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Domain repeats
A multi-domain architecture offers the potential of reuse trough domain repeats. Domain
duplication is essential for the formation of the active sites of enzymes such as the aspartic
proteinases, the serine proteinases and porphobilinogen deaminase or the binding sites of
transport proteins such as lactoferrin, transferrin and sulfate binding protein.
Domain repeats can also allow duplication of active sites or binding sites such as the
calcium binding sites in calmodulin or the 9 DNA-binding zinc fingers of the transcription
factor TFIIID.
A few examples:
1. an immunoglobulin Fab fragment which contains two ig
constant domains and two ig variable domains
2. the two lobes of lactoferrin, each being composed of two
similar domains
3. the bi-lobal monomeric aspartic proteinase, pepsin, and the
dimeric HIV proteinase which possesses similar activities
4. zinc fingers
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