Mechanistic investigation of the RedG-catalysed oxidative carbocyclisation of undecylprodigiosin to streptorubin B

Prodiginines are a large family of red-pigmented antibiotics produced by a range of actinomycetes and other eubacteria that contain the highly-conjugated 4-methoxypyrrolyldipyrromethene core. It has recently been shown that the Rieske oxygenase-like non-haem iron-dependent enzyme RedG is responsible for catalysing the oxidative carbocyclisation of undecylprodigiosin to form the carbocyclic derivative streptorubin B, via functionalisation of an unactivated C-H bond. The mechanism of this chemically challenging reaction has been investigated using a mutasynthesis approach, employing analogues of key biosynthetic intermediates that bear specific functional groups, designed to probe different aspects of the proposed catalytic mechanism. Analogues of the biosynthetic intermediate 2-undecylpyrrole bearing sulphides, ethers, cyclopropanes, methyl substituents and deuterium labels in the undecyl chain have been synthesised and fed to Streptomyces coelicolor mutants blocked in the biosynthesis of 2-undecylpyrrole. These 2-undecylpyrrole analogues have also been fed along with synthetic 4-methoxy-2,2’-bipyrrole carboxaldehyde to Streptomyces albus expressing redH and redG. The results of these feeding experiments have confirmed that RedG utilises molecular oxygen, is capable of catalysing dealkylation of an ether and revealed that the pro-R hydrogen atom from C-7’ of the undecyl chain is specifically abstracted during the oxidative carbocyclisation. The results also showed that RedG is not able to tolerate substrate analogues with increase steric bulk close to the site of cyclisation