Carboxypeptidase: the Active Site

Figure 1. Location of the active site in an enzyme. The folding of the long chain of amino acids of carboxypeptidase brings together the zinc atom and three of the four amino acids that bind to the substrate at the active site, even though - as their numbers indicate - they are located at different places in the chain. The zinc atom itself is held in place by three different amino acids - nos. 69, 72, and 196 (not shown). The fifth part of the site folds into place (arrow) when the substrate binds to the enzyme. The region in dark pink is the active site, while the blue area is the hydrophobic entrance to the cleft. [source: Keeton and Gould, 1986. Biological Sciences, 4th ed. p. 80]

Figure 2. Model of an active site of an enzyme. Shown here in schematic form is the base of the cleft where the active site of carboxypeptidase is located. (The hydrophobic entrance to the cleft is out of the drawing, to the bottom. The entire enzyme, with the active site highlighted is depicted in the figure above.) Part of a substrate molecule is shown in the cleft, linked to the enzyme by five weak bonds (red bands). Seven of the amino acids of the active site are indicated by their abbreviated names; the numbers beside the names refer to the positions of the amino acids in the enzyme polypeptide chain. The function of this enzyme is to separate the terminal amino acid (top) from the amino acid chain (extending down out of the figure) at the covalent bond indicated by the arrow. The highly electronegative zinc ion and the charged oxygen draw away the electrons of this bond and so initiate its rupture. [source: Keeton and Gould, 1986. Biological Sciences, 4th ed. p. 79]

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