Question

The \(\alpha\) -helical parts of myoglobin and other proteins stop whenever a proline residue is encountered in the chain. Why is proline never present in a protein \(\alpha\) helix?

Short answer

Proline's rigid structure disrupts the alpha helix formation.

Step-by-step solution

Understanding the Alpha Helix Structure

The alpha helix is a common structural motif in proteins. It is a right-handed coil where each amino acid corresponds to a 100-degree turn in the helix. The stability of the alpha helix is due to hydrogen bonds formed between the carbonyl oxygen of one amino acid and the amide hydrogen of another amino acid four residues away.

Role of Proline in Protein Structure

Proline is unique among the amino acids because its side chain forms a cyclic structure, bonding back to the nitrogen in the amino acid backbone. This creates a rigid ring that limits the rotational freedom of the preceding peptide bond.

Effect of Proline's Structure on the Helix

Proline's rigid structure introduces a kink in the protein chain, disrupting the alpha helix. It also lacks the amide hydrogen to participate in the necessary hydrogen bonding, which is critical for the helix stability.

Conclusion on Proline in Alpha Helices

Proline's cyclic structure and inability to form hydrogen bonds are why it is rarely found in alpha helices. When encountered, proline causes the helix to bend or terminate.

Key concepts

Understanding the Alpha Helix

Proteins are complex molecules that fold into specific shapes to perform their functions. One such shape, or structure, often seen in proteins, is the alpha helix. It resembles a tightly-coiled spring or helix. The alpha helix is stabilized by hydrogen bonds, which are critical for maintaining its shape.
These bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another amino acid located four residues earlier in the chain.
Each turn of the helix involves 3.6 amino acids, creating a consistent spiral structure. This right-handed helical twist is an elegant solution nature has found to pack amino acids tightly while allowing proteins to be both strong and flexible. The predictability of this structure makes the alpha helix a common feature in many proteins.

• Helices are right-handed and have 3.6 amino acids per turn.
• Stability is achieved through hydrogen bonds between amino acids.
• Alpha helices are common in proteins due to their stable structure.

The Importance of Hydrogen Bonding

Hydrogen bonding plays a vital role in stabilizing protein structures like the alpha helix. These bonds occur when a hydrogen atom is weakly attracted to a more electronegative atom, such as oxygen or nitrogen. In the alpha helix, hydrogen bonds provide the necessary stability by linking the peptide chain together at regular intervals.
These bonds help to maintain the helical shape, preventing the structure from unraveling. Without these interactions, the protein would lose its functional form. Hydrogen bonding is not just limited to the alpha helix; it is an essential feature in many protein structures.

• Hydrogen bonds occur between carbonyl oxygen and amide hydrogen in the helix.
• They provide stability and maintain the protein's functional form.
• Hydrogen bonding is crucial for various secondary structures in proteins.

Amino Acid Structural Properties and Proline's Role

Each amino acid in a protein has unique structural properties that affect how it behaves in sequences. Proline is an unusual amino acid due to its distinctive cyclic structure. Its side chain bonds back to the nitrogen atom in the backbone, forming a rigid ring.
This makes proline unique because it restricts the rotation of the peptide bond before it. Such rigidity introduces a kink when proline occurs in an otherwise flexible alpha helical chain, disrupting the regular hydrogen bonding pattern essential for helix integrity.
Moreover, proline lacks an amide hydrogen required for forming hydrogen bonds, a critical component keeping the helix intact. Consequently, protein sequences often avoid placing proline within their alpha helices, as its presence may cause the helix to kink or terminate prematurely, disrupting the overall protein structure.

• Proline's cyclic structure restricts bond rotation, introducing a kink.
• It cannot contribute to hydrogen bonding in the helix.
• Proline's unique properties often mark the end of alpha helical segments.