Multiple modes of substrate hydrolysis-induced covalent modification of strigolactone receptors

The strigolactone signaling pathway in plants is unconventional among plant hormone signaling pathways in that the receptor also acts as an enzyme that hydrolyzes the strigolactone substrate. While the canonical view of strigolactone hydrolysis is that it occurs via a nucleophilic attack on the butenolide ring of strigolactone, an alternative Michael addition mechanism in which hydrolysis occurs via a nucleophilic attack on the enol-ether bridge has been proposed. Furthermore, while it is known that a hydrolysis-induced covalent modification to the receptor promotes strigolactone receptor activation, the nature of this covalent modification has been disputed. Here, we employ QM/MM string method simulations to determine the favored pathway of strigolactone hydrolysis and the nature of the covalent modification that acts as a promoter of strigolactone receptor activation. Our simulations show that strigolactone hydrolysis occurs via an acyl substitution pathway beginning with nucleophilic attack on the butenolide ring, which is well corroborated by previous experimental literature. Additionally, we show that multiple possible modes of covalent modifications to the catalytic residues by the butenolide ring are able to form and interconvert, reconciling several seemingly conflicting views on the hydrolysis-induced covalent modification to strigolactone receptors.