Intramolecular transferase

These catalysts facilitate the interconversion of isomeric compounds and include racemases, optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramolecular transferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triose phosphate isomerase. 

Thus, ihere are intramolecular oxiduredticiases le.g.. kelolisumerascs). intramolecular transferases te.g., phosphumuluses), and intramolecular lyases. About fifty isomerases are known,... 

Class 5. Isomerases interconvert isomeric structures by intramolecular rearrangements. They include racemases, epimerases, cis- and trans-isomerases, intramolecular transferases (mutases), and intermolecular lyases. 

Isomerases catalyze molecular isomerizations and include the epimerases, racemases, and intramolecular transferases. An example is shown in Figure 4.8, with the xylose isomerase (EC 5.3.1.5 systematic name, D-xylose ketol-isomerase commonly called glucose isomerase) transformation of a-D-glucopyranose to a-o-fructofuranose. 

Type of isomerization (1) racemases and epimerases, (2) cis-trans, (3) intramolecular oxidoreductases, (4) intramolecular transferases, (5) intramolecular lyases... 

Either the aglycone resulting from the action of a P-glucosidase or the glucoside, or possibly both, undergo cis-trans isomerization under influence of UV-light or possibly mediated by a dimethylallyl transferase. The last step of the biosynthetic pathway is an intramolecular esterification reaction, which can occur spontaneously, to yield coumarin (3.96). The enzymes that involved in these reactions have not been purified. 

In this section some examples are discussed in order to prepare nucleoside analogs by base interchange through a transglycosylation reaction. In these processes two different types of intramolecular enzymes are usually employed nucleoside phos-phorylases and N-2 -deoxyribosyl transferases. 

Figure 43-1 I Schematic view of the role of NAT enzymes in the metabolism of aromatic amines. N-acetylation might be a detoxification reaction in a number of cases however, after N-hydroxylation of aromatic amines (e.g., by CYP enzymes), NAT enzymes can bioactivate these intermediates by either 0-acetylation or intramolecular N,0-acety transfer, leading to the formation of nitrenium ions, which might react with DNA or alternatively be detoxified by, for example, GST enzymes. Importantly, it is shown that a number of other biotransformation enzymes are also involved in the metabolism of aromatic amines as well. (Redrawn from Wormhoudt LW, Commandeur jNM, Vermeuien NPE. Genetic polymorphisms of human N-acetyitransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes relevance to xenobiotic metabolism and toxicity. Crit Rev Toxicol 1999 29 59-124. Reproduced by permission from Taylor and Francis, Inc.)...

With the preceding reviews of the enzymology of monoterpene cyclization and of model studies relevant to the cyclization process, it is possible to formulate a unified stereochemical scheme for the enzymatic cyclization of geranyl pyrophosphate (Figure 4). The proposal which follows is consistent with the implications of parallel advances in related fields, most notably the contributions of Cane (8,16,24,25,52), Arigoni (67) and Coates (68,69) on the stereochemistry of sesquiterpene and diterpene cyclizations, and of Poulter and Rilling (29,70) on the stepwise, ionic mechanism of prenyl transferase, a reaction type of which several monoterpene, sesquiterpene and diterpene cyclizations are, in a sense, the intramolecular equivalents. 

Cholesterylation Protein C-terminal domain undergoes an intein like process, and acts as an intramolecular cholesterol transferase C-terminal of Gly n.d. Extracellular plasma membrane Hedgehog... 

So far, no specific mono(ADPR) transferase was found which could be responsible for the ADP-ribosylation of histone HI and H2B in response to alkylation. This certainly does not exclude the existence of such an enzyme. However, the possibility of an intramolecular shift of mono(ADPR) or poly(ADPR) residues from an energy-rich ester linkage to a more stable type of bond must also be taken into consideration. 

---