Scheme 1 shows the plausible mechanisms of decarboxylation vs hydroxylation, which have been considered in peroxygenases. As seen, the hydroxylation in P450BSβ performs the standard hydrogen atom transfer (HAT) by Cpd I which is converted to Cpd II, followed by a rebound step to yield a hydroxylated product. However, three alternative pathways were proposed for the decarboxylation mechanism in P450OleT (Scheme 1). Pathway 1 involves a single electron transfer from the substrate to Cpd I followed by HAT to the so formed porphyrin-oxoiron(IV), Cpd II. However, this pathway was ruled out by the absence of OleT accumulation for reactions with a protonated long-chain fatty acid, and at the same time, the reaction of OleT exhibited a significant substrate deuterium kinetic isotope effect (KIE). Pathway 2, in Scheme 1, was ruled out too due to spectroscopic observation of a metabolite with a Soret band maximum at 426 nm and hyperporphyrin (i.e. a species having an additional band between 300 nm and 426 nm) character, corresponding closely to Cpd II. Scheme 1. Plausible mechanisms of hydroxylation by P450BSβ and decarboxylation by P450OleT. The question marks in pathways 2 and 3 refer to the mechanistic puzzle of the nature of the proton sources in these mechanistic pathways. This puzzle is targeted in this study.