Difference Between Polypeptide and Protein

Polypeptide vs Protein
 

Amino acid is a simple molecule formed with C, H, O, N and may be S. It has the following general structure.

There are about 20 common amino acids. All the amino acids have a –COOH, -NH₂ groups and a –H bonded to a carbon. The carbon is a chiral carbon, and alpha amino acids are the most important in the biological world. The R group differs from amino acid to amino acid. The simplest amino acid with R group being H is glycine. According to the R group, amino acids can be categorized into aliphatic, aromatic, non polar, polar, positively charged, negatively charged, or polar uncharged, etc. Amino acids present as zwitter ions in the physiological pH 7.4. Amino acids are the building blocks of proteins. When two amino acids join together to form a dipeptide, the combination takes place in a -NH₂ group of one amino acid with the –COOH group of another amino acid. A water molecule is removed, and the formed bond is known as a peptide bond.

Polypeptide

The chain forms when a large number of amino acids are joined together is known as a polypeptide.  Proteins consist of one or more of these polypeptide chains. The primary structure of a protein is known as a polypeptide. From the two terminals of the polypeptide chain, N-terminus is where the amino group is free, and the c- terminus is where the carboxyl group is free. Polypeptides are synthesized at ribosomes. The amino acid sequence in the polypeptide chain is determined by the codons in mRNA.

Protein

Proteins are one of the most important types of macromolecules in living organisms. Proteins can be categorized as primary, secondary, tertiary and quaternary proteins depending on their structures. The sequence of amino acids (polypeptide) in a protein is called a primary structure. When polypeptide structures fold into random arrangements, they are known as secondary proteins. In tertiary structures proteins have a three dimensional structure. When few three dimensional protein moieties bound together, they form the quaternary proteins. The three dimensional structure of proteins depends on the hydrogen bonds, disulfide bonds, ionic bonds, hydrophobic interactions and all the other intermolecular interactions within amino acids. Proteins play several roles in living systems. They participate in forming structures. For example, muscles have protein fibers like collagen and elastin. They are also found in hard and rigid structural parts as nails, hair, hooves, feathers, etc. Further proteins are found in connective tissues like cartilages. Other than the structural function, proteins have a protective function too. Antibodies are proteins, and they protect our bodies from foreign infections. All the enzymes are proteins. Enzymes are the main molecules which control all the metabolic activities. Further, proteins participate in cell signaling.  Proteins are produced on ribosomes. Protein producing signal is passed onto the ribosome from the genes in DNA. The required amino acids can be from the diet or can be synthesized inside the cell. Protein denaturation results in the unfolding and disorganization of the proteins’ secondary and tertiary structures. This can be due to heat, organic solvents, strong acids and bases, detergents, mechanical forces, etc.