Which of the following best distinguishes the hydrogen bonding patterns in alpha helices from those in beta sheets?

Question

BlackTom AI · Accepted Answer

To tackle this question, I’ll compare what characterizes hydrogen bonding in alpha helices versus beta sheets and then evaluate each statement.
Option 1: 'In alpha helices, hydrogen bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of an amino acid four residues ahead in the sequence, whereas in beta sheets, hydrogen bonds form between backbone groups of amino acids that are distant in the primary sequence but align side-by-side.' This aligns with the classic textbook description: within an alpha helix, the C=O of residue i bonds with the N–H of residue i+4, giving a stable helical turn. In beta sheets, hydrogen bonds form between backbone amide and carbonyl groups of neighboring strands, which may be distant in the sequence but are aligned in the sheet, enabling interstrand H-bonds. This option correctly captures the distinct spacing and pattern of H-bonds for the two motifs.
Option 2: 'Both alpha helices and beta sheets form hydrogen bonds only between immediately adjacent amino acids in the sequence.' This is incorrect because, in alpha helices, the i to i+4 hydrogen bonding involves residues separated by three intervening residues, not immediate neighbors. In beta sheets, contacts occur between residues on different strands that can be far apart in the primary sequence, not simply adjacent in sequence.
Option 3: 'Alpha helices rely on hydrogen bonds formed within a single turn of the helix, while beta sheets do not involve hydrogen bonding at all.' This is false: beta sheets are stabilized by inter-strand hydrogen bonds between backbone groups; beta sheets do involve hydrogen bonding. The claim that beta sheets lack H-bonding is a fundamental misunderstanding.
Option 4: 'Hydrogen bonding in alpha helices occurs exclusively on the external surface of the helix, whereas in beta sheets it occurs solely within the protein’s core.' This is not accurate: hydrogen bonds in alpha helices occur along the helix interior along the backbone, not limited to an external surface, and beta-sheet hydrogen bonds are between strands and can be exposed or buried depending on folding; the statement misrepresents the spatial distribution.
Option 5: 'In both alpha helices and beta sheets, hydrogen bonds form randomly along the backbone, with no distinct pattern based on the sequence order.' This is incorrect because there is a very clear, highly regular pattern: i to i+4 hydrogen bonds in helices and organized interstrand hydrogen bonding in sheets, not a random distribution.
In summary, option 1 accurately reflects the established distinction in H-bonding patterns between alpha helices and beta sheets, while options 2, 3, 4, and 5 contain specific inaccuracies about adjacency, presence of H-bonds, spatial distribution, and patterns along the backbone.

Which of the following best distinguishes the hydrogen bonding patterns in alpha helices from those in beta sheets?单项选择题

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