How are Proteins Distinguished from Each Other

Different combinations of amino acids give different properties to proteins. Therefore, the main feature that should be used in distinguishing proteins from each other is the amino acid sequence of proteins. However, proteins can be distinguished by their size and the biochemical properties as well.

Proteins are the main type of biological molecules that serve as structural, functional as well as regulating molecules inside the cell. Each protein is made up of a polypeptide chain folded into secondary and tertiary structures to produce a three-dimensional molecule. While being a functional molecule, the tertiary structure of proteins interacts with other protein molecules or cofactors, forming the quaternary structure. The main determinant of the structure and the function of a particular protein molecule is its amino acid chain. Thus, the differences between proteins stem from their amino acid sequence. This article describes how are proteins distinguished from each other.

Key Areas Covered

1. What are Proteins
      – Definition, Structure, Function
2. How are Proteins Distinguished from Each Other
     – Amino Acid Sequence, Size, Biochemical Properties

Key Terms: Amino Acid Sequence, Biochemical Properties, Polypeptide Chain, Proteins, Size

What are Proteins

Proteins are large organic compounds with nitrogen atoms. They may be composed of one or two amino acid chains. Each amino acid chain is made up of the alternative assembly of universal amino acids. Proteins are also called polypeptide chains since a peptide bond is formed between amino groups and carboxyl acid groups of adjacent amino acids. Typically, a natural polypeptide may contain approximately 50 – 2000 amino acids. Proteins are very complex and dynamic type of molecules, consisting of four structural levels: primary, secondary, tertiary, and quaternary structure. The 3-D structure of proteins is shown in figure 1.

Figure 1: Protein Structure

Humans have 20,000 to 25,000 protein-coding genes. About 2 million of different protein types can be synthesized from this. However, the human body consists of around 50,000 proteins.

Proteins can be consumed as complete proteins or incomplete proteins. Complete proteins consist of all the essential amino acids. However, incomplete proteins lack some essential amino acids. Dietary proteins are broken down into amino acids during digestion. Proteins serve as a structural component of a cell. They also regulate the functions of the body as hormones and enzymes. They serve as transporting molecules such as hemoglobin as well. Some proteins are components of the immune system.

How are Proteins Distinguished from Each Other

Different combinations of amino acids give different properties to proteins. Therefore, the main feature that should be used in distinguishing proteins from each other is the amino acid sequence of proteins. However, proteins can be distinguished by their size and the biochemical properties as well.

Amino Acid Sequence

Proteins are made up of a series of amino acids. The sequence of the amino acid chain is determined by the sequence of nucleotides of the gene encoded for that particular protein. Therefore, in order to differentiate between two proteins, the corresponding gene sequences can be used. The secondary, tertiary, and quaternary structures of proteins differ based on their amino acid sequence.

Size

Proteins can be distinguished by separating them based on their size. The size of the protein depends on the number of amino acids in the polypeptide chain. SDS-PAGE is the technique used in the separation of proteins based on their size. Protein separation in SDS-PAGE is shown in figure 2.

Figure 2: SDS-PAGE

Biochemical Properties

Biochemical properties of different proteins such as enzymatic activity can be used in order to distinguish proteins from each other. Based on the biochemical properties of amino acids in the polypeptide chain of a particular chain its biochemical properties vary. On that account, the isoelectric point of the proteins may also differ from each other. Isoelectric point is the pH at which the net charge of the polypeptide chain becomes zero. At the isoelectric point, proteins get precipitated from their solutions.

Conclusion

Proteins are organic compounds made up of a polypeptide chain. The structure of protein depends on the amino acid sequence of the protein. The properties of the proteins depend on the type of amino acids in the polypeptide chain. Therefore, in order to distinguish proteins from each other, factors such as amino acid sequence, size, and the biochemical properties of proteins can be used.

Reference:

1. Lodish, Harvey. “Functional Design of Proteins.” Molecular Cell Biology. 4th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.

Image Courtesy:

1. “Main protein structure levels en” By LadyofHats (Public Domain) via Commons Wikimedia
2. “SDS-PAGE” By User Magnus Manske on en.wikipedia – Originally from en.wikipedia; description page is (was) here (CC BY-SA 3.0) via Commons Wikimedia