I've been hearing quantum superposition and tunneling as a factor in enzyme mechanisms at least since I was a grad student (circa 1988ish). It was all the rage that perhaps it was the main enzymatic mechanism; but, it grew a little less popular as more 'conventional' explanations seemed to provide acceptable kinetic models. "sources"? too many to list, but just search for quantum effects and enzymes and set your last date to something like 1990ish, like here: https://scholar.google.com/scholar?q=enzyme+mechanism+quantu...