James Batcheller Sumner was an American biochemist. He discovered that enzymes can be crystallized, for which he shared the Nobel Prize in Chemistry in 1946 with John Howard Northrop and Wendell Meredith Stanley. He was also the first to prove that enzymes are proteins.

Enzymes are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps. The study of enzymes is called enzymology and the field of pseudoenzyme analysis recognizes that during evolution, some enzymes have lost the ability to carry out biological catalysis, which is often reflected in their amino acid sequences and unusual 'pseudocatalytic' properties.