Linkage specificity, enzyme

The first group is comprised that of compounds which potentially could be degraded to form G-l-P by phosphorolysis by beta-linkage specific enzymes. They include IPTGlu, cellobiose, sophorose, salicin, and sucrose. Addition of these compounds, or exogenous G-l-P, to Solka Floe fermentations improved maximum cellulase yields from 171 to 309%, and the time period for enzyme synthesis was reduced from 95 to 59% compared with using Solka Floe only. 

Linkage specificity, in which the enzyme alters the reactivity of a particular type of bond. 

This great structural variety, however, complicates the specific biosynthesis of complex oligosaccharides. In general, the formation of each saccharide linkage requires specific enzymes ( one linkage—more than one enzyme ) and thus, in comparison with the enzymic synthesis of proteins and nucleic acids, much more effort is needed. 

Recent developments have led to agents with a built-in functional group that allows more rapid metabolism. Initially, the presence of ester groupings, as in suxamethonium, allowed fairly rapid metabolism in the body via esterase enzymes that hydrolyse these linkages. The enzyme involved appears to be a non-specific serum acetylcholinesterase (see Box 13.4). Even better is the inclusion of functionalities that allow additional degradation via an elimination reaction. Such an agent is atracurium. 

The plasma fucosyltransferase to Gal produces a-L-Fuc-( 1—>-2)-Gal linkages, and is the H-dependent, blood-group-specific enzyme. Its selective inhibition by thiol-blocking agents has permitted its discrimination245 from other fucosyltransferases. 

E represents the enzyme, S the substrate or reactant, and P the product. For a specific enzyme, only one or a few different substrate molecules can bind in the proper manner and produce a functional ES complex. The substrate must have a size, shape, and polarity compatible with the active site of the enzyme. Some enzymes catalyze the transformation of many different molecules as long as there is a common type of chemical linkage in the substrate. Others have absolute specificity and can form reactive ES complexes with only one molecular structure. In fact, some enzymes are able to differentiate between D and L isomers of substrates. 

A second novel aspects approach is substrate directed synthesis. The point of this aspect is that wild-type enzymes could be used to generate more diversity without engineering the enzyme. Therefore acceptor substrates were designed which allow the control of linkage specificity by the enzyme and further chemical reactions and... 

Hydrolysis is the reverse process of condensation as a water molecule and specific enzymes break all the glycosidic linkages in disaccharides and polysaccharides into their constituting monosaccharides. 

The relative lack of reactivity of the amide bond is notable in proteins, which are polymers of amino acids connected by amide linkages (Section 22.6B). Proteins are stable in aqueous solution in the absence of acid or base, so they can perform their various functions in the aqueous cellular environment without breaking down. The hydrolysis of the amide bonds in proteins requires a variety of specific enzymes. 

Some of the vitamins in the coeiizymc form associate tightly wdth specific enzymes, but not via a covalent linkage. Immediately after biosynthesis on the ribosome, enzymes do itot contain their cofactor, and these are called apoenzymes. An eitzyme containing its required cofactor is called a hoLoenzyme, With removal of the cofactor, the enzyme is also called an apoenzyme. The enzymes that exist in apoenzyme and holoenzyme forms include those that use vitamin Bx2, vitamin B, thiamin, and riboflavin-based cofactors. Enzymes that use niacin-based cofactors, folate, ascorbate, and vitamin K are not said to exist in apoenzyme and holocn-zyme forms. These enzymes bind their cofactors relatively weakly, and the cofactors behave in a manner similar to substrates. 

Although some commercially available enzymes are linkage specific, the majority of linkages cannot be defined solely using this approach and/or using the chemical approaches. Hence, linkage analysis experiments are often necessary for rigorous stmeture analysis. 

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