Difference Between Polypeptide and Protein

Main Difference – Polypeptide vs Protein

Polypeptides and proteins are polymers of amino acids. Amino acids are organic compounds composed of an amine group, carboxyl group, a hydrogen atom and an alkyl group attached to the same carbon atom. Amino acids are linked together via peptide bonds in order to form polypeptides and proteins. A peptide bond is a covalent bond that is made by the condensation of two amino acids, eliminating a water molecule. This water molecule is formed by the combination of a hydrogen atom from an amino group of one amino acid and the hydroxyl group of the carboxyl group of the other amino acid. Peptides are short amino acid chains. Polypeptides are long amino acid chains. Proteins are produced from two or more polypeptide chains. Both polypeptides and proteins can be found in biological systems. The main difference between polypeptides and proteins is that polypeptides have lower molecular weight than proteins.

Key Areas Covered

1. What is a Polypeptide
       – Definition, Properties, Functions, Examples
2. What is a Protein
      – Definition, Structures, Properties, Functions, Examples
3. What is the Difference Between Polypeptide and Protein
      – Comparison of Key Differences

Key Terms: Condensation, Peptide, Peptide Bonds, Polymer, Polypeptide, Protein

What is a Polypeptide

A polypeptide is a long unbranched chain made out of amino acids. A polypeptide, as indicated by its name, is a collection of several peptides. A peptide is a short chain amino acid sequence. Therefore, a polypeptide is a long chain amino acid sequence.

Amino acids are linked covalently to form this long unbranched polypeptide chains. This covalent bond is called a peptide bond. A peptide bond is formed from a condensation reaction between two amino acid molecules. Here, the carboxyl group of one amino acid will react with the amino group of another amino acid. This eliminates a water molecule by combining the –OH of carboxyl group and H from an amino group. Then the resulting bond will be –CONH- bond. Therefore, in order to identify the presence of peptide bonds, we can find the presence of –CONH- bonds.

Figure 1: Formation of a Peptide Bond

Polypeptides are used to form proteins. A protein can be composed of one or more polypeptide chains. Therefore the primary structure of a protein is an unbranched polypeptide chain. In biological systems, we can find both small and large polypeptide chains. For example, most vertebrate hormones are small polypeptides such as insulin, glucagon, corticotrophin, etc.

What is a Protein

A protein is a complex structure made out of a large number of amino acids. Amino acids bind together to form polypeptide chains. These chains can combine to form a protein. Proteins are very large and complex molecules having a high molecular weight. There are four levels of the arrangement of a protein.

Figure 2: Four Levels of Protein Structures

Levels of Protein Structures

Primary Structure

The primary structure of a protein is simply a long polypeptide chain. There can be sulfide bonds in between amino acids present in this chain.

Secondary Structure

The secondary structure of a protein is known as the alpha helix structure since this is a well-ordered, folded structure (a spiral structure). Sometimes, there are secondary structures called anti-parallel beta pleated structure. Here, polypeptide chains are arranged like a beta-pleated sheet.

Tertiary Structure

The tertiary structure is a complex structure than the primary structure and the secondary structure. It is a 3D structure. This structure is formed from the combination of polypeptide chains through disulfide bonds, ionic bonds, Van Der Waal bonds and hydrogen bonds.

Quaternary Structure

The quaternary structure is composed of several subunits or polypeptides. Here, hydrophobic interactions are used to hold these subunits together. A good example for this is Hemoglobin.

Difference Between Polypeptide and Protein

Definition

Polypeptide: A polypeptide is a long unbranched chain made out of amino acids.

Proteins: A protein is a complex structure made out of a large number of amino acids.

Subunits

Polypeptide: Polypeptides are formed from amino acids.

Proteins: Proteins are made from polypeptides.

Molecular Weight

Polypeptide: The molecular weight of a polypeptide is lower than that of a protein.

Proteins: The molecular weight of a protein is higher than a polypeptide.

Chemical Bonding

Polypeptide: Polypeptides are composed of peptide bonds.

Proteins: Proteins are composed of several types of bonds such as peptide bonds, disulfide bonds, ionic bonds and Van Der Waal attractions.

Conclusion

Both polypeptides and proteins are organic compounds composed of amino acids. These are categorized as polymers made out of amino acid monomers. These amino acids are linked to each other via peptide bonds. The main difference between polypeptide and protein is that polypeptides have lower molecular weight than that of the proteins.

References:

1. Helmenstine, Ph.D. Anne Marie. “Know the Four Conformation Levels of Protein Structure.” ThoughtCo, Available here. Accessed 14 Sept. 2017.
2. “Chapter 5 : Amino Acids and Peptides.” Bioinfo.org, Available here. Accessed 14 Sept. 2017.
3. “Polypeptides.” Biology-Pages.info, Available here. Accessed 14 Sept. 2017.

Image Courtesy:

1. “224 Peptide Bond-01” By OpenStax College – Anatomy & Physiology, Connexions Web site.  Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
2. “Levels of structural organization of a protein” By Scurran15 – Own work (CC BY-SA 4.0) via Commons Wikimedia