McMurry's Organic Chemistry (7th Ed.) states, that Tyrosine is iodinated by mechanism of electrophilic aromatic substitution and the iodinating agent is $\ce{I+}$ or $\ce{HIO}$ formed by thyroid peroxidase. The mechanism is quite clear in this case.

Wikipedia article about thyroid peroxidase states that:

thyroid peroxidase oxidizes iodide to atomic iodine ($\ce{I}$) or iodinium ($\ce{I+}$).

From this figure from previous wikipedia article it moreover seems like $\ce{I^{0}}$ would be able to function as an iodinating agent.

I would like to ask whether atomic iodine produced by thyroid peroxidase could act as an iodinating agent by itself (if yes, by what mechanism, because I think electrophilic aromatic substitution cannot be used, morover, this reaction is supposed to happen in the colloid without any additional enzymatic support) or if I just didn't understand the article and figure correctly and additional oxidation to $\ce{I^{+}}$ is indeed needed (atomic iodine is just an irreactive intermediate).

I tried to find some references for this, but I was unable to do so.

NOTE: I originally posted this in biology.stackexchange.com but got no answers, so I thought here it might be more suitable.


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