Several protein domain are thought to have a purely structural role, allowing functional domains to be advantageously situated or providing the overall protein with the necessary flexibility. But beware, a domain may be considered to have a purely structural role be simply because of the lack of knowledge about its true function. The immunoglobulin domains family provides examples of this such as the constant heavy 1 and 2 domains and the constant light domain of immunoglobulin.
A few examples:
1. an immunoglobulin Fab fragment which contains two "spacer" ig constant domains and two "functional" ig variable domains
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