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Aspartic acid proteinase inhibitor SQAPI, aspartic acid inhibitor first isolated from squash, inhibits a wide range of aspartic proteinases. This particular family of PAAPIs (proteinaceous aspartic acid inhibitors) seems to have evolved quite recently from an ancestral cystatin. Structurally it consists of a four-stranded anti-parallel beta-sheet gripping an alpha-helix in much the same manner that a hand grips a tennis racket. The unstructured N-terminus and the loop connecting beta-strands 1 and 2 are important for pepsin inhibition, but the loop connecting strands 3 and 4 is not. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",