Problem 69 - Chemistry Principles and Practice

Question

Draw the structures of two of the tripeptides that can be formed from alanine, glycine, and cysteine.

Step-by-Step Solution

Answer

Two possible tripeptides are AGC and GCA, drawn with specific peptide bond sequences.

Step 1: Identify the Amino Acids

Begin by recognizing the three given amino acids: alanine (Ala, A), glycine (Gly, G), and cysteine (Cys, C). Each amino acid is represented by a three-letter abbreviation or a one-letter symbol: Alanine is 'A', Glycine is 'G', and Cysteine is 'C'.

Step 2: Determine Possible Arrangements

The tripeptides can be arranged in different sequences since the order of amino acids matters in peptides. For these tripeptides, focus on arranging three amino acids, which gives us combinations such as: AGC, ACG, GAC, GCA, CAG, and CGA.

Step 3: Draw the Tripeptide Structure

For each selected sequence, draw the peptide chain starting from the N-terminus (left side) to the C-terminus (right side). Connect the amino acids via peptide bonds (-CO-NH-). Ensure the side chains (R groups) are represented for each amino acid in the sequence.

Step 4: Example Structure 1: AGC

For AGC (Alanine, Glycine, Cysteine), start with Alanine: N-terminus -> NH2-CH-COOH (with methyl side chain CH3). Follow with Glycine (no side chain): NH-CH2-CO. Lastly, add Cysteine: NH-CH-COOH (with thiol side chain CH2SH). Connect the amino acids with peptide bonds.

Step 5: Example Structure 2: GCA

For GCA (Glycine, Cysteine, Alanine), start with Glycine: N-terminus -> NH2-CH2-COOH. Add Cysteine: NH-CH-COOH (with thiol side chain CH2SH). Finally, add Alanine: NH-CH-COOH (with methyl side chain CH3). Connect amino acids with peptide bonds.

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins. They play a crucial role in biology, being the organic compounds that combine to form proteins. Each amino acid contains a basic amino group \((\text{-NH}_2)\), an acidic carboxyl group \((\text{-COOH})\), and a unique side chain (R group) attached to a central carbon atom called the alpha carbon. This side chain determines the characteristics and function of each amino acid.
For instance, in the exercise, we have three amino acids: alanine, glycine, and cysteine.

• Alanine (Ala, A) contains a methyl group (CH3) as its side chain. This makes it non-polar and hydrophobic, influencing how it behaves in the protein structure.
• Glycine (Gly, G) is the simplest amino acid with just a hydrogen atom as its side chain, making it quite flexible and able to fit into tight spaces within protein structures.
• Cysteine (Cys, C) contains a thiol group (CH2SH) in its side chain, which is polar and can form disulfide bonds, important for the stabilization of a protein's structure.

Understanding these features helps in predicting how peptides and proteins will fold and function in the body.

Tripeptide

A tripeptide is a small peptide consisting of three amino acids linked by peptide bonds. In biological systems, tripeptides can contribute to significant biological functions by forming part of larger protein structures or even functioning independently. The order and composition of these amino acids dictate the tripeptide's properties and biological activity.
In the exercise, we explore combinations of the amino acids alanine, glycine, and cysteine to form possible tripeptides such as AGC (Alanine-Glycine-Cysteine) and GCA (Glycine-Cysteine-Alanine).

• The sequence determines not only the chemical structure but the functional properties. For example, AGC and GCA are different not just because of the order but because it influences potential interactions with other molecules.
• These tripeptides serve as a particular example of how amino acid sequence dictates the biology of the resulting peptide.

Understanding tripeptides offers insights into the complexity of proteins, as they are essentially just longer chains of amino acids.

Peptide Bonds

Peptide bonds are the chemical bonds that link amino acids together in a protein. They are formed through a dehydration reaction between the carboxyl group \((-COOH)\) of one amino acid and the amino group \((-NH_2)\) of another.This bonding releases a molecule of water \((H_2O)\) and creates a covalent bond known as a peptide or amide bond \((-CO-NH-)\).
The peptide bond is crucial because it determines the primary structure of proteins and peptides. Combining the three amino acids in this exercise requires forming two peptide bonds linking them into a tripeptide.

• For example, to form the tripeptide AGC, peptide bonds connect Alanine to Glycine and then Glycine to Cysteine.
• Peptide bonds are strong and stable, resisting hydrolysis under normal physiological conditions, contributing to the stability of protein structures.

Ultimately, the correct formation of peptide bonds is essential for the proper function and integrity of proteins in living organisms.