Enzyme deactivation which is called denaturization is when an enzyme's physical structure is changed due to either high pH, or temperature. If enzymes convert the sugar to starch and make the corn less sweet, then boiling the enzymes will cause them to deactivate, which doesn't allow them to convert the sugar into starch. Therefore, the corn will remain sweeter.
Consider that when the enzyme is heated that its physical structure changes, meaning that it may not be able to fit the sugar molecule in its activation site. Meaning that the sugar can no longer bind to the enzyme to create starch, which again leaves it sweeter.
*activation site - site where the substrate (sugar) bonds to the enzyme in order for a chemical change to occur to the substrate.