Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

88 Chapter 5
Other serine proteases such as trypsin, elastase and subtilisin have very
similar active site structures (Figure 5.7) and employ the same type of mechanism,
even though in the case of subtilisin the primary sequence of the enzyme
bears no resemblance to that of chymotrypsin. This may be a rare example of
convergent evolution, where the same active site structure has arisen from two
evolutionary origins, suggesting that this particular three-dimensional align-
(a)
(b)
(c)
Figure 5.7 Comparison of the structures of: (a) a-chymotrypsin (PDB Wle 5CHA, catalytic triad Asp-102, His-57,
Ser-195); (b) trypsin (PDB Wle 1BTX, catalytic triad Asp-102, His-57, Ser-195); and (c) subtilisin Carlsberg (PDB Wle
1AV4, catalytic triad Asp-32, His-64, Ser-125). In (a) and (b) the speciWcity loops are shown in red. Selectivity for a
basic side chain in trypsin is provided by Asp-189, which is found in a-chymotrypsin as Ser-189.