4. Which of the following amino acids to you expect to find in higher proportion within
histones compared to a cytoskeletal protein?
a. K and R
b. L and A
c. D and E
d. F and W
The correct answer is a. K and R (lysine and arginine), because histones are rich in basic, positively charged amino acids that interact tightly with the negatively charged DNA backbone, unlike most cytoskeletal proteins which are not as enriched in these residues.
Introduction: Histones vs Cytoskeletal Proteins
Histones are small, highly basic nuclear proteins that bind and package DNA into nucleosomes, and they are characteristically rich in the basic amino acids lysine (K) and arginine (R). Cytoskeletal proteins such as actin and tubulin primarily provide structural support and motility and do not show such a strong enrichment of positively charged residues, instead having more mixed or neutral amino acid compositions. This difference in amino acid composition underlies the correct choice in the MCQ about which amino acids are expected in higher proportion within histones compared to a cytoskeletal protein.
Correct Option: a. K and R
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K (lysine) and R (arginine) are basic amino acids whose side chains are positively charged at physiological pH, making histones overall highly basic proteins.
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DNA is negatively charged due to its phosphate backbone, so the positively charged lysine and arginine residues in histones form strong electrostatic interactions with DNA, enabling tight binding and efficient packaging of chromatin.
In core histones (H2A, H2B, H3, H4) and linker histone H1, lysine and arginine are present in high proportions, which is repeatedly emphasized in cell biology and genetics texts as the reason histones are described as “highly basic” proteins. In contrast, typical cytoskeletal proteins like actin and tubulin do not show such selective enrichment of K and R because their primary function is structural support and motility rather than binding to polyanionic DNA.
Option b: L and A (Leucine and Alanine)
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Leucine (L) is a hydrophobic, aliphatic amino acid frequently found in the interior of globular proteins and in transmembrane helices, contributing to hydrophobic core stability rather than electrostatic interactions with DNA.
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Alanine (A) is a small, nonpolar amino acid that is chemically neutral at physiological pH and does not provide a positive charge for DNA binding.
Many proteins, including cytoskeletal proteins, contain substantial amounts of hydrophobic residues like leucine and alanine for structural stability, but these amino acids do not explain the strong DNA-binding capacity and basic character of histones. Therefore, while L and A may be present in histones to some extent, they are not present in higher proportion compared to cytoskeletal proteins in a way that defines histone function, making option b incorrect in this context.
Option c: D and E (Aspartate and Glutamate)
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Aspartate (D) and glutamate (E) are acidic amino acids that carry negative charges on their side chains at physiological pH.
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If histones were enriched in acidic residues like D and E, their net charge would be negative or less positive, which would repel the negatively charged DNA instead of promoting tight association.
Indeed, it is explicitly noted in many explanations that if histone proteins were rich in acidic amino acids rather than basic ones, DNA packaging into chromatin would be severely compromised because two negatively charged species would repel each other. Hence, option c is directly opposite to the known chemistry of histone–DNA interaction and is clearly incorrect.
Option d: F and W (Phenylalanine and Tryptophan)
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Phenylalanine (F) and tryptophan (W) are bulky, aromatic, hydrophobic amino acids, important for hydrophobic packing and sometimes for specific binding pockets, but they are uncharged at physiological pH.
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While aromatic residues can contribute to protein stability and sometimes to stacking interactions with nucleic acids, they do not confer the strong positive charge density required for tight, non‑specific electrostatic binding to DNA along its phosphate backbone.
Cytoskeletal and many structural proteins commonly contain aromatic residues for stability, but histone identity and function are not defined by enrichment in F and W; instead, they are defined by the abundance of basic residues K and R. Therefore, option d is also incorrect relative to what distinguishes histones from typical cytoskeletal proteins.
Why Histones Need Lysine and Arginine
Histones are described as “highly basic proteins” specifically because they contain a high proportion of lysine and arginine residues, which are positively charged at nuclear pH. These positively charged side chains neutralize and bind the negatively charged DNA phosphate groups, enabling DNA to wrap around histone octamers and form nucleosomes that compact the eukaryotic genome. Additionally, many key epigenetic modifications—such as acetylation and methylation—occur on the ε‑amino group of lysine and on the guanidinium group of arginine in histone tails, making these residues central to chromatin regulation and gene expression.
