Preproinsulin

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Ribosomes feed the growing amino acid chain (preproinsulin) directly into the ER where the signal peptide (red) is immediately clipped off by the signal peptidase (red triangle) to yield proinsulin. This is later processed further to mature and active insulin. Cell components and proteins in this image are not to scale.
Ribosomes feed the growing amino acid chain (preproinsulin) directly into the ER where the signal peptide (red) is immediately clipped off by the signal peptidase (red triangle) to yield proinsulin. This is later processed further to mature and active insulin. Cell components and proteins in this image are not to scale.

Preproinsulin is the primary translation product of the insulin gene. It is a peptