Many scientists who studied enzymes in the 1800's studied reactions caused by yeast enzymes. An American biochemist, James B. Sumner, was the first person to isolate a pure enzyme in the form of crystals. In 1926, he extracted the enzyme urease from beans and proved that enzymes are protein molecules. In 1969, scientists first chemically synthesized an enzyme, ribonuclease, from amino acids. Today, scientists are trying to make new synthetic enzymes that carry out reactions not found in nature.
Enzymes are too tiny to be seen even with the most powerful light microscopes.
However, scientists know through various research techniques that enzymes occur in a number of shapes and sizes. Although enzymes of different plants and animals have different protein structures, they function in similar ways. The structure of any particular enzyme enables it to cause chemical reactions in other molecules. Scientists sometimes change the structure of enzyme molecules to study how altered enzymes function or to make enzymes carry out different chemical reactions. An enzyme's structure can easily be destroyed by heat, acids, or alkalis.