#Question id: 723
#Unit 1. Molecules and their Interaction Relevant to Biology
Common structural elements in proteins such as α-helices or β-sheets are characterized by uniformly repeating, energetically favorable main chain conformations which additionally exhibit a completely saturated hydrogen-bonding network of the main chain NH and CO groups. Although polyproline or polyglycine type II helices (PPII or PGII ) are frequently found in proteins, they are not considered as equivalent secondary structure elements. Which of the following statements are correct about PP and PGs.
A. PGII -like helices form hexagonal bundles which appear to fulfill the criterion of a (largely) saturated hydrogen-bonding network of the main-chain groups
B. main chain NH and CO groups of the central PGII -helix are saturated by either intra- or intermolecular hydrogen-bonds, resulting in a self-contained hydrogen-bonding network
C. The formation of the right-handed PPI helix is possible only with proline residues because of the required cis conformation.
D. PPII helices seem to be stabilized by main chain-water hydrogen bonds (in the absence of main chain- main chain H-bonds), and tend to have a regular pattern of hydrogen bonds with water. After this, it is not surprising, that PPII helices are found mostly on the protein surface.
#Question id: 724
#Unit 1. Molecules and their Interaction Relevant to Biology
In 1988, Richardson and Richardson observed that certain amino acid residues are statically favored as the amino-terminal caps for the alpha helices in 45 globular proteins. Which of the following statement stand correct with stability of helix?
A. The introduction of a negatively charged side chain at the N-cap, which can neutralize the partial dipole created by the unpaired amide protons, has been shown to increase stability
B. In particular, Gly is often observed at the both C-cap position and N-cap positions
C. In particular, Gly is often observed at the C-cap position only
D. In particular, Gly is often observed at the C-cap position, Asn at the N-cap position, Pro at the Ncap+1 and Asp and Glu at N2 and N3
#Question id: 725
#Unit 1. Molecules and their Interaction Relevant to Biology
At 25°C values of [θ]222, the mean residue ellipticity at 222 nm, are - 33,000 and -3,000 deg cm2 dmol-1 for a polypeptide existing in α- helical (α) and β-structure (β), respectively. If this polypeptide undergoes a two-state heat-induced α-β transition, and a value of [θ]222 = -15,000 deg cm2 dmol-1 is observed at 60°C, then this observation leads to the conclusion that the α helix conversion to β- structure is:
#Question id: 726
#Unit 1. Molecules and their Interaction Relevant to Biology
The structure of a protein is known from X-ray diffraction studies which gave 30% - helix, 50% -sheet and 20% random coil. Circular dichroism (CD) measurements gave 50% -helix, 40% -sheet and 10% random coil. What could not be a possible explanation for these observations.
#Question id: 727
#Unit 1. Molecules and their Interaction Relevant to Biology
26-residue peptide composed of alanine and leucine shows a circular dichroism (CD) spectrum characteristic of α-helix at 50°C in 5 mM phosphate buffer at pH 7.4. When the peptide solution is cooled gradually to 25°C, and the CD spectra are recorded at different temperatures, the most likely observation will be that
#Question id: 728
#Unit 1. Molecules and their Interaction Relevant to Biology
Match the following tile peaks with corresponding circular dichroism spectra
|
A. Alpha helix |
I. negative bands at 222 nm and 208 nm and a positive one at 190 nm. |
|
B. Beta sheet |
II. negative band at 218 nm and a positive one at 196 nm. |
|
C. Random coil |
III. positive band at 212 nm and a negative one around 195 nm. |