Question

Write full structures for the following peptides: (a) C-H-E-M (b) E-A-S-Y (c) P-E-P-T-I-D-E

Short answer

Show fully drawn peptide chains for C-H-E-M, E-A-S-Y, P-E-P-T-I-D-E using amino acid structures connected by peptide bonds.

Step-by-step solution

Understanding the Peptide Notation

The given sequences represent peptides, where each letter corresponds to the one-letter abbreviation of an amino acid. For example, C stands for Cysteine, H for Histidine, E for Glutamic Acid, M for Methionine, A for Alanine, S for Serine, Y for Tyrosine, P for Proline, T for Threonine, and D for Aspartic Acid, and I for Isoleucine.

Convert Each Letter to Amino Acid Structure

Convert each letter in the sequence to its full amino acid structure. Amino acids have a common backbone: the amino group (NH₂), the carboxyl group (COOH), and a specific side chain (R-group) which differs for each amino acid. List the R-groups for sequence individual components as follows:\[\begin{align*}\text{Cysteine (C)} & : \, \text{-CH}_{2}\text{-SH} \\text{Histidine (H)} & : \, CH_2-\text{imidazole ring} \\text{Glutamic Acid (E)} & : \, CH_2CH_2COOH \\text{Methionine (M)} & : \, CH_2CH_2SCH_3 \\text{Alanine (A)} & : \, CH_3 \\text{Serine (S)} & : \, CH_2\text{-OH} \\text{Tyrosine (Y)} & : \, CH_2\text{-phenol ring} \\text{Proline (P)} & : \, \text{pyrrolidine ring} \\text{Threonine (T)} & : \, CH(OH)CH_3 \\text{Isoleucine (I)} & : \, CH(CH_3)CH_2CH_3 \\text{Aspartic Acid (D)} & : \, CH_2COOH\end{align*}\]

Combining Amino Acids into Peptide Chains

Link the amino acids together in the sequence provided. The N-terminus of the first amino acid is a free NH₂ group, while the C-terminus of the last amino acid is a free COOH group. Between each pair of amino acids, form peptide bonds by linking the NH group of the incoming amino acid to the CO group of the preceding amino acid. For sequence (a) C-H-E-M, write out:\[\text{NH}_2\text{-}\text{Cysteine}\text{-}\text{Peptide Bond}\text{-}\text{Histidine}\text{-}\text{Peptide Bond}\text{-}\text{Glutamic Acid}\text{-}\text{Peptide Bond}\text{-}\text{Methionine}\text{-}\text{COOH}\]Repeat similarly for sequences (b) E-A-S-Y and (c) P-E-P-T-I-D-E.

Confirming the Structure

Double-check the peptide structure by matching each chain with the correct sequence of R-groups and peptide bonds. Ensure there are no additional atoms at the termini other than a free NH₂ group and a free COOH group. Review that this step aligns with chemical rules for peptide synthesis shown previously.

Key concepts

Amino Acid Sequence

Amino acid sequences are like the words of a sentence, each composed of distinct letters that represent different amino acids. This sequence, often denoted using one-letter or three-letter abbreviations, dictates the function and properties of the resulting peptide or protein.
Understanding these sequences is crucial as they define the primary structure of proteins. Each letter in a sequence stands for a specific amino acid; for example, "C" signifies Cysteine, while "M" stands for Methionine.
These letters can be found in a variety of sequences within peptides, offering insights into both their composition and potential biological activities.

To convert these sequences into full structures, it's essential to comprehend the unique characteristics of each amino acid — notably their R-groups, which play a significant role in defining how peptides fold and function. As such, memorizing these abbreviations and corresponding elements can serve as a vital tool in protein biology.

Peptide Bond Formation

Peptide bond formation is the process through which amino acids link together to form peptides. This bond is a form of covalent linkage, specifically between the carboxyl group of one amino acid and the amino group of another.
This resulting bond, an amide linkage, is fundamental in building polypeptides and proteins.

During peptide bond formation, a molecule of water is released as the carboxyl and amino groups join. The reaction can be simplified as:

• The NH group of one amino acid connects to the CO group of another, focusing on the loss of water in the linking.
• This leaves behind a repeating unit of the backbone: (-NH-CHR-CO-), where "R" represents the side chain of the amino acid.

The significance of peptide bonds extends beyond the mere connection of amino acids; they also provide stability and allow for the unique folding necessary for functional proteins.

Amino Acid R-groups

Amino Acid R-groups, or side chains, are the differentiating factor among amino acids. They greatly influence the structure and function of the peptides they compose.
Specifically, these R-groups determine the chemistry and physical properties unique to each amino acid — whether they are hydrophobic, hydrophilic, acidic, or basic, among other qualities.

Consider the variety in R-groups as follows:

• **Cysteine** has a thiol group (-SH) making it capable of forming disulfide bonds with other Cysteines.
• **Histidine** possesses an imidazole ring, allowing it to act as a proton exchange participant.
• **Glutamic Acid and Aspartic Acid** include carboxyl groups, classifying them as acidic amino acids capable of forming ionic bonds.
• **Tyrosine** features a phenolic group which can participate in hydrogen bonding due to its hydrophilic nature.

In summary, the functional capabilities and interactions of peptides and proteins rely heavily on these versatile side chains, which drive the diverse range of activities seen in living organisms.