Cancer cells exhibit several unique metabolic phenotypes that are critical for cell growth and proliferation. Specifically, they over-express the M2 isoform of the tightly regulated enzyme pyruvate kinase (PKM2), which controls glycolytic flux, and they are highly dependent on de novo biosynthesis of serine and glycine. Here we describe a novel rheostat-like mechanistic relationship between PKM2 activity and serine biosynthesis. We show that serine can bind to and activate human PKM2 and that following serine deprivation, PKM2 activity in cells is reduced. This reduction in PKM2 activity shifts cells to a fuel-efficient mode where more pyruvate is diverted to the mitochondria and more glucose derived carbon is channelled into serine biosynthesis to support cell proliferation.