What Are Peptides Definition Types Peptide Bond Formation and Examples
What are Peptides?
Peptides are short chains of amino acids. In peptides, two or fifty amino acids get linked by peptide bonds.
When two consecutive - amino acid molecules get linked together by removal of one water molecule and by formation of covalent between C1 of one - amino acid and N2 of another - amino acid, then this type of covalent bond is called peptide bond. Peptide bond formation is given below –
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Thus, we can say peptides are formed by linking two and fifty amino acids by peptide bonds and removal of water molecules.
Word peptide has been taken from Greek word peptos , which means ‘digested’.
Chains of peptides which contain less than ten or fifteen amino acids are called oligopeptides. Dipeptides, tripeptides and tetrapeptides etc. come under the class oligopeptides only.
Polypeptides are long, unbranched chains of peptides up to approximately 50 amino acids. Peptides are essential for proper functioning of cells. These are also studied under the molecular biology. They come under the class of biological polymers, nucleic acids, oligosaccharides, polysaccharides, and oligomers.
Although peptides and proteins are the same as both are made up of amino acids, but the peptide chains are shorter than proteins. When a polypeptide contains more than 50 amino acids then it is known as protein.
All peptides have an N – terminal and C – terminal residue at the ends of the peptide chains. Cyclic peptides are exceptions in this case. N – terminal represents the start of polypeptide and referring to the free amine group. C – terminal represents the end of the polypeptide chain and referring to the free carboxyl group.
Types or Classes of Peptides
Peptides have been classified according to their sources and functions. Few of its main classes have been discussed below –
Ribosomal Peptides – Ribosomal peptides are subjected to proteolysis. They function in higher organisms as hormones. These are of many types such as antimicrobial peptides, tachykinin peptides, vasoactive intestinal peptides, pancreatic polypeptide etc.
Peptones – They are formed during the proteolysis of milk and meat. They are used for growing fungi, bacteria for various purposes.
Milk Peptides – These are formed by digestion of milk protein. They also formed during fermentation of milk.
Dipeptides – These peptides are formed by linkage of two amino acids by one peptide bond. Example – carnosine, anserine etc.
Tripeptides – These peptides are formed by linkage of three amino acids by two peptide bonds. Example – glutathione, ophthalmic acid etc.
Oligopeptides – These peptides are formed by linkage of more than two and less than 20 amino acids by peptide bonds. Example – netropsin, amanitin etc.
On the basis of number of amino acids peptides can be of many types such as monopeptide, dipeptide, tripeptide (as discussed above), tetrapeptide, pentapeptide, hexapeptide, heptapeptide, octapeptide, nonapeptide and decapeptide.
Formation of Peptides
Peptides are formed by the peptide linkage between amino acids. let us understand the peptide formation by taking an example of dipeptide. For the formation of dipeptide, two amino acids approach each other and form a covalent bond between C1 – carbon of carboxylic acid and N2 – nitrogen atom of amino acid group by removal of the water molecule. One amino acid loses its hydrogen atom and another its hydroxyl group. It is a condensation reaction. The covalent linkage formed between two amino acids is called peptide bond (-CO-NH-) or peptide linkage. Formation of peptide bonds consumes energy. As the water molecule is getting removed, so it is a dehydration synthesis process.
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Formation of a Dipeptide
When these peptide bonds of peptide break, they release energy. These can be broken down by hydrolysis, but it is a very slow process. Enzymes such as peptidases or proteases can catalyze the process.
Although peptide and protein both are composed of amino acids and carry out various important functions in cells but differ in their various aspects. Difference between peptide and protein is given below –
Difference Between Peptide and Protein
Functions of Peptides
Peptides are fundamental organic compounds of cells. They carry out various important functions in the cell. In human body, they perform key roles in various biological reactions. Few of its functions are listed below –
They are important for muscle building.
GHRP 6 (Growth hormone releasing peptide 6) is useful in the growth of our body.
Neuropeptides are used by neurons to communicate with each other.
Various peptides act as hormones such as peptide hormone.
Opioidergic agent is a peptide which functions to directly modulate the opioid neuropeptide systems in the brain.
Copper – peptides are useful for flawless skin and protection against harsh sunny conditions.
Some peptides are used for hunger issues as well.
They are used for healing purposes as well.
They are useful in increasing the level of prolactin hormone in females.
They function as antibiotics and prevent the growth of microorganisms.
They work as anticancer agents as well. Although many evidences are not there to support that peptides have an active role as anticancer agents.
They play a role of mediator in various reactions.
Some peptides work as a transporter. They selectively allow certain molecules to pass through the cell membrane.
