Amidino transfer

Fig. 3.5 Biosynthesis pathway of saxitoxin as proposed by Shimizu et al. [224]. The reaction steps are (1) Claisen-condensation between acetate and arginine (2) amidino transfer from a second arginine to the a-amino group of intermediate B (3) and (4), cyclization (5), introduction of S-adenosyl methionine (SAM) methyl-derived side chain, involving the loss of one methionine methyl hydride (6) 1,2-H shift (7) epoxidation of methylene side chain (8) opening of epoxide to an aldehyde (9) reduction of the aldehyde to hydroxyl (10) carbamoyl transfer and dihydroxylation of C-12...

A major class of enzymes that catalyze the transfer of a group or moiety from one compound to another. The groups being transferred can be one-carbon units such as methyl, hydroxyhnethyl, carbamoyl, or amidino moieties. Enzymes transferring aldehyde or ketonic groups such as transketolase are members of this class. Other examples include acyltransferases, glycosyltransferases, aminotransferases, phosphotransferases, and sulfotrans-ferases. 

The terminal amidino group of arginine is transferred intact to a number of other substances in simple displacement reactions. An example is the formation of guanidinoacetic acid (Eq. 24-27, steps a and b). 

Korosi94 proposed that the initial amidino acid is formed as shown in Scheme 14. According to this scheme the reaction is initiated by proton transfer from the amino acid to the lactim ether, being easier with aromatic and heterocyclic than with aliphatic amino acids. The... 

The functions of the guanidine-modifying enzymes further subdivide the family into three distinct groups hydrolases, dihydrolases, and amidinotransferases. Hydrolases catalyze the hydrolysis of guanidine derivatives to form ureido compounds, whereas dihydrolases catalyze a hydrolysis reaction to yield a primary amine, ammonia, and bicarbonate (or carbon dioxide). The amidinotransferases transfer an amidino group from one substrate to an amine. Although these are distinct reactions, they are characterized by common structural and mechanistic themes. 

Another phosphorylated spin trap, 5-diethoxyphosphoryl-5-methyl-l-pyrroline-(V-oxide [DEPMPO (16)], has been employed to distinguish radical reactions that produce HO from those that proceed via one-electron transfer followed by hydration or nucleophilic substitution, to give apparent radical adducts. The same spin trap was one of several nitrones employed to study the decomposition of three water-soluble amidino-azo initiators (proposed as potential sources of alkylperoxy and alkoxyl radicals). ... 

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