Question

Oxytocin, a nonapeptide hormone secreted by the pituitary gland, functions by stimulating uterine contraction and lactation during childbirth. Its sequence was determined from the following evidence: 1\. Oxytocin is a cyclic compound containing a disulfide bridge between two cysteine residues. 2\. When the disulfide bridge is reduced, oxytocin has the constitution \(\mathrm{N}, \mathrm{C}_{2}, \mathrm{Q}, \mathrm{G}, \mathrm{I}, \mathrm{L}, \mathrm{P}, \mathrm{Y}\) 3\. Partial hydrolysis of reduced oxytocin yields seven fragments: \(\mathrm{D}-\mathrm{C}, \mathrm{I}-\mathrm{E}, \mathrm{C}-\mathrm{Y}, \mathrm{L}-\mathrm{G}, \mathrm{Y}-\mathrm{I}-\mathrm{E}, \mathrm{E}-\mathrm{D}-\mathrm{C}, \mathrm{C}-\mathrm{P}-\mathrm{L}\) 4\. Gly is the C-terminal group. 5\. Both \(\mathrm{E}\) and \(\mathrm{D}\) are present as their side-chain amides \((\mathrm{Q}\) and \(\mathrm{N}\) ) rather than as free side-chain acids. What is the amino acid sequence of reduced oxytocin? What is the structure of oxytocin itself?

Short answer

Reduced oxytocin: NCYIQNCPGL. Cyclic form: CYIQNCPLG.

Step-by-step solution

Identifying the N-terminal and C-terminal

From the information given, the C-terminal amino acid is Gly (present in the fragment L-G), and the N-terminal can be inferred from the fragments. Since Gly is the C-terminal, we identify the pattern of fragments leading to it.

Analyzing the Fragment Patterns

We have the fragments D-C, I-E, C-Y, L-G, Y-I-E, E-D-C, and C-P-L. Start by considering fragment connectivity: - E-D-C suggests a sequence in sequence E follows D-C. - I-E can connect to Y-I-E because E could follow Y-I and also I may directly appear before L corresponding to fragment I-L which then connects to L-G Both N and Q replace E and D, so Y-I-E makes Q isomer connecting to D ending the sequence at C before forming a disulfide loop back to E.

Assembling the Chain

Begin constructing continuous peptide: - C-Y is straightforward, leading from cysteine to tyrosine. - Follow the chain from C-Y to Y-I-E which connects with fragment E-D-C. - The middle needs to be filled with C-P-L leading finally to C-Y. - C-P-L follows from end of D-C and feeds back into disulfide bridge to `E-D`

Folding into a Disulfide-Cylinder

Final sequence between fragments encompasses: C-Y-I-E-N/C-P-D-C-L-Q-G forming linear before linking Cysteines back to N-terminal with disulfide given sequence: NC9876-QPLD-YIE-CY forming a circle, ensuring cysteine provides a bridge in started end.

Final Sequence and Cyclic Formation

The reduced oxytocin linear sequence is: `N` - `C` - `Y` - `I` - `Q` - `Q` - `C` - `P` - `L` - `G`. After forming a disulfide bond between the two cysteine residues, oxytocin sequence is CYIQNCPLG forming a full cyclic structure due to loop.

Key concepts

Nonapeptide structure

Oxytocin is an example of a nonapeptide, which is a type of peptide composed of nine amino-acid residues. This unique structure plays a crucial role in its biological activity. Peptides consist of short chains of amino acids linked by peptide bonds, and nonapeptides feature nine such links.

In terms of structure, nonapeptides can accommodate a wide variety of functionalities due to their specific sequence of amino acids. Oxytocin, in particular, is cyclic because it includes a disulfide bridge that connects two parts of the peptide chain, creating a loop. This loop is essential for oxytocin's ability to function correctly within the body.

Understanding the nonapeptide structure helps in grasping how oxytocin performs its physiological roles, such as stimulating uterine contractions during labor and promoting milk ejection during breastfeeding.

Amino acid sequence

The amino acid sequence of a peptide like oxytocin defines its properties and functions. The sequence is essentially the order in which amino acids are linked together in the peptide chain. For oxytocin, this sequence is crucial for forming its biologically active structure.

In the case presented, the amino acid sequence for reduced oxytocin is - `N` - `C` - `Y` - `I` - `Q` - `Q` - `C` - `P` - `L` - `G`.

This linear sequence undergoes a transformation into a cyclic structure upon the formation of a disulfide bond, which we will explore further. Each letter in this sequence represents a specific amino acid, where, for instance, `C` stands for Cysteine and `Y` for Tyrosine. Accurately determining this sequence is critical as it dictates the peptide's three-dimensional shape and its ability to interact with other molecules in the body.

Disulfide bridges

Disulfide bridges are covalent bonds that form between sulfur atoms of two cysteine residues within a protein or peptide. These bonds play a critical role in stabilizing the three-dimensional structure of peptides like oxytocin.

In the case of oxytocin, the disulfide bridge is responsible for the peptide's cyclic nature. The bridge forms a loop that connects the two cysteine residues at positions `C` in the sequence: - C-Y-I-Q-Q-C-P-L-G.

This disulfide bond is essential for the functional integrity of oxytocin. It holds the peptide in a specific shape necessary for its biological activity. Without this disulfide bond, oxytocin would not be able to effectively bind to its receptor sites, rendering it inactive. Thus, understanding disulfide bridges is crucial for those studying peptide chemistry and biochemistry.

Peptide chain analysis

Peptide chain analysis involves determining the sequence of amino acids in a peptide or protein and understanding how they are linked to perform biological functions. In this context, it was necessary to analyze the fragments obtained from partial hydrolysis of reduced oxytocin.

This analysis involves piecing together peptide fragments such as D-C, C-Y, and L-G, which reveal parts of the larger chain. This method helps scientists determine the complete linear sequence before the formation of any intramolecular bonds like disulfide bridges.

Once the individual segments are identified, researchers can reconstruct the linear sequence of oxytocin and later restore it to its native cyclic structure. This step is essential for comprehending how the peptide folds and interacts within biological systems, leading to its specific functionality.