereochemical retention. The series of events is reflected in each of the inverting glycosidases listed above. The reactions are unique not only because they involve hydrolysis of both (albeit truncated) substrate anomers, but the enzymes also catalyze an uncommon nonhydrolytic condensation reaction using the activated glycosyl fluoride. 

In Step 2, adenylosuccinase (also known as adenylosuccinate lyase, the same enzyme that catalyzes one of the steps in the purine pathway) carries out the nonhydrolytic removal of fumarate from adenylosuccinate, leaving AMP. 

ARS lyase catalyzes a reversible (A, 10 ), nonhydrolytic cleavage of ARS to arginine and fumaric acid [Eq. (10)] involving a transelimination (Ratner, 1973). The reversibility of the plant enzyme was shown by the production of ARS frorti arginine by Pisum sativum (Davison and Elliot,... 

A third enzyme, ADP-ribosyl protein lyase, catalyzes the nonhydrolytic cleavage of protein proximal ADPR residues yielding the unique nucleotide ADP-3"-deoxy-pentos-2"-ulose (ADP-DP). In addition to the basic enzymology shown in Figure 1, related enzyme activities... 

More structural data for cellulose-type linkage geometries are available from crystals of proteins that are complexed with either cellulose fragments or molecules that contain a lactose moiety. Figure 15-10 shows a complex of cellotetraose and an endoglucanase (Sakon et al. 1996), as well as the structure of the fragment in more detail. That protein is an enzyme that catalyzes the hydrolysis of cellulose molecules, but it also has nonhydrolytic binding sites. Other proteins do... 

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