Edwin Gerhard Krebs (1918 - present) was awarded half of the Nobel Prize in Physiology or Medicine in 1992 for describing how reversible phosphorylation -- the addition of a phosphate group to a protein -- works as a switch to activate proteins and regulate various cellular processes. Krebs, along with fellow Nobel Laureate Edmond Fischer, began investigating the process by which muscle cells obtain energy from glycogen in the 1950s. Gerty and Carl Ferdinand Cori had previously shown that cells use an enzyme called phosphorylase to release glucose from glycogen. Krebs and Fischer showed that phosphorylase could be converted from an inactive to an active form by the addition of a phosphate group taken from adenosine triphosphate. The reaction is catalyzed by an enzyme called protein kinase. Krebs and Fischer also showed that phosphorylase is inactivated by the removal of a phosphate group in a process catalyzed by another enzyme called phosphatase.