Protein structures have several different levels of organization. The primary structure of a protein is its amino acid sequence. The secondary and tertiary structures are more complicated. Consider the definitions below and select the one that best fits the term "protein domain."
(a) a small cluster of a helices and ß sheets
(b) the tertiary structure of a substrate-binding pocket
(c) a complex of more than one polypeptide chain
(d) a protein segment that folds independently
Answer: D