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Functional Considerations: Insulin Synthesis, An Example of Posttranslational Processing

Insulin is a small protein consisting of two polypeptide chains joined by disulfide bridges. Its biosynthesis presents a clear example of the importance of posttranslational processing in the achievement of the final, active structure of a protein.

Insulin is originally synthesized as a single 110-amino acid polypeptide chain with a molecular weight of about 12,000 Da. This polypeptide is called preproinsulin. Preproinsulin is reduced to a polypeptide with a molecular weight of about 9000 Da, called proinsulin, as the molecule is inserted into the cis-ternae of the rER. Proinsulin is a single polypeptide chain of 81 to 86 amino acids that has the approximate shape of the letter G. Two disulfide bonds connect the bar of the G to the top loop.

During packaging and storage of proinsulin in the Golgi apparatus, a cathepsin-like enzyme cleaves most of the side of the loop, leaving the bar of the G as an A chain of 21 amino acids cross-linked by the disulfide bridges to the top of the loop, which becomes the B chain of 30 amino acids. The 35-amino acid peptide removed from the loop is called a Cpeptide (connecting peptide). It is stored in the secretory vesicles and released with the insulin in equimolar amounts. No function has been identified for the C peptide. However, measurement of circulating levels of C peptide sometimes provides important clinical information on the secretory activity of B cells.

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Supplements For Diabetics

Supplements For Diabetics

All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.

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