Protein structures adopt1 many different folds - or shapes. These can be classified under various schemes, but it is sometimes difficult to separate the historical name (globin for example) from a group. That is, a protein structure is solved and the fold is compared to others of its basic type (the highest level of description is usually that of the SSE composition). However, subsequent proteins with similar2 folds will tend to be classified under the name of the origional3. These 'attractors' that happened to be easy to crystallise or important pharmaceutical targets have biased some naming schemes.

In any case, protein structures are objects that defy proper classification since they have a blend of designs and might even contain parts of each other. It has been shown that single mutatations can alter the SSEs of a protein - and presumably alter the fold. Prions are the best known examples of proteins with more than one 'stable' fold (well, one stable and one 'meta-stable'). It seems likely that they are a rare exception, but it does show that fold is not a rigid platonic form that protein sequence aspires to - even though quite diverse sequences adopt the same fold.

Some common folds include:

1Sorry, this is slightly jargony. Meaning : 'to take on' as in 'adopt a style'.
2the question of 'what is a similar fold' is crucial. Intuitively it is something like structural superposition, but there are undoubtedly more sophisticated procedures.
3Which, kowing molecular biologists is bound to be someting ridiculous like son of sevenless.

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