This ends our coverage on the topic “Peptide”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.
FAQs on Peptides Structure Classification and Biological Importance
1. What are peptides in chemistry?
A peptide is a molecule formed when two or more amino acids are linked together by peptide bonds (amide bonds). In chemistry, peptides are short chains of amino acids joined by a condensation reaction between the –COOH group of one amino acid and the –NH2 group of another. They are smaller than proteins and typically contain fewer than 50 amino acid residues. Peptides play key roles in biological processes such as hormone signaling and enzyme regulation.
2. What is a peptide bond and how is it formed?
A peptide bond is a covalent amide linkage (–CO–NH–) formed between two amino acids during a condensation reaction. It forms when:
- The carboxyl group (–COOH) of one amino acid reacts with the
- Amino group (–NH2) of another amino acid.
During this reaction, one molecule of water is eliminated. For example:
H2N–CH(R1)–COOH + H2N–CH(R2)–COOH → H2N–CH(R1)–CO–NH–CH(R2)–COOH + H2O
This reaction is also called a dehydration synthesis.
3. What is the difference between a peptide and a protein?
The main difference between a peptide and a protein is the length and complexity of the amino acid chain. In general:
- Peptides contain fewer than about 50 amino acids.
- Proteins contain more than 50 amino acids and have complex three-dimensional structures.
Proteins also exhibit higher levels of structural organization (primary, secondary, tertiary, and quaternary structure), whereas peptides are shorter and usually simpler in structure.
4. What are the types of peptides?
Peptides are classified based on the number of amino acid residues they contain. The main types of peptides are:
- Dipeptides – contain 2 amino acids.
- Tripeptides – contain 3 amino acids.
- Oligopeptides – contain 2–10 amino acids.
- Polypeptides – contain more than 10 amino acids (but fewer than typical proteins).
They may also be classified as linear peptides or cyclic peptides depending on their structure.
5. How do you name a peptide?
A peptide is named by listing the amino acids from the N-terminus to the C-terminus, changing the suffix of each amino acid (except the last) from “-ine” to “-yl.” For example:
- Glycine + Alanine forms glycylalanine.
Steps to name a peptide:
- Identify the amino acids in sequence.
- Start from the free –NH2 end (N-terminus).
- Modify all but the last amino acid name to end in “-yl.”
This systematic naming is important in organic and biochemistry.
6. What is the general structure of a peptide?
The general structure of a peptide consists of a repeating backbone unit –NH–CH(R)–CO– linked by peptide bonds. Each amino acid contributes:
- An α-carbon (Cα)
- A hydrogen atom
- A variable R group (side chain)
- A peptide linkage –CO–NH–
The chain has two ends:
- N-terminus (free –NH2 group)
- C-terminus (free –COOH group)
The sequence of R groups determines the peptide’s chemical and biological properties.
7. How are peptides synthesized in the laboratory?
Peptides are commonly synthesized using solid-phase peptide synthesis (SPPS), a stepwise method developed by Merrifield. The process involves:
- Attaching the first amino acid to an insoluble resin.
- Protecting reactive groups using protecting groups (e.g., Fmoc).
- Activating and coupling the next amino acid.
- Repeating the cycle until the desired sequence is formed.
- Cleaving the completed peptide from the resin.
SPPS allows precise control over peptide sequence and is widely used in organic chemistry and biochemistry.
8. What happens during peptide bond hydrolysis?
During peptide bond hydrolysis, the amide bond is broken by the addition of water, splitting the peptide into individual amino acids. The reaction is the reverse of condensation:
R–CO–NH–R′ + H2O → R–COOH + R′–NH2
Hydrolysis can occur:
- Under strong acidic conditions (e.g., 6 M HCl).
- Under strong basic conditions.
- With enzymes such as proteases.
This reaction is important in digestion and protein analysis.
9. Why is the peptide bond planar?
The peptide bond is planar because it has partial double-bond character due to resonance between the carbonyl group and the nitrogen lone pair. Resonance structures delocalize electrons:
- O=C–NH ↔ O−–C=N+H
This partial double-bond character:
- Restricts rotation around the C–N bond.
- Makes the peptide bond rigid and flat.
Planarity is essential for the formation of protein secondary structures such as α-helices and β-sheets.
10. What are some examples and functions of peptides?
Examples of peptides include glutathione, oxytocin, and vasopressin, which perform important biological functions. Their roles include:
- Hormonal regulation (e.g., oxytocin).
- Antioxidant activity (e.g., glutathione).
- Neurotransmission and cell signaling.
Peptides are widely studied in chemistry, biochemistry, and pharmaceutical science due to their therapeutic and industrial importance.
