These concepts are schematically illustrated in Figure 3.16. Many enzymes are composed of discrete domains as well. For example, the crystal structure of the integral membrane enzyme prostaglandin synthase was recently solved by Garavito and his coworkers (Picot et al., 1994). The structure reveals three separate domains of the folded enzyme monomer: a -sheet domain that functions as an interface for dimerization with another molecule of the enzyme, a membrane-incorporated -helical domain that anchors the enzyme to the biological membrane, and a extramembrane globular (i.e., compact folded region) domain that contains the enzymatic active site and is thus the catalytic unit of the enzyme.