 This is achieved mainly by blocking formation of ammonia until PRPP is bound and channelling the ammonia to the PRTase active site.Initial activation of the enzyme by PRPP is caused by a conformational change in a "glutamine loop", which repositions to be able to accept glutamine.This results in a 200-fold higher Km value for glutamine binding Once glutamine has bound to the active site, further conformational changes bring the site into the enzyme, making it inaccessible.