Peptides: Videos & Practice Problems

Peptides are formed when two or more amino acids bond together through a peptide bond, also known as an amide bond. This bond occurs when the carboxyl group of one amino acid covalently links with the amino group of another amino acid. In this process, the carboxyl group is typically in its anionic form (negatively charged), while the amino group is in its cationic form (positively charged).

The formation of a peptide bond involves a condensation reaction, which results in the loss of a water molecule. For example, when the amino acids alanine and threonine bond, water is released. Specifically, one oxygen atom and two hydrogen atoms are removed from the reacting groups, leading to the formation of the peptide bond. The remaining structures combine to create a dipeptide, which is represented by the three-letter codes of the amino acids connected by a hyphen (e.g., Ala-Thr).

The number of amino acids in a peptide is indicated by specific prefixes: "di-" for two amino acids (dipeptide), "tri-" for three (tripeptide), and "tetra-" for four (tetrapeptide). When a peptide consists of a larger number of amino acid residues, it is referred to as a polypeptide. A residue is defined as an individual amino acid within a peptide chain. Understanding these terms and the process of peptide bond formation is crucial for grasping the complexity of protein structures and functions.

A tetrapeptide consists of four amino acid residues linked together by peptide bonds, which are a specific type of amide bond. A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of another, resulting in a carbonyl group (C=O) bonded to a nitrogen atom (N). In the structure of a tetrapeptide, you will find three peptide bonds connecting the four amino acids.

To identify the peptide bonds, look for the carbonyl carbon that is single-bonded to a nitrogen atom. In the given tetrapeptide, there are three distinct peptide bonds:

• The first peptide bond connects the first and second amino acids.
• The second peptide bond connects the second and third amino acids.
• The third peptide bond connects the third and fourth amino acids.

Each of these peptide bonds is crucial for maintaining the structure and function of the protein. The sequence and arrangement of the amino acid residues determine the properties and biological activity of the peptide. Understanding the formation and significance of peptide bonds is essential in biochemistry, particularly in the study of protein synthesis and structure.

Understanding the structure and naming of peptides is crucial in biochemistry, particularly due to their inherent directionality. Peptides are chains of amino acids linked by peptide bonds, and they are always oriented from the N terminus to the C terminus. The N terminus refers to the end of the peptide that has a free amino group (-NH₂), while the C terminus is the end with a free carboxyl group (-COOH).

When drawing or naming peptides, it is essential to arrange the amino acids from left to right, starting with the N terminus on the left and ending with the C terminus on the right. For example, in a peptide consisting of two amino acids, the N terminus will be positioned on the left side, indicating the presence of the free amino group, and the C terminus will be on the right side, where the carboxyl group is located. This consistent orientation helps in accurately representing the peptide's structure and facilitates clear communication in scientific contexts.

Always remember to maintain this directionality when writing or naming peptides, as it is fundamental to understanding their properties and functions in biological systems.

A dipeptide is formed by the condensation of two amino acids, where the carboxyl group of one amino acid reacts with the amino group of another, resulting in the release of a water molecule. In this case, we will construct a dipeptide from tyrosine and cysteine, following the order from the N-terminus to the C-terminus.

To visualize the structure, we start with tyrosine, which has a hydroxyl group (-OH) attached to a benzene ring. The structure can be represented as follows:

Tyrosine: C_{9}H_{11}NO_{3}

Next, we have cysteine, which is characterized by a thiol group (-SH). Its structure is:

Cysteine: C_{3}H_{7}NO_{2}S

When these two amino acids combine, the carboxyl group of tyrosine interacts with the amino group of cysteine, forming a peptide bond. The reaction can be summarized as:

R_{1}C_{9}H_{11}NO_{3} + R_{2}C_{3}H_{7}NO_{2}S \rightarrow R_{1}C_{9}H_{10}N_{2}O_{3}S + H_{2}O

In this equation, R_{1} represents the side chain of tyrosine, and R_{2} represents the side chain of cysteine. The water molecule is released during the formation of the peptide bond.

The resulting dipeptide structure can be depicted with the peptide bond highlighted. The dipeptide formed from tyrosine and cysteine is named using their three-letter codes. Tyrosine is abbreviated as TYR and cysteine as CYS. Therefore, the name of the dipeptide is TYR-CYS.

This dipeptide showcases the unique properties of both amino acids, with tyrosine contributing a polar hydroxyl group and cysteine providing a reactive thiol group, which can participate in further biochemical reactions.