Question

Determine the sequence of a peptide consisting of 14 amino acids on the basis of the following data. Amino acid composition: \((4 S, 2 L, F, G, I, K, M, T, W, Y)\) Amino-terminal analysis: \(\mathrm{S}\) Carboxypeptidase A digestion: \(\mathrm{L}\) Trypsin digestion: \((3 S, 2 L, F, I, M, T, W)(G, K, S, Y)\) Chymotrypsin digestion: \((\mathrm{F}, \mathrm{I}, \mathrm{S})(\mathrm{G}, \mathrm{K}, \mathrm{L})(\mathrm{L}, \mathrm{S})(\mathrm{M}, \mathrm{T})(\mathrm{S}, \mathrm{W})\) \((\mathrm{S}, \mathrm{Y})\) Amino-terminal analysis of \((\mathrm{F}, \mathrm{I}, \mathrm{S})\) peptide: \(\mathrm{S}\) Cyanogen bromide treatment: \(\left(2 \mathrm{S}, \mathrm{F}, \mathrm{G}, \mathrm{I}, \mathrm{K}, \mathrm{L}, \mathrm{M}^{*}, \mathrm{T}, \mathrm{Y}\right)\) \((2 S, L, W)\) \(\mathrm{M}^{*},\) methionine detected as homoserine

Short answer

The peptide sequence is SFIYSGKLTWMSL.

Step-by-step solution

Review Amino Acid Composition

The given peptide is composed of 14 amino acids with the following counts: 4 S, 2 L, and one each of F, G, I, K, M, T, W, and Y. This list specifies the total amount and types of amino acids present in the peptide.

Analyze Amino-terminal & Carboxypeptidase A Data

From amino-terminal analysis, the sequence begins with a serine (S) residue. Carboxypeptidase A digestion shows that the C-terminal residue is leucine (L). This information suggests that the sequence starts with S and ends with L.

Use Trypsin Digestion Information

Trypsin cuts on the carboxyl side of lysine (K) or arginine (R)—however, there is no R in the sequence. After trypsin digestion, the resulting peptides are (3 S, 2 L, F, I, M, T, W) and (G, K, S, Y), indicating that G, K, S, and Y form another segment of the sequence.

Interpret Chymotrypsin Digestion

Chymotrypsin cleavage yields smaller peptides: (F, I, S), (G, K, L), (L, S), (M, T), (S, W), and (S, Y). This pattern suggests the peptide consists of aromatic amino acids like F, W, and Y. Furthermore, (G, K, L) and (L, S) indicate specific connections between these residues.

Consider Cyanogen Bromide Treatment

Cyanogen bromide cleaves at methionine, generating peptides: (2 S, F, G, I, K, L, M*, T, Y) and (2 S, L, W). M* indicates cleavage occurs here, suggesting segments (after M) starting with L and containing W.

Assemble Sequence Using All Data

Combining the information: the peptide starts with S and ends with L. After incorporating all the digestion results, the most likely sequence is SFIYSGKLTWMSL. This accommodates the start and end conditions and spans all the cleavage data.

Key concepts

Amino Acid Analysis

To solve a peptide sequencing problem, we first need to understand the amino acids that compose the peptide. This is known as amino acid analysis. In this exercise, our peptide consists of 14 amino acids. These amino acids are four serines (S), two leucines (L), and one each of phenylalanine (F), glycine (G), isoleucine (I), lysine (K), methionine (M), threonine (T), tryptophan (W), and tyrosine (Y). This analysis sets the stage for understanding the peptide's composition, helping us know what to expect in the subsequent sequencing steps.
This comprehensive accounting of the amino acids is crucial. It serves as a checklist to ensure that all components are considered when determining the sequence. It's like knowing all the pieces of a puzzle before attempting to put them together.

Enzyme Digestion

Enzyme digestion is a technique used to break down proteins into smaller fragments or peptides. This process helps in determining the sequence of the peptide. Specifically, enzymes such as trypsin and chymotrypsin are used to achieve specific cleavage patterns. Understanding how these enzymes cleave proteins can provide critical insights into the arrangement of amino acids in the peptide.
For instance, in our exercise, various digestion methods are used: carboxypeptidase A digestion helps us identify the terminal ends of the peptide, while trypsin and chymotrypsin provide specific cleavage points through their enzymatic activity. Using these methods, one can infer which segments are next to each other based on which fragments are produced.

Trypsin Cleavage

Trypsin is an enzyme that cleaves at the carboxyl side of basic amino acids, specifically lysine (K) and arginine (R). However, in this problem, there is no arginine, so our focus is solely on lysine. Upon trypsin digestion, we observe two main peptides from our original sequence: one contains (3S, 2L, F, I, M, T, W) and another with (G, K, S, Y).
This cleavage pattern indicates that the sequence has a section ending with lysine connecting to glycine, serine, and tyrosine. The presence of the peptide with multiple serines suggests these residues must be considered when assembling the final sequence. Thus, trypsin digestion clarifies where certain amino acids might be located relative to others in the sequence.

Chymotrypsin Mapping

Chymotrypsin is another enzymatic tool, cleaving at aromatic amino acids like phenylalanine (F), tryptophan (W), and tyrosine (Y), as well as leucine (L). In this problem, chymotrypsin digestion yields even smaller peptide fragments: (F, I, S), (G, K, L), (L, S), (M, T), (S, W), and (S, Y).
Each resulting fragment suggests proximity of these residues in the original peptide. For instance, (G, K, L) implies a connection between glycine, lysine, and leucine, while (S, W) suggests serine is adjacent to tryptophan. Using these insights, chymotrypsin helps confirm the position of aromatic residues and how non-aromatic residues fit into the larger sequence, aiding in the puzzle of sequencing this peptide.