# Formation of Human Insulin Protein in Bacteria This document outlines the process of producing human insulin using recombinant DNA technology, where the human insulin gene is inserted into bacteria (*e.g., E. coli*) to synthesize the protein. ## 1. The Human Insulin Gene (INS) - **Function:** Codes for the hormone insulin, which regulates blood sugar. - **Structure:** Consists of two polypeptide chains, *chain A* (21 amino acids) and *chain B* (30 amino acids), linked by disulfide bonds. - **Location:** Located on human chromosome 11 at position **11p15.5**. - **Genetic Makeup:** The gene is approximately 1.5 Kbp long, containing 3 exons and 2 introns. - **Synthesis Pathway in Humans:** 1. *Pre-pro-insulin* (109 amino acids) is first synthesized. 2. It becomes inactive *pro-insulin* (86 amino acids) inside the cell. 3. Proteolytic enzymes cleave it into active *insulin* (51 amino acids). ## 2. Steps in Recombinant Insulin Production ### Step 1: Identification and Isolation of the Insulin Gene The gene responsible for producing human insulin (the INS gene) is identified and isolated from a human DNA sample. Because bacteria cannot process introns, synthetic genes corresponding to the final A and B chains are created. This is often done using reverse transcriptase to create cDNA from mRNA. ### Step 2: Selection of a Suitable Plasmid Vector A **plasmid** (a small, circular DNA molecule from bacteria) is chosen to act as a *vector* to carry the insulin gene into the host bacterium. - **Common Plasmids:** - *pBR322*: Contains resistance genes for ampicillin and tetracycline. - *pUC18/pUC19*: Derivatives of pBR322, contain an ampicillin resistance gene. - **Key Features of a Good Vector:** - Small size - Origin of replication (allows it to be copied by the host cell) - High copy number (many copies per cell) - Multiple unique restriction sites (for gene insertion) ### Step 3: Creation of Recombinant DNA - The synthetic genes for insulin's *chain A* and *chain B* are inserted into two separate plasmids. - Each gene is inserted next to the *lacZ* **gene**, which codes for the enzyme **β-galactosidase**. - This creates a **fusion gene**. A promoter sequence is also included to ensure the gene is expressed by the bacterium. - DNA Ligase is used to seal the DNA fragments together. ### Step 4: Insertion into Host Cells (Transformation) The recombinant plasmids (one with the gene for chain A, one with the gene for chain B) are introduced into separate host bacteria, typically *Escherichia coli*. This process is called **transformation**. ### Step 5: Production of Insulin Chains (Expression) - The transformed bacteria are grown in large fermentation tanks with optimal culture conditions. - As the bacteria multiply, they replicate the plasmids, **cloning** the insulin gene. - The bacteria then express the fusion gene, producing a fusion protein: - *β-galactosidase-A chain* - *β-galactosidase-B chain* - The large β-galactosidase protein protects the small insulin chains from being destroyed by the bacteria's own protease enzymes. ### Step 6: Extraction and Separation - The bacterial cells are harvested, and the fusion proteins are isolated. - The insulin chains are separated from the β-galactosidase protein by treatment with **cyanogen bromide**. This chemical specifically cleaves at the amino acid methionine, which was intentionally engineered to be at the junction between the two proteins. ### Step 7: Assembly of Functional Insulin - The purified A and B chains are mixed together *in-vitro*. - They are chemically treated (e.g., with sodium disulphonate and sodium sulphite) to form the correct **disulfide bridges** between cysteine residues. - This assembly process creates biologically active human insulin. ### Step 8 and 9: Final Purification, Testing, and Packaging - The active insulin is further purified to ensure it is free of any bacterial by-products. - It undergoes rigorous testing for quality, safety, and effectiveness. - Finally, the pure insulin is packaged into vials or insulin pens for medical use. ## Biological Significance This technology provides a reliable and scalable source of human insulin for treating diabetes mellitus, a global health issue. It avoids the immunological reactions and supply limitations associated with animal-derived insulin (e.g., from pigs or cows).