Hair is comprised largely of keratin, a fibrous protein consisting of two a-helices wrapped around each other in a left-handed coiled-coil. The diagram below compares a partial alignment of the amino acid sequences of a form of human keratin and of ovine keratin. For each pair of dissimilar aligned amino acids, (i) indicate if a side chain is associated with a discrete charge by writing or (at pH 7) adjacent to the letter; and (ii) indicate whether each amino acid is polar (p) or apolar (a) in the case of aligned, electrostatically-neutral, side chains. In the diagram, 219 and 158 designate se-quence positions of the first aligned residue in each polypeptide strand. human II 5 219 ovine II 7c 158 FEQYINNLRRQLDSIVGERGRLDSELRNMQDLVEDFKNKYEDEINKRTTAE FEGYIETLRREAECVEADSGRLSSELNHVQEVLEGYKKKYEQEVALRATAE

What would you predict about the ratio of hydrophilic to hydro- phobic amino acid residues in a series of monomeric globular proteins that range in size from a molecular weight of 10,000 g/mole to a molecular weight of 100,000 g/mole? Note that the volume of a sphere is 4/3nr³, while the sur- face area of the outside of a sphere is 4пr².

Histidine is an amino acid with three titratable groups: an -NH3+ group (pKa = 9.2), a -COOH group (pKa = 1.8), and an imidazole (amine-like) group (pKa = 6.0). The titration curve for histidine is shown below with four points highlighted. (a) Identify which point on the titration curve corresponds to the pKa for each of the titratable groups, and which point corresponds to the pI. Explain your choices. (b) Calculate the value of pI for histidine

Consider a protein in which a negatively charged glutamic acid side chain (pKa = 4.2) makes a salt bridge (ion-ion interaction) with a positively charged histidine side chain (pKa = 6.5). Part A Do you predict that this salt bridge will become stronger, become weaker, or be unaffected as pH increases from pH = 7.2 to pH = 7.8? The salt bridge will become stronger. The salt bridge will become weaker. The salt bridge will be unaffected. Submit Part B Previous Answers Correct At pH = 7.2 the glutamic acid (Glu) side chain will carry a charge of ~ -1 (at 3 pH units above the pKa for Glu, the side chain will be almost fully ionized); whereas the histidine (His) side chain will carry a charge of < +0.5 (at pH = pK₂ the charge on His would be +0.5; since pH = 7.2 is above its pKa, it will carry less (+) charge as it becomes more deprotonated). As the pH increase to 7.8, the charge on Glu will remain ~ -1 and the charge on His will decrease; thus, this salt bridge is predicted to become weaker…