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 dimmeric HIV proteinase which possesses similar activities
4. zinc fingers
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