Question

Draw the structures of the tripeptides gly–ala–ser and ser–ala–gly. How many other tripeptides are possible using these three amino acids?

Short answer

The structures of the tripeptides gly–ala–ser and ser–ala–gly are as follows: gly-ala-ser: \(NH_2-CH_2-CO-NH-CH(CH_3)-CO-NH-CH(CH_2OH)-COOH\) ser-ala-gly: \(NH_2-CH(CH_2OH)-CO-NH-CH(CH_3)-CO-NH-CH_2-COOH\) Using the given amino acids glycine (Gly), alanine (Ala), and serine (Ser), there are 27 possible tripeptides.

Step-by-step solution

Understanding the structures of amino acids

To draw the structures of the tripeptides, we first need to understand the basic structures of the three amino acids involved – glycine (Gly), alanine (Ala), and serine (Ser). Glycine (Gly): R group = H Alanine (Ala): R group = CH3 Serine (Ser): R group = CH2OH The general structure of an amino acid is, \[NH_2-CH(R)-COOH\]

Drawing the structure of gly–ala–ser

In the given order, we have glycine (Gly) followed by alanine (Ala) and finally serine (Ser). We will form a peptide bond between the amine group of the first amino acid and the carboxyl group of the next amino acid. 1. Gly: NH2-CH2-COOH 2. Ala: NH2-CH(CH3)-COOH 3. Ser: NH2-CH(CH2OH)-COOH To form gly–ala–ser, peptide bond forms between Gly and Ala, and then between Ala and Ser: \[NH_2-CH_2-CO-NH-CH(CH_3)-CO-NH-CH(CH_2OH)-COOH\]

Drawing the structure of ser–ala–gly

Similarly, in the order ser–ala–gly, we have serine (Ser) followed by alanine (Ala) and finally glycine (Gly). We will form a peptide bond between the amine group of the first amino acid and the carboxyl group of the next amino acid. 1. Ser: NH2-CH(CH2OH)-COOH 2. Ala: NH2-CH(CH3)-COOH 3. Gly: NH2-CH2-COOH To form ser–ala–gly, peptide bond forms between Ser and Ala, and then between Ala and Gly: \[NH_2-CH(CH_2OH)-CO-NH-CH(CH_3)-CO-NH-CH_2-COOH\]

Finding the total number of possible tripeptides with the given amino acids

We have 3 amino acids: glycine (Gly), alanine (Ala), and serine (Ser). To find the total number of possible tripeptide combinations, we need to calculate the permutations for these amino acids in 3 possible positions. Total number of permutations = 3 × 3 × 3 = 27 possible tripeptides.

Key concepts

Tripeptides

Tripeptides are small protein structures composed of three amino acids linked in a chain. Their linear arrangement influences the properties and functions of the tripeptide. In the current context, we are examining tripeptides made from the amino acids glycine (Gly), alanine (Ala), and serine (Ser).

One intriguing aspect of tripeptides is the way they can vary. Even with just three distinct amino acids, the number of different tripeptides that can be formed is quite substantial. You calculate the permutations by considering each position in the peptide chain independently, allowing for any of the three amino acids to occupy a position.

Thus, for three positions filled with any of the three amino acids, you have 3 choices for the first position, 3 for the second, and 3 for the third, resulting in 3 x 3 x 3 = 27 unique tripeptide combinations. Understanding how tripeptides are ordered is crucial because even a simple swap in the sequence changes its properties.

Peptide Bonds

Peptide bonds are the glue that holds the amino acids together in a tripeptide. These are special chemical bonds formed through a condensation reaction. During this reaction, the amino group of one amino acid reacts with the carboxyl group of the next, releasing a molecule of water. As such, peptide bonds are also known as amide bonds.

Visually, it might be easier to see this process with an example. Let's consider the formation of a peptide bond in the tripeptide gly–ala–ser. The bond between Gly and Ala forms when the amine group of Gly links to the carboxyl group of Ala in the sequence:

• Glycine nitrogens bond to the carbon of alanine's carboxyl group.
• Alanine subsequently links to serine in a similar manner.

The structure continues in this chain-like manner for as many amino acids as are present in the sequence. The chain formed exhibits its unique behavior in biological systems largely due to these peptide bonds. Peptide bonds also add stability to proteins by maintaining their linear structure.

Amino Acid Structures

To fully appreciate the function and formation of tripeptides, understanding the basic structure of amino acids is essential. Each amino acid features a central carbon atom, known as the alpha carbon ( \(\alpha\)-carbon), bonded to an amino group ( \(NH_2\)), a carboxyl group ( \(COOH\)), a hydrogen atom, and a unique side chain (R-group). This R-group is what differentiates one amino acid from another.

For instance, the side chain of glycine (Gly) is a simple hydrogen ( \(H\)), which is why it's one of the smallest amino acids. Alanine (Ala) has a methyl group ( \(CH_3\)) as its side chain, while serine (Ser) includes a hydroxymethyl group ( \(CH_2OH\)).

• Glycine is non-polar and small, influencing how proteins fold.
• Alanine is also non-polar, slightly larger due to its methyl group.
• Serine, with its polar side chain, can form hydrogen bonds, affecting solubility.

Each amino acid's structure plays a significant role in shaping the properties of proteins and peptides, impacting how they function in various physiological processes.