Question

Peptide bond in proteins is strutcurally represented as

• A. -CO-NH-
• B. -COO-NH-
• C. -C-O-C-
• D. -C-O-CO-

Hint:

Key linkages in biomolecules:
- Proteins: Peptide linkage (\(-\text{CO}-\text{NH}-\)).
- Carbohydrates: Glycosidic linkage (\(-\text{C}-\text{O}-\text{C}-\)).
- Nucleic acids: Phosphodiester linkage.

Answer 1

The Correct Option is A

Concept:
Proteins are linear polymers of \(\alpha\)-amino acids connected by peptide bonds.
A peptide bond is a covalent amide linkage formed by a condensation reaction between adjacent amino acid residues.

Step 1: Condensation of Amino Acid Residues:
When two \(\alpha\)-amino acid molecules react, the carboxylic acid group (\(-\text{COOH}\)) of one amino acid condenses with the amino group (\(-\text{NH}_2\)) of the neighboring amino acid.
This condensation eliminates a molecule of water:
\[ \text{H}_2\text{N}-\text{CHR}_1-\text{COOH} + \text{H}_2\text{N}-\text{CHR}_2-\text{COOH} \rightarrow \text{H}_2\text{N}-\text{CHR}_1-\overset{\text{O}}{\overset{\parallel}{\text{C}}}-\text{NH}-\text{CHR}_2-\text{COOH} + \text{H}_2\text{O} \]

Step 2: Structural Characteristics of the Peptide Linkage:
The resulting amide bond connecting the two amino acid units is represented as:
\[ -\text{CO}-\text{NH}- \] Resonance between the carbonyl group and the nitrogen atom gives the peptide bond partial double bond character:
\[ -\overset{\text{O}}{\overset{\parallel}{\text{C}}}-\overset{\cdot\cdot}{\text{N}}\text{H}- \longleftrightarrow -\overset{\text{O}^-}{\overset{\parallel}{\text{C}}}=\overset{+}{\text{N}}\text{H}- \] This partial double bond character makes the peptide bond planar, rigid, and resistant to rotation, which is important for the folding of protein secondary structures.

Step 3: Verification of Other Options:
- Option (B), \(\text{-COO-NH-}\), represents a nonexistent or unstable carbamate-like linkage.
- Option (C), \(\text{-C-O-C-}\), represents an ether or glycosidic bond.
- Option (D), \(\text{-C-O-CO-}\), represents an acid anhydride linkage.
Final Answer:
The peptide bond in proteins is represented as -CO-NH-, corresponding to option (A).