A metal or a redox cofactor like flavin can readily accomplish these subsequent steps.
#Oxygen reactivity series#
This latter biradical species must be activated by a series of one-electron stepwise transfer reactions leading sequentially to superoxide anion, hydrogen peroxide, hydroxyl radical, and, finally, water (Klinman 2007). It is known that molecular oxygen could be present as an excited singlet and a ground triplet state, the former being highly reactive, the latter more stable, since its reaction with organic substrates is a spin-forbidden process. 1971), that other enzymes were reported to catalyze an oxygenase side reaction and raised the issue of how such activity could occur.Īctually, the question of the reactivity with oxygen for enzymes lacking a proper metal or redox cofactor is still puzzling. The paper of Abell and Schloss represented a milestone, since it was the first time, after the identification of the oxygenase reaction catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) (Bowes et al. These enzymes belong to diverse families and catalyze different chemical reactions. These new PLP activities are not paracatalytic and could only scratch the surface on a wider and unexpected catalytic capability of PLP enzymes.Īpproximately 30 years ago, it was reported (Abell and Schloss 1991) that some enzymes that do not bind metals/cofactors typical of redox reactions are able to consume oxygen.
These PLP enzymes belong to the bacterial and fungal kingdoms and are present in organisms synthesizing bioactive compounds. Moreover, other recent reports revealed the existence of new oxidase activities catalyzed by new PLP enzymes, MppP, RohP, Ind4, CcbF, PvdN, Cap15, and CuaB.
In fact, some plant PLP decarboxylases have been reported to catalyze oxidative reactions producing carbonyl compounds. However, the reactivity of PLP enzymes with dioxygen is not confined to mammals/animals. These side paracatalytic reactions could be particularly relevant for human health, also considering that some of these enzymes are responsible for the synthesis of important neurotransmitters such as γ-aminobutyric acid, dopamine, and serotonin, whose dysregulation under oxidative conditions could have important implications in neurodegenerative states. A significant novelty appeared approximately 30 years ago when it was reported that some PLP-dependent decarboxylases are able to consume molecular oxygen transforming an amino acid into a carbonyl compound. From this point, the different reaction pathways diverge leading to multiple activities: transamination, decarboxylation, racemization, elimination, and synthesis. The breakage of a specific bond of the external aldimine gives rise to a carbanionic intermediate.
Reaction specificity is controlled by the orientation of the external aldimine intermediate that is formed upon addition of the amino acidic substrate to the coenzyme. PLP is necessary for many reactions involving amino acids. The versatility of reactions catalyzed by pyridoxal 5′-phosphate (PLP) enzymes is largely due to the chemistry of their extraordinary catalyst.
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