Are Proteins Peptides ? 5 Facts You Should Know


In this post, you will find the 6 facts about, are protein peptides and functions and characteristics of various proteins.

Are proteins peptides or we can say proteins are peptides or polypeptides made of different amino acid residues joined by amide linkage and further they get folded variously to form different protein structures.

Are proteins peptides?

Peptides are short chains of 2-50 amino acids joined covalently by peptide linkage while proteins are large polymers or heteropolymers having different types of α-amino acids. These amino acids are  joined by peptide linkage (-NH.CO-) in a linear chain and gets various folding to form proteins. Therefore, proteins are formed from peptides and polypeptides.

are proteins peptides
tetrapeptide (example ValGlySerAla) with green marked amino end (L-Valine) and blue marked carboxyl end (L-Alanine) from Wikipedia

Are proteins polypeptides ?

A proteins can be made of single polypeptide or multiple polypeptide. Single polypeptide protein are known as monomeric protein while protein having more than two  polypeptide are called oligomeric protein.

  • Examples of monomeric proteins are : myoglobin and lysozyme.
A representation of the 3D structure of the protein myoglobin from Wikipedia
  • Examples of oligomeric proteins are : immunoglobulins (consists two light chain and two heavy chain), Insulin (it consist only two polypeptides), pyruvate dehydrogenase (it contains 72 polypeptides).
Two heavy chains (blue, yellow) and the two light chains (green, pink) from Wikipedia

How many peptides are there in a protein?

Peptides differentiates from proteins on the basis of size and structure. Basically, proteins are larger than peptides. But if we talk about the proteins, all proteins have peptides or polypeptides joined by peptide linkage. If  the amino acid residues is ten or less than ten , it is simply called peptide or oligopeptide whereas a polypeptide has number of amino acid residues, basically more than twenty. A protein  can be made up of single polypeptide like lysozyme or it may have 72 polypeptides in pyruvate dehydrogenase.

The molecular weight of the smallest protein is 4500 daltons( adenocorticotrophin hormone) and the highest molecular weight protein is pyruvate dehydrogenase which is about 4600000 daltons.

How are proteins polypeptides?

Proteins contains single polypeptide or more than one polypeptide. Polypeptide consists of number of amino acid residues in the chain that form proteins. A polypeptide can be smaller or equivalent to a protein.

Proteins vs Peptides

ProteinsPeptides
They are large size consist of single or multiple polypeptides of several kinds of amino acids joined through peptide bond.Peptide term is used to denote amino acids residues which are less than ten in number.
Chemically, they are formed of carbon, Hydrogen, Oxygen, Nitrogen and in many cases Sulphur.When two amino acids join, peptide bond is formed by liberating water molecule.
Protein contains large number of reactive groups that can react with and absorb the other smaller substances over its surface.Condensation reaction occurs when two amino acids joins and produces a free carboxylic group at one end, so it can form a chain of amino acid residues.
Animal proteins are complete proteins as they are able to provide all the essential and semi-indispensable amino acids.  Peptides are not diverse as proteins in terms of functions carried out, but they are indispensable for biological functions.

What are the properties of Proteins?

Proteins are found in large number in every organisms. This is possible because of variations of in length of amino acid residues. A

polypeptide of only 30 amino acid can make 2030  possible combinations to make different proteins with different functions.

  • Specificity : Every organism posses a unique protein specific to it, proteins can be similar in case of closely related species and that can also be determined using serum test.
  • Polypeptide: A proteins can be made of single polypeptide or multiple polypeptide. Single polypeptide protein are known as monomeric protein while protein having more than two  polypeptide are called oligomeric protein. Monomeric proteins are : myoglobin and lysozyme.
  • Amino acid residues: A polypeptide consists of few to numerous of amino acid residues. Spinach ferrodoxin has 97 amino acid residues, ribonuclease has 124, myoglobin has 153 while alpha and beta polypeptides of hemoglobin has 141 and 146 amino acid residues respectively.
  • Molecular Weight : The molecular weight of the smallest protein is 4500 daltons( adenocorticotrophin hormone) and the highest molecular weight protein is pyruvate dehydrogenase which is about 4600000 daltons.
  • Solubility : Proteins form protoplasmic structures in water as some of them are insoluble. Some form colloidal solution in water.
  • Permeation : Proteins cannot pass through cell membrane because they are the component of it. But they can pas somehow by exocytosis and endocytosis.
  • Electrical reaction : At some pH there is a point where protein is electrically neutral or their charge is zero due to the presence of equal positive and negative charges. This point is called isoelectric point.
  • Reactivity : Protein contains large number of reactive groups that can react with and absorb the other smaller substances over its surface.
  • Denaturation : Proteins can be denatured by various factors like UV radiations, heavy metal, strong alkali etc but the sequence of amino acid usually not affected. Denaturation can be temporary or permanent, however, denatured protein is not biologically functional.

Give some examples of proteins and their functions.

Proteins Functions
CollagenIntercellular ground substance
TrypsinEnzyme
InsulinHormone
AntibodyFights contagious agents
ReceptorSensory reception (smell, taste, hormone etc)
GLUT-4Enables glucose transport into cells

Q3. Give some examples of Proteins and their characteristics.

ProteinsCharacteristics
AlbuminsWater soluble proteins, precipitated in high salt concentration and coagulated by heat
GlobulinsThey can dissolve in diluted salt solution and coagulated by heat.
ProlaminesThey dissolve in 70-80% of IPA
GlutelinsThey get dissolved in diluted acid or alkali and coagulated by heat.
HistonesThey are easily dissolved in water and not precipitated by heat
ProtaminesWater soluble and not coagulated by heat
ScleroproteinsWater insoluble, not precipitated but some changed to gelatin by heating.

Classification of proteins on the basis of quality.

Proteins are classified into two types:

  • Complete ProteinsProteins consists of all the essential amino acid are called first class proteins or complete proteins. Protein derived from animals are usually complete proteins. Among plants, soya protein is an example of complete protein. E.g., egg, meat, fish, milk.
  • Incomplete proteins They are also called second class proteins because they do not have one or more essential amino acids that limits to fulfill the body requirement in humans.

Conclusion

To wrap up this post, we conclude that proteins can be made of single polypeptide or multiple polypeptide. Single polypeptide protein are known as monomeric protein while protein having more than two  polypeptide are called oligomeric protein. Small peptides joined together to form large polypeptides and these polypeptides further folded variously to form different proteins.

Saif Ali

Hi, I am Saif Ali. I obtained my Master's degree in Microbiology and have one year of research experience in water microbiology from  National Institute of Hydrology, Roorkee. Antibiotic resistant microorganisms and soil bacteria, particularly PGPR, are my areas of interest and expertise. Currently, I'm focused on developing antibiotic alternatives. I'm always trying to discover new things from my surroundings.  My goal is to provide readers with easy-to-understand microbiology articles. If you have a bug, treat it with caution and avoid using antibiotics to combat SUPERBUGS. Let's connect via LinkedIn.

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