Halo-organic & Lignocellulosic Recycle

Chloroperoxidase is an extracellular heme-thiolate enzyme produced by Caldariomyces, a fungus. It is known to be responsible for the formation and recycle of the vast majority of halo-organics in nature. A rugged protein within the acidic pH ranges, this protein is well-characterized and explored for green-chemistry applications. This enzyme is one of the most prolific catalysts known, which could catalyze a bevy of reactions like epoxidation, sulfoxidation, hydroxylation, (de)halogenation, hydrogen atom or electron abstraction, hetero-atom dealkylation, etc.

The image shows the topography and structure of the globular soluble protein chloroperoxidase (CPO) bound to different dye molecules of dimensions that exceed the active site channel to the heme center. Until the turn of 21st century, it was believed that enzymes such as the above were deactivated by DRS like peroxide and all the molecules that CPO converted in reaction accessed the active site. We demonstrated that there were two different types of reactions- one that occurring within the active site and another that could occur outside the active site, mediated by a DRS released from the active site. We also demonstrated that CPO could turnover millions of DRS molecules in its catalatic mode, prior to getting deactivated (the deactivation is also real, but occurs only at very high concentrations of DRS). We demonstrated that what the earlier researchers assumed as "deactivation" was in fact a depletion of peroxide (the reaction initiator or electron donor/acceptor) in milieu! We reasoned out that while several oxygen insertion reactions occurred at suitable small substrates with high enantioselectivity at the heme-center active site, the signature activity of several one-electron abstractions and chlorine atom insertions occurred outside the active site. Therefore, diverse dyes of varying dimensions and topologies could get decolorized even without gaining access to the active site, by being present in the vicinity of the enzyme and reacting with the DRS before it could meet another one of its own kind or peroxide (thereby leading to dismutations or quenching). Studies with this enzyme also demonstrated how diverse substrates could give "end product inhibition" with various redox proteins even though lacking multiple binding sites (which was required by the Belanger assumption used to explain such observations under the extension of Michaelis-Menten paradigm). In this system, we demonstrated that the redox-active components of such a DRS-mediated system could play multiple roles in the overall process. Therefore, molecules that were too large (like lignocellulosics) or that did not have any evolutionary relevance with the enzyme's active site (thereby presenting Fischer's lock-key type or Koshland's induced type of topological complementation) could also get broken down and recycled. Thus, the versatility and complexity of the reaction system was explained with the murburn perspective.