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(a and b) Comparison of monomeric pepsin (a) with two symmetry-related lobes (61) and dimeric HIV proteinase (b) (4) inspired by the analyses of Blundell and coworkers (61–63). The arrows indicate the positions of approximate and perfect 2-fold axes relating domains (pepsin) and protomers (HIV proteinase), respectively. (c) The two topologically equivalent catalytic groups (the “hands” Asp-32 and Asp-215 of pepsin), which have an approximate 2-fold axis shown as an arrow.
