Since enzymes are largely protein, enzyme characteristics are associated with the nature of proteins. The shape of the protein is important in determining the catalytic properties of the enzyme. Two common models that describe the specific shape of enzyme are the lock-in-key model [local] and (on the same page) the induced fit model [local]. The lock-in-key theory is the earlier model. It seems to imply that an enzyme had to be a perfect fit with the substrate molecule and that the enzyme shape was rigid. The modified lock-in-key model is the induced fit model which suggests the active site has a small amount of elasticity.

The shape of the protein and subsequent activity of the enzyme are effected by pH and temperature. Since enzymes have a specific shape some molecules can compete for the active site of the enzyme with the intended substrate.