1. Indicate the probable effects of the following mutational changes on the structure of a protein. Provide a brief rationale for your answer. (a) Changing a Leu for a Phe
(b) Changing a Lys for a Glu
(c) Changing a Val for a Thr
(d) Changing a Gly for an Ala
2. For each of the statements below, indicate if it pertains best to primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) protein structure.
(a) Limited trypsin treatment of protein A generates two polypeptide fragments, one with ATP-binding activity and the other with Ca2+-binding activity; both fragments can be fractioned on a gel filtration column and show activity.
(b) The iron-binding protein ferritin has a native molecular weight of -450,000 and consists of 12 heavy chains and 12 light chains which when assembled exhibit two-fold, three-fold and four-fold subunit symmetry.
(c) Normal hemoglobin and sickle-cell hemoglobin differ only by a single amino acid substitution, a Val for Glu6.
(d) A 310 helix is more elongated than an a-helix which, in turn, is more elongated than a at-helix.
(e) The cleft formed between two pairs of -helices in a four-helix bundle often comprises the active sites of enzymes.
(f) The enzyme urokinase, which is involved in dissolution of blood clots, is a single polypeptide that contains two serine protease domains, several Kringle domains, and an epidermal growth factor-like domain.
(g) An hydropathy plot of cytochrome c shows three distinct hydrophobic segments and four hydrophilic segments.
(h) An immunoglobulin G molecule contains two 50,000 Da heavy chains and two 25,000 Da light chains that are held together by multiple disulfide linkages to form a native protein with a molecular weight of 150,000 Da.