Hugo Theorell (1903-1982) was awarded the 1955 Nobel Prize in Physiology or Medicine for his discoveries about the nature and mode of action of oxidation enzymes. He was the first to isolate crystalline myoglobin – a protein that is structurally related to blood-borne hemoglobin and is the primary oxygen-carrying pigment of muscle tissues -- and the “old yellow enzyme” (now called NADPH hydrogenase) which is necessary for the oxidative interconversion of sugars by cells. Theorell found that the enzyme contained a nonprotein coenzyme--the yellow riboflavin (vitamin B2) phosphate--and a protein apoenzyme. His discovery that the coenzyme facilitates glucose oxidation by binding a hydrogen atom at a specific site on the riboflavin molecule marked the first time that the effect of an enzyme was attributed to the chemical activity of specific atoms in the enzyme. Later, Theorell studied cytochrome c, a protein located in the inner membrane of mitochondria, showing that its core was made of an iron atom surrounded by a cyclic macromolecule called porphyrin. His investigation of alcohol dehydrogenase led to the development of sensitive blood tests that have helped legally define alcohol intoxication.