Assignment 2- Insulin Structure

Insulin is a polypeptide which includes an A chain comprised of 21 amino acids and a B chain of containing 30 amino acids. The two chains forming this dimer are attached via two disulfide bonds; one located between the 7th aa of the A chain and 7th aa of the B chain and 20th aa of the A chain and 19th aa of the B chain. Another intra-chain disulfide bond is located between the sixth and eleventh amino acids in the A chain.Insulin is synthesized from proinsulin which is derived from a preproinsulin precursor.Insulin is the active form which is derived when the c terminal 23 amino acid sequence is removed from the preproinsulin, the proinsulin molecule folds to provide disulfide linkage between the chains. The connecting peptide of the pro insulin is then cleaved by enzymes resulting in the final insulin product. (Hadley, and Levine 241-244)


Using NCBI, Blast and ClustalW, the protein sequences of human preproinsulin and preproinsulin of the chimpanzee and field rat were assessed and alligned. The allignment results can be viewed here: 

CLUSTAL 2.0.12 multiple sequence alignment


gi|4557671|ref|NP_000198.1|         MALWMRLLPLLALLALWGPDPAAAFVNQHLCGSHLVEALYLVCGERGFFY 50

gi|57113877|ref|NP_001008996.1      MALWMRLLPLLVLLALWGPDPASAFVNQHLCGSHLVEALYLVCGERGFFY 50

gi|82749718|gb|ABB89743.1|          MALWMRFLPLLALLVVWEPKPAQAFVKQHLCGPHLVEALYLVCGERGFFY 50

                                    ******:****.**.:* *.** ***:*****.*****************


gi|4557671|ref|NP_000198.1|         TPKTRREAEDLQVGQVELGGGPGAGSLQPLALEGSLQKRGIVEQCCTSIC 100

gi|57113877|ref|NP_001008996.1      TPKTRREAEDLQVGQVELGGGPGAGSLQPLALEGSLQKRGIVEQCCTSIC 100

gi|82749718|gb|ABB89743.1|          TPKSRREVEDPQVPQLELGGSPEAGDLQTLALEVARQKRGIVDQCCTSIC 100

                                    ***:***.** ** *:****.* **.**.**** : ******:*******


gi|4557671|ref|NP_000198.1|         SLYQLENYCN 110

gi|57113877|ref|NP_001008996.1      SLYQLENYCN 110

gi|82749718|gb|ABB89743.1|          SLYQLENYCN 110

                                    **********

Figure 1: Allignment of protein sequence of Insulin from various species listed in Figure 2.


Figure one shows that the protein sequence of human preproinsulin is very similar and mostly identical to the protein sequences of preproinsulin in the chimpanzee as well as the field rat.

Sequence 1: gi|4557671|ref|NP_000198.1|      110 aa Human
Sequence 2: gi|57113877|ref|NP_001008996.1   110 aa Chimpanzee
Sequence 3: gi|82749718|gb|ABB89743.1|       110 aa Lesser rice-field rat

Figure 2: List of sequences corresponding to each species and number of amino acids

Sequences (1:2) Aligned. Score:  98
Sequences (1:3) Aligned. Score:  80
Sequences (2:3) Aligned. Score:  80

Figure 3: Comparison of protein sequence of each species by percent similarity score.

As shown in figure 3, Human preproinsulin is 98% similar to preproinsulin of the Chimpanzee and 80% with the field rat. This shows how much the structure of insulin is conserved among various species. 

Human insulin most often differs from other mammalian insulin in the 8th, 9th and 10th positions within the intra-chain disulfide bond of the A chain and the 30th position of the B chain. (Hadley, and Levine 241-244)

Insulin acts by activating plasma membrane receptors that have tyrosine kinase activity. The insulin receptor has alpha subunit that contains the insulin binding domain and a beta subunit that contains a tyrosine kinase domain. It contains two alpha and two beta subunits covalently attached by inter and intra subunit disulfide bridges. Insulin binds to the alpha subunits causing a conformational change in the receptor complex and the beta subunit is autophosphorylated  and becomes an activated tyrosine kinase which then phosphorylates multiple intracellular proteins. 

The major enzyme responsible for insulin degradation in the body is hepatic gluthione insulin dehydrogenase which acts by breaking insulin into its seperate A and B chains. The enzyme acts with glutathione which reduces the individual half cysteine moieties of the interchain disulfide bonds and acts as a cofactor for the dehydrogenase enzyme. (Hadley, and Levine 241-244)

References:

Hadley, Mac E., and Jon E. Levine. Endocrinology. 6th ed. Upper Saddle River, NJ: Pearson, 2006. 241-244. Print. 


http://www.ebi.ac.uk/Tools/clustalw2/index.html?

http://www.ncbi.nlm.nih.gov/BLAST/

 http://www.ncbi.nlm.nih.gov/

First Assignment: My Favorite Hormone- Insulin

Insulin is a peptide hormone produced by the beta cells of the islet of Langerhans in the pancreas. The term insulin comes from the Latin word for islet/island.  It was  first isolated by Nicolae Polescue in 1921. In 1958, the primary structure of insulin was discovered by Frederick Sanger, a British molecular biologist. Since it was the first protein sequence to be determined, he was awarded the Nobel prize in chemistry for his work. (De Meyts)

 Figure 1: Structure of Human Insulin.(http://rst.gsfc.nasa.gov/Sect20/A12.html)

 The biologically active circulating form of insulin is a monomer, which consists of two chains, an A chain with 21 amino acids and a B chain of 30 amino acids. (in humans). The A and B chains are linked by two disulfide bridges, A7-B7 and A20-B19. The A chain also contains an intra-chain disulfide bridge connecting A6 and A11. Insulin is produced and stored in the body as a hexamer. It is far more stable and non-reactive than the monomer form. The structure of insulin is well preserved in vertebrate animals, with slight variations in structure. Cow insulin differs from human insulin in only three amino acid residues, and pig insulin differs in only one. (Mayer, Zhang and DiMarchi)


Insulin functions to control blood glucose concentration. When released it stimulates cells in the liver, muscle and adipose tissue to take up glucose from the blood and store it as glycogen in the liver and muscle tissue. This also functions to stop the use of lipids as an energy source. When insulin is absent, glucose is not taken up by the body and lipids instead will be mobilized to the liver as an energy source. Insulin has also been shown to have effects on the brain such as improving cognition. Once in the brain, insulin enhances both learning and memory.  (Soria, Tuduri, Gonzalez, Martin, and Nadal 52-60)

References:

De Meyts, Pierre. "Insulin and its Receptor: Structure, Function and Evolution." BioEssays 26.12 (2004): n. pag. Web. 11 Oct 2010.

Mayer, John P., Faming Zhang, and Richard D. DiMarchi. "Insulin Structure and Function." Peptide Science 88.5 (2007): n. pag. Web. 11 Oct 2010.

Soria, Bernat, Eva Tuduri, Alejandro Gonzalez, Franz Martin, and Angel Nadal. "Pancreatic Islet Cells: a Model for Calcium-dependent Peptide Release." HFSP Journal 4.2 (2010): 52-60. Web. 11 Oct 2010.


http://www.microscopyu.com/galleries/pathology/index.html