What is the enzyme that makes apples's pectin heat resistant? And can it be added to other fruits to achieve similar results? In some recipes for apple and cherry pie, it is suggested to pre-cook the filling at low temperature in order to make pectin heat resistant and so preserve the shape and structure of the fruit.
This is explained in McGee's "On Food and Cooking" at page 283 as:


  Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 120F/ 50C (and inactivated above 160F/70C), and alters the cell-wall pectins so that they’re more easily cross-linked by calcium ions


Now my question is, what is this enzyme? Is this enzyme available for purchase? And if so, if I were to add it to a, say, strawberry pie filling, would it help maintain its structure?
I don't know for sure if it is the same enzyme naturally that naturally occurs in apples, but there is a commercial product called NovoShape that serves this purpose. It is a pectin esterase. You can find it in small quantities at Modernist Pantry: http://www.modernistpantry.com/novoshape.html