Features of the peptide bond
Amino acids in the chain are joined by a peptide bond, which is formed between the carboxyl group of one molecule and the amino group of another. The repeating line of atoms forms the peptide backbone, from which side chains (radicals) extend outward.
Key characteristics of the bond:
- Coplanarity: all atoms of the peptide group lie in the same plane.
- Trans-configuration: hydrogen and oxygen atoms are located on opposite sides of the bond.
- Stability: the bond is stable and under laboratory conditions is destroyed only by harsh acid hydrolysis.
- To detect peptide bonds, the specific biuret test is used, which yields a characteristic purple coloration in an alkaline medium.
Structure-function relationship using hormones as an example
To understand the importance of the amino acid sequence, it is sufficient to compare hypothalamic nonapeptides — oxytocin and vasopressin. Both hormones consist of 9 amino acids and contain a cyclic region due to a disulfide bridge.
The differences lie in just two positions:
- Position 3: oxytocin has isoleucine (Ile), while vasopressin has phenylalanine (Phe).
- Position 8: oxytocin contains leucine (Leu), whereas vasopressin contains arginine (Arg).
These minimal changes radically alter the conformation of the molecules and their ability to interact with receptors. Oxytocin is responsible for uterine muscle contraction and lactation, while vasopressin regulates blood pressure and promotes water reabsorption in the kidneys.
Molecular pathology and heredity
The order of amino acids in a protein is strictly determined by the genetic code. Errors in the primary structure can lead to severe diseases.
A classic example is sickle cell anemia. In this disease, the structure of hemoglobin S undergoes a substitution of just a single amino acid: at the sixth position of the $eta$-chain, glutamic acid is replaced by valine. As a result, the solubility of the protein drops a hundredfold, leading to the deformation of erythrocytes and impaired function.
Remembering peptide reading: "Start — Fasting" (N-terminus is always first, C-terminus is next).
Class and exam questions
What is the peptide backbone of a protein?
This is a repeating chain of atoms (amino group — alpha-carbon — carboxyl group), from which the side chains of amino acids extend in different directions.
Which enzymes cleave peptide bonds?
Peptide bonds are very strong and are cleaved by specialized enzymes called proteases (or proteolytic enzymes of the digestive tract).
How are peptides correctly read and written?
Peptide chains are always read and written strictly in one direction — from the N-terminus to the C-terminus.
What else to study on this topic
- Rules for peptide nomenclature and the use of the suffix "-yl"
- Comparison of keto and enol forms of the peptide bond
- The role of proline in the formation of peptide backbone hydrogen bonds
- Detailed analysis of the four levels of protein structural organization
- Classification of proteins into simple and conjugated
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