Oxytocin, a nonapeptide hormone secreted by the pituitary...

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 Asn, Cys $_{2},$ Gln, Gly, Ile, Leu, Pro, Tyr. 3. Partial hydrolysis of reduced oxytocin yields seven fragments: Asp-Cys, Ile-Glu, Cys-Tyr, Leu-Gly, Tyr-lle-Glu, Glu-Asp-Cys, Cys-Pro-Leu. 4. Gly is the C-terminal group. 5. Both Glu and Asp are present as their side-chain amides (Gin and Asn) rather than as free side-chain acids. What is the amino acid sequence of reduced oxytocin? What is the structure of oxytocin itself?

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 Asn, Cys $_{2},$ Gln, Gly, Ile, Leu, Pro, Tyr.
3. Partial hydrolysis of reduced oxytocin yields seven fragments: Asp-Cys, Ile-Glu, Cys-Tyr, Leu-Gly, Tyr-lle-Glu, Glu-Asp-Cys, Cys-Pro-Leu.
4. Gly is the C-terminal group.
5. Both Glu and Asp are present as their side-chain amides (Gin and Asn) rather than as free side-chain acids. What is the amino acid sequence of reduced oxytocin? What is the structure
of oxytocin itself?

Key Concepts

Peptide Sequencing

Peptide sequencing is the process of determining the order of amino acids in a peptide or protein. It involves techniques such as chemical cleavage and analysis of overlapping fragments, which help in reconstructing the full sequence. This process is crucial for understanding the structure–function relationships of peptides and proteins and for applications in areas like drug design and molecular biology.

Disulfide Bonds in Protein Structure

Disulfide bonds are covalent linkages formed between the sulfur atoms of two cysteine residues, contributing significantly to the folding and stability of many proteins and peptides. In sequencing studies, these bonds can complicate analysis because they create a cyclic or cross-linked structure; therefore, reducing these bonds to break the cycle is an important step to obtain linear sequences for easier fragmentation and analysis.

Partial Hydrolysis

Partial hydrolysis is a controlled chemical or enzymatic process that cleaves peptide bonds to generate a set of overlapping peptide fragments. These fragments retain parts of the original sequence, and by analyzing the overlapping regions, scientists can deduce the complete amino acid sequence. This method has been fundamental in the pre-mass spectrometry era for peptide and protein structure determination.

Cyclic Peptide Structure and Its Impact

Cyclic peptides are peptides in which the amino or carboxyl termini, or side-chain functionalities, are linked to form a ring structure. The formation of cycles, often stabilized by disulfide bridges, affects the physical and chemical properties of the peptide. In structural determination, recognizing and correctly accounting for this cyclic nature is vital because it influences the fragmentation pattern and the interpretation of sequencing data.

Amino Acid Side?Chain Modifications

Amino acid side-chain modifications, such as the conversion of acidic groups to amides (e.g., aspartic acid to asparagine and glutamic acid to glutamine), can alter the chemical behavior and molecular weight of the residues. Recognizing these modifications is important in sequence determination since they affect both the overall charge and the fragmentation patterns observed during analysis, allowing for accurate interpretation of the peptide's true constitution.

Transcript

00:01 Okay, so we start with the nano -peptide and given a bunch of information about it's digest.

00:06 The way i'm going to start this is by just kind of drawing out blanks for the nine.

00:16 And so let me keep track of the order.

00:18 So it does tell us that a glycine is the c -terminal residue, which is very helpful.

00:24 We additionally have quite a few fragments that this produces, which i'm going to write out pretty quickly right here.

01:19 All right.

01:22 So the first thing to notice is that from this fragment, i have leuclycine.

01:29 I'll have one glycine that appears in this entire peptide, which means that this amino acid must be leucine.

01:36 So similarly, i've got a cysteine pro -lucine right here that lines up with the leucine, considering there's only one leucine.

01:44 So this must be pro and a cysteine.

01:48 So there's our first of the two cistines in here.

01:53 So notice that these two fragments, this one right here, aspsis and glue aspsis overlap.

02:00 So i'm going to add that, obviously, next.

02:04 The important thing also to notice is that it tells us that these are in their basic forms.

02:08 So instead of writing as, i'm going to write asn and gln.

02:16 Okay.

02:19 So very next, we have ile and glu, which is the same thing as gln in this case.

02:27 So the first one, or the one before this, must be isolucine.

02:33 Kind of working backwards, i can see i've got isolucine and glute mate here by tyrosine, which means tyrosine must be here.

02:42 And there's really only one opportunity for alasmino acid, but just to be sure we do have a cysteine tyrosine right here.

02:49 So that is the identity of this noopeptide.

02:53 So i'm going to erase this and start working on drawing the destructuring.

03:00 So the important thing to notice is that it tells us that this forms a ring structure.

03:06 And the way it forms that is through these disulfide bridges.

03:10 So this structure itself is going to be very strange, and typically just unlike most structures that we draw normally.

03:18 But i am going to start by just drawing my n terminal, just like i kind of normally would...

Please give Ace some feedback