Passage
Many cephalopods can change color to blend in with their surroundings by altering the conformations of particular proteins within their skin cells. The reflectin proteins play a major role in this camouflage mechanism. Reflectins are a group of intrinsically disordered proteins that do not adopt clearly defined secondary or tertiary structures. Under certain conditions reflectins aggregate, allowing cells to reflect various wavelengths of light.Skin cells from the squid Doryteuthis opalescens were cultured and three reflectins-DoRA, DoRB, and DoRC-were purified and analyzed to determine amino acid composition. They were found to contain an abundance of arginine and serine but relatively few cysteine, leucine, isoleucine, valine, phenylalanine, or tryptophan residues. Figure 1 shows the relative hydrophobicity of each reflectin variant as a function of amino acid position.
Figure 1 Hydrophobicity plots of three reflectins isolated from D. opalescens. (Note: Negative numbers indicate hydrophilic regions.) In D. opalescens skin cells (pH ≈ 7.8) , reflectins aggregate in response to acetylcholine (ACh) , which causes a signal cascade that changes the phosphorylation state of the proteins. Cells were incubated with or without ACh, and DoRA and DoRB were purified and assessed for phosphorylation by mass spectrometry. DoRA became phosphorylated in the presence of ACh whereas DoRB became dephosphorylated. After examining the isoelectric points of unmodified DoRA and DoRB, as shown in Figure 2, scientists proposed that reflectins aggregate upon charge neutralization.
Figure 2 Isoelectric focusing of DoRA and DoRB
Adapted from Demartini DG, Izumi M, Weaver AT, Pandolfi E, Morse DE. Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells. J Biol Chem. 2015;290(24) :15238-49.
-In the absence of ACh, which of the following bond types best characterizes the structure of reflectins?

Question

Passage
Many cephalopods can change color to blend in with their surroundings by altering the conformations of particular proteins within their skin cells. The reflectin proteins play a major role in this camouflage mechanism. Reflectins are a group of intrinsically disordered proteins that do not adopt clearly defined secondary or tertiary structures. Under certain conditions reflectins aggregate, allowing cells to reflect various wavelengths of light.Skin cells from the squid Doryteuthis opalescens were cultured and three reflectins-DoRA, DoRB, and DoRC-were purified and analyzed to determine amino acid composition. They were found to contain an abundance of arginine and serine but relatively few cysteine, leucine, isoleucine, valine, phenylalanine, or tryptophan residues. Figure 1 shows the relative hydrophobicity of each reflectin variant as a function of amino acid position.

Figure 1 Hydrophobicity plots of three reflectins isolated from D. opalescens. (Note: Negative numbers indicate hydrophilic regions.) In D. opalescens skin cells (pH ≈ 7.8) , reflectins aggregate in response to acetylcholine (ACh) , which causes a signal cascade that changes the phosphorylation state of the proteins. Cells were incubated with or without ACh, and DoRA and DoRB were purified and assessed for phosphorylation by mass spectrometry. DoRA became phosphorylated in the presence of ACh whereas DoRB became dephosphorylated. After examining the isoelectric points of unmodified DoRA and DoRB, as shown in Figure 2, scientists proposed that reflectins aggregate upon charge neutralization.

Figure 2 Isoelectric focusing of DoRA and DoRB
Adapted from Demartini DG, Izumi M, Weaver AT, Pandolfi E, Morse DE. Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells. J Biol Chem. 2015;290(24) :15238-49.
-In the absence of ACh, which of the following bond types best characterizes the structure of reflectins?

Quizplus · Accepted Answer

11ecba2d_0ed8_64c9_a3bf_13b4de8803c4_MD0008_00 Protein structure is organized in four levels as follows: Primary structure is the sequence of amino acids in a polypeptide chain. The amino acids are linked together by covalent bonds between the carbonyl carbon of one amino acid and the backbone nitrogen of another, called peptide bonds. Secondary structure is the set of local folds within a polypeptide and usually consists of α-helices and β-sheets. These structures are primarily stabilized by hydrogen bonds between the carbonyls and amides of the polypeptide backbone. The passage states that reflectins have few defined secondary structures, so hydrogen bonds between backbone carbonyls and amides are rare in reflectins (Choice A). Tertiary structure is the overall three-dimensional structure of the protein and is stabilized by interactions between R groups, including hydrogen bonding, ionic, and hydrophobic interactions as well as disulfide bonds. The passage states that reflectins do not have well-defined tertiary structures, and therefore hydrogen bonds between R group alcohols are rare (Choice C). The passage also states that reflectins have few cysteine residues, so covalent bonds between sulfur atoms (disulfide bonds) are also rare (Choice B). Quaternary structure is the interaction of two or more polypeptide chains with each other and is stabilized by the same forces as tertiary structure. The aggregation of reflectins may be thought of as a type of quaternary structure, but the question asks for bond types in the absence of ACh, in which aggregation does not occur. Because reflectins do not fold, they only participate in primary structure, characterized by the covalent carbon-nitrogen linkages of peptide bonds. Educational objective: Protein structure is organized into primary, secondary, tertiary, and quaternary levels. Primary structure is the sequence of amino acids linked together by peptide bonds. Secondary structure consists of local structures such as α-helices and β-sheets stabilized by backbone hydrogen bonds. Tertiary and quaternary structures describe the three-dimensional form of a protein, mediated by hydrophobic, ionic, and hydrogen bond interactions.

Passage Many Cephalopods Can Change Color to Blend in with Their