A protein fold is an arrangement of secondary structure element of a domain or protein. Alpha helices sandwiching a beta sheet, that is a fold.
A domain in an independently folding unit of a protein, that is, a part of the protein that can fold itself into the same thing independent of the rest of the protein.
A motif is a commonly repeating arrangement of a few secondary structure element.
The number of protein folds discovered plateau at about 1,500 folds, but there are still many protein structures solved every year, this implies it is no a solving problem, but rather there is simple a limited number of folds available in nature (the upcoming question basically acknowledged that)
We had an interesting case of myoglobin and hemoglobin, sharing only 30% of primary structure (i.e. sequence) but yet have remarkably similar globular fold tertiary structure, this implies proteins of highly divergent sequences can still adopt the same fold. We have found many protein structures, implying nature does sample a lot of possible arrangements, and the linearity does not limit the number of combination in any way because of combinatoric explosion. The last choice seems plausible, if there is only a limited number of stable fold, we might already sampled all of them.
The correct answer of this one is
Proteins of highly divergent sequences can still adopt the same fold.
It is likely that there are a limited number of stable, functional protein fold, nature utilizes most of them.
The mystery of this question is the first choice, I feel like it is a correct statement, yet it is not part of answer, probably because it is simply not a "reason" why we have limited number of folds, not because it is a wrong statement, but I don't know.
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