33. Phosphorylation of ADP to ATP occurs through energy metabolism, comprising oxidative phosphorylation or substrate level phosphorylation or photo- phosphorylation (in plants). ATP can also be formed from ADP through the action of adenylate kinase. Crystal structure determination of adenylate kinase shows that the C-terminal region has the sequence – val-asp-asp-val-phe-ser-gln-val-cys-thr-his-leu-asp-thr- leu-lys. What can be a possible conformation of the sequence?

    (1) A helix that is not amphipathic
    (2) Amphipathic helix
    (3) Leucine zipper helix
    (4) Beta helix

Structural Analysis of Adenylate Kinase: Predicting the Conformation of Its C-terminal Sequence

Adenylate kinase plays a crucial role in ATP metabolism by catalyzing the interconversion of adenine nucleotides. The structural features of this enzyme, particularly its C-terminal region, determine its function and stability. Understanding these features is essential for CSIR NET Life Science aspirants.

Correct Answer:

The correct option is (2) Amphipathic helix.

Explanation:

(1) A helix that is not amphipathic ❌

The given sequence contains both hydrophilic (Asp, Thr, Ser, Lys) and hydrophobic (Val, Phe, Leu) residues arranged in a manner that suggests an amphipathic nature. A purely hydrophilic or hydrophobic helix would lack this characteristic.

(2) Amphipathic Helix ✅

An amphipathic helix has distinct hydrophilic and hydrophobic sides. The presence of alternating hydrophobic (Val, Phe, Leu, Cys) and hydrophilic (Asp, Ser, Thr, Lys) residues supports the possibility of an amphipathic helix. This structural motif allows interactions with both hydrophobic and hydrophilic environments, aiding in protein-protein and membrane interactions.

(3) Leucine Zipper Helix ❌

Leucine zippers require a repeating Leu (L) residue at every seventh position, forming a dimerization domain in transcription factors. The given sequence does not exhibit this pattern.

(4) Beta Helix ❌

Beta helices are characterized by stacked beta-strands forming a spiral. The given sequence lacks the alternating polar and nonpolar pattern typical of beta-strand formation, making it unlikely to adopt a beta-helix conformation.

Nearby Topics for Better Understanding

1. Amphipathic Helices and Their Functional Significance

• Found in membrane proteins (e.g., transporters, receptors).
• Play roles in protein-protein interactions.
• Enable enzyme-substrate binding.

2. Structural Features of Adenylate Kinase

• Converts ADP to ATP using phosphate transfer.
• Contains helices and loops for structural flexibility.
• Regulates cellular energy balance.

3. Difference Between Alpha-Helix, Beta-Helix, and Leucine Zipper

4. Importance of Protein Secondary Structure in Function

• Determines binding specificity.
• Affects enzyme catalysis.
• Influences protein folding and misfolding (e.g., in diseases like Alzheimer’s).

Conclusion

The C-terminal sequence of adenylate kinase likely forms an amphipathic helix, facilitating interactions with both hydrophilic and hydrophobic regions. This structural insight helps in understanding protein folding and function, a key topic for CSIR NET Life Science aspirants.