Intradiol cleavage

It is interesting to note that dioxygenases catalyzing the intradiol cleavage contain the ferric form of iron as the sole cofactor, whereas those catalyzing the extradiol cleavage contain the ferrous form. 

Pyrocatechase31 and protocatechuate 3,4-dioxygenase 86, both of which are typical examples of the intradiol cleavage, catalyze the reaction shown in Eqs. (2) and (12), respectively. 

The enzyme catalyzes the intradiol cleavage of catechol with the insertion of 2 atoms of molecular oxygen to form e/s.c/s-muconic acid. The enzyme also catalyzes the oxidation of various catechol derivatives, including 4-methylcatechol, 4-chloro-catechol, 4-formylcatechol (protocatechualdehyde), 4,5-dichlorocatechol, 3,5-di-chlorocatechol, 3-methylcatechol, 3-methoxycatechol, and 3-hydroxycatechol (pyro-gallol). All of these substrates give products having an absorption maximum at around 260 nm characteristic of t7s-c/s-muconic acid derivatives. However, when 3-methylcatechol is used as a substrate, the product formed shows an absorption maximum at 390 nm besides that at 260 nm. These two absorption maxima are found to be attributable to two different products, 2-hydroxy-6-oxo-2,4-heptadienoic acid (7), and 5-carboxy-2-methyl-2,4-pentadienoicacid(2-methylmuconic acid), (2) [Eq. (19)]96. ... 

Component 13 has been suggested to arise from prior hydroxylation of the ring followed by intradiol cleavage and lactonization. Though plausible, this mechanism does not satisfactorily explain why 15 is not observed at all in several cases. A more reasonable suggestion involves the extradiol cleavage of the ring, subsequent oxidation of the aldehyde function, and lactonization. 

Finally, it is interesting to note that only 13 is formed in cases 1 and 8 and only 15 is formed in cases 3,4, and 9. If 13 arises from extradiol cleavage while 15 results from intradiol cleavage, the above experiments have provided clues as to how such specific cleavages may be effected by the active sites of dioxygenases. 

In some strains of pseudomonads, the degradation of the intermediate catechol produced by the activity of salicylate hydroxylase may proceed by the alternative intradiol cleavage pathway (Barnsley 1976). Reference has been made to the alternative gentisate pathway for the degradation of the intermediate salicylate (Grund et al. 1992). 

Protocatechuate 3,4-dioxygenase. Crystalline preparations of the enzyme were obtained as described previously (2). The enzyme catalyzes the intradiol cleavage of protocatechuic acid with the insertion of two atoms of molecular oxygen to form /8-carboxy cis,cis-muconic acid (Reaction 3). 

Intradiol cleavage products Extradiol cleavage products Oxygenation products... 

Catechol dioxygenase - intradiol cleavage Arene dihydroxylation 175)... 

EVIDENCES FOR SUBSTRATE ACTIVATION OF COPPER CATALYZED INTRADIOL CLEAVAGE IN CATECHOLS... 

A different kind of transformation occurs when ternary copper(II) catecholato complexes [Cu(3,5-DTBC)(phen)] and [Cu(3,5-DTBC)(bpy)] react with 0 in DMSO [55], affording the intradiol cleavage product... 

For example, the enzyme pyrocatechase effects intradiol cleavage of catechol to muconic acid [72,73,74], whereas protocatechuate 3,4-dioxygenase promotes that of protocatechuic acid (R = COOH in 37) [75,76]. Metapyrocatechase is a catalyst for proximal extradiol cleavage of catechol to a-hydroxymuconic acid e-semialdehyde (38) [77] ... 

As for the role of bipy and py, subsequent work has established that they are not essential for the oxygenation (dehydrogenation) to occur. Iron(III) salts, like chloride, nitrate or perchlorate, hydrated or anhydrous, are effective catalysts for 3,5-DTBC oxidation in THF or DMF [108]. 3,5-DTBQ is the predominant product but intradiol cleavage to 15, DBMUA and 53 (Scheme 7) also takes place. Additives like py. 

The products of a 60-hour reaction at room temperature were 3.5-DTBQ (24 %), DBMUA (45 %) and BUP (6 %). As TCQ is not oxidized, the TCC ligand is apparently displaced by 3,5-DTBC from the coordination sphere before any oxidation takes place. Extensive intradiol cleavage occurs with the incorporation of one 0-atom, similarly to a ruthenium-catalyzed reaction [103] but in contrast to pyrocatechase and its iron [105] and copper models [32], where 2 0-atoms are inserted. 

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