Normal Hemoglobin Is a Tetramer, Consisting of Two Molecules of β

Normal hemoglobin is a tetramer, consisting of two molecules of β hemoglobin and two molecules of α hemoglobin. In sickle-cell disease, as a result of a single amino acid change, the mutant hemoglobin tetramers associate with each other and assemble into large fibers. Based on this information alone, we can conclude that sickle-cell hemoglobin exhibits ________.

A) only altered primary structure
B) only altered secondary structure
C) only altered tertiary structure
D) only altered quaternary structure
E) altered primary structure and altered quaternary structure; the secondary and tertiary structures may or may not be altered