Question

(18 pts) Answer True or False for each of the following questions (a) An amino acid is in the zwitterionic form only if the pll is well below 7 (b) Salting out takes advantage of the fact that the solubility of proteins varies with concentration. (c) Proteins usually consists of amino acids of both L-isomers (d) Essentially all a-helices found in proteins are left-handed globular protein is thermodynamically the most stable structure. ( Isoclectric focusing can be used to separate proteins based on their pl values and D-isomers (e) Christian Anfinsen's experiment using ribonuclease proves that the native conformation of a (B) DNA is a linear polymer composed of monomers consisting of deoxyribose sugar, a phosphate and one of four bases. (Ch) In gel-filtration chromatography, small proteins move rapidly through the column, while large proteins move slowly () Edman degradation is an accurate method to sequence polypeptides which are longer than 50 amino acids. D. (8 pts) Answer each of the following questions regarding the tetrapeptide Asp-Cys- Ala-Leu Amino acidpk Aspartic acid Cysteine Ala Leu Anporia mad 19 10.78 9.87 9.62 08% 8.33 1.92 2.32 a) Name the carboxy-terminal residue. b) Give the net charge at pH 1. c) Give the net charge at pH 12 The peptide contains four of His and seven of Asp residues. Estimate the peptide's net charge at plH 6.0. Explain. E. (4 pts) A soluble peptide has two types of charged side chains: His (pk 6.00) and Asp (pKR 3.9).

Answer 1

a. False - In case of Histidine the isoelectric point (PI) or pH at which it exists as zwitterion form is 7.59 (Slightly Basic).

b. True

c. False - Usually proteins consist of only L-amino acids. D-amino acids are toxic for life.

d. False - $\aa$ -helix which is a common motif in the secondary structure of a protein is right-handed.

e. True

f. True - Isoelectric focusing is used to separate charged molecules like usually proteins and peptide.

g. True

h. False - In gel filtration, bigger molecule moves faster since less volume is accessible. The smaller molecule stuck inside the beads, so they move slower.

i. False- By Edman degradation, we can sequence polypeptide which has less than 50 residues/amino acids (in practice 30 residues/ amino acids).