The following are examples of active site:

Example 1: Chymotrypsin active site

The proteolytic enzyme chymotrypsin cleaves peptide chains on the carboxyl-terminal side of an aromatic amino acid subunit. In this surface contour image of chymotrypsin, the active site is shaded red and a bound tripeptide substrate is shown as a red stick structure.


Example 2: Chymotrypsin active site

Specific residues in the active site of chymotrypsin position the substrate and act as nucleophiles and electrophiles to promote cleavage of the peptide bond. In this close-up view of the active site, the side chains of Ser195 and His57 are shown as stick structures, and the substrate is shown as a green stick structure. Note the pocket on the surface of the enzyme that fits the aromatic side chain of the substrate (blue).


Example 3: The active site promotes catalysis by lowering activation energy

Amino acid side chains in the active site of an enzyme, interact with the substrate(s), lowering the activation needed to reach the transition state, and thereby promoting catalysis. This reaction coordinate diagram compares the progress of an uncatalyzed (black) and an enzyme-catalyzed reaction (blue). Much less energy is needed to reach the transition state in the catalyzed reaction.



The following are examples of binding sites similar to active sites, which may be confused with active sites.

Not an example Antigen-antibody binding

Antibodies bind their antigens with great affinity and specificity, but the binding does not lead to a chemical transformation. This human class I MHC protein, shown as a surface contour model in a top view, tightly and specifically binds its substrate, an HIV-derived peptide (shown in red), but does not covalently alter it. The antibody is not an enzyme; the peptide is not a substrate; the binding site is not an active site.


Not an example Active sites and allosteric sites in phosphofructokinase-1

Allosteric regulators bind to sites other than the active site. Two of the four identical subunits of phosphofructokinase-1 are shown as red and gray ribbon models. At each of the two active sites, the substrates ADP (blue) and fructose 1,6-bisphosphate (yellow) are nearly in contact. ADP also acts as an allosteric modulator of the enzyme action, binding to allosteric sites created at the interface between subunits.