The human insulin gene contains (3) exons; exon (1) encodes the signal peptide, exon (2) encodes the B chain, and partial C-peptide, while exon (3) encodes the A chain and the remainder of the C-peptide. Insulin biosynthesis begins with translocation of the precursor preproinsulin to proinsulin through a sequential physiochemical process that occurs on the endoplasmic reticulum and results in mature insulin with (53) residues stored on secretory granules 1, 5.
This study revealed seven novel mutations through the coding sequence of the proinsulin gene with no evidence of their biological effect on human samples. All the mutated residues reduced protein stability and they were bigger in size than the wild residues with the different charges that affect the physicochemical property of the residues. The majority of mutated amino acids are distributed on both exon (2) and (3) of the interesting gene which might have a negative effect on the protein folding process.
Normally, AMP kinase (AMPK) plays an important role in glucose uptake and metabolism in skeletal muscle. Previous studies reported a reduced insulin-stimulated metabolic flexibility in women with PCOS, which linked to low levels of both pyruvate dehydrogenase (PDH) and AMPK. In addition, early gene-chip studies suggested that mitochondrial dysfunction, as well as ER stress, may contribute to the PCOS-specific insulin resistance because there was reduction of oxidative phosphorylation among women with PCOS that associated with hyperinsulinemia and excess androgens. Besides, that PCOS women had high serum levels of LH and insulin express the polymorphism rs5030775 (A18T) which is associated with more potent stimulation of the IP3 response than a cAMP pathway 26–28.
Additionally, there are remarkable differences in physicochemical properties between the wild type and mutant amino acids which results in loss of hydrophobicity interactions, increase the electroneutral reactions which might produce bumps, unusual torsion angles, and formation of abnormal binding sites.. Mutant proinsulin (C96Y) expressed in Chinese hamster ovary cells results on the accumulation of mutant proinsulin in the ER which prompt beta-cell dysfunction This phenotypic pattern differs from that of any human insulin mutation, however, was found in a family thought to have MODY 29–31.
Based on conservation scores, the substitutions of glycine (rs765512575 (G44R) and rs1313322794 (G47R)) and Proline (SNP ID: rs145038693 (P52R)) on different positions might probably damage protein structure because they prevent normal folding and progression of proinsulin into insulin secretory pathway. Therefore, abnormally folded proinsulin molecules may result in degradation in the endoplasmic reticulum, leading to severe endoplasmic reticulum stress and potentially beta-cell death by apoptosis, as has been described in both the Akita and Munich mouse models 3, 32.
While these mutations are exceptional, their molecular analysis has provided important insights into the biochemical bases of the hormone’s pathway of biosynthesis and mechanism of receptor binding. Denaturation of secondary, tertiary or quaternary structure of the protein due to exposure to a stress factor such as presence of hydrophobic surface, temperature or pH will results in unfolding of the protein into a random or misfolded shape 10.
Hydrogen bonds afford the furthermost role of directional interactions that support the molecular recognition with essential impact on protein folding, and protein structure. Occurrence of SNP ID: rs1182567488 (Q28H), SNP ID: rs1460766978 (P52T), SNP ID: rs781016664 (R56W), and SNP ID: rs760425445 G90D have no effect on the biological potency of protein despite their role on losing of the hydrogen interactions a protein and its ligands 35.
In patients with induced diabetes of youth (MIDY), characterized by decreased insulin secretion because the mutant proinsulin expression prevents WT proinsulin from exiting the endoplasmic reticulum (ER), which is essential for insulin production.
Historically, MIDY mutants exhibit misfolding in proinsulin formation and therefore cannot form bioactive insulin. Moreover, the initial onset of clinical insulin deficiency appears to be determined by the foremost folding of proinsulin which may lead to impair its release from ER and then reduce bioactive insulin production. This might cause hyperglycemia which may evoke β-cells to further upregulate proinsulin synthesis36.
In conclusion, Insulin protein plays an essential role through different metabolic pathways and any abnormal expression and/or damaging of its molecular background can affect its metabolic functions and results on numerous phenotypic picture of disease.