Proteins – A‑Level Biology 9700Proteins
Learning Objective
Describe and draw the general structure of an amino acid and explain the formation and breakage of a peptide bond.
1. General Structure of an Amino Acid
An amino acid consists of a central (α) carbon atom bonded to four different groups:
- Amino group \$-\mathrm{NH2}\$ (or \$-\mathrm{NH3^+}\$ at physiological pH)
- Carboxyl group \$-\mathrm{COOH}\$ (or \$-\mathrm{COO^-}\$ at physiological pH)
- Hydrogen atom \$-\mathrm{H}\$
- Variable side chain \$-\mathrm{R}\$ which determines the identity of the amino acid
The α‑carbon is a chiral centre (except for glycine, where \$R = \mathrm{H}\$).
Suggested diagram: General structure of an α‑amino acid showing the α‑carbon, amino group, carboxyl group, hydrogen and side chain \$R\$.
2. Formation of a Peptide Bond (Condensation Reaction)
When two amino acids join, the carboxyl group of one reacts with the amino group of the next. The reaction is a dehydration (condensation) synthesis:
\$\mathrm{R1{-}CH(NH2)COOH + H2N{-}CH(R2)COOH \;\longrightarrow\; R1{-}CH(NH2)C(O)NH{-}CH(R2)COOH + H2O}\$
- The \$-\mathrm{OH}\$ from the carboxyl group and a \$-\mathrm{H}\$ from the amino group are lost as water.
- The resulting covalent link \$-\mathrm{C(O)–NH-}\$ is the peptide (amide) bond.
- The new molecule is called a dipeptide; further additions give polypeptides and proteins.
Suggested diagram: Formation of a peptide bond between two amino acids, showing the loss of water and the –C(O)–NH– linkage.
3. Breakage of a Peptide Bond (Hydrolysis)
Peptide bonds can be cleaved by hydrolysis, the reverse of condensation. Water adds across the bond, regenerating the original amino‑acid termini:
\$\mathrm{R1{-}CH(NH2)C(O)NH{-}CH(R2)COOH + H2O \;\longrightarrow\; R1{-}CH(NH2)COOH + H2N{-}CH(R2)COOH}\$
- Water provides a hydroxyl (\$\mathrm{OH^-}\$) to the carbonyl carbon and a proton (\$\mathrm{H^+}\$) to the nitrogen.
- The peptide bond is broken, yielding the free carboxyl and amino termini.
- Enzymes such as proteases catalyse hydrolysis in living organisms.
4. Summary Table
| Process | Reactants | Products | Key Feature |
|---|
| Peptide‑bond formation | Two amino acids | Dipeptide + \$H_2O\$ (released) | Dehydration (condensation) |
| Peptide‑bond hydrolysis | Dipeptide + \$H_2O\$ | Two amino acids | Hydrolysis, enzyme‑catalysed |
5. Key Points to Remember
- All amino acids share the same backbone; diversity comes from the \$R\$ side chain.
- The peptide bond is planar and has partial double‑bond character, giving rigidity to the polypeptide chain.
- Formation of peptide bonds is energetically unfavourable; cells couple it to ATP hydrolysis (via amino‑acyl‑tRNA synthetases).
- Hydrolysis of peptide bonds releases energy; it is the basis of protein catabolism.