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/