Question

For cats, the main determinant of a food's palatability is its protein content. for cats, then, food tastes especially good when it activates _____ receptors in the gustatory system

Answer 1

When it activates the G protein-coupled receptors. (Or glutamate receptors more specifically.) These receptors are in the base of the savory/umami taste that is usually associated to a more meaty/protein taste. Cats tend to like all that is described having this umami taste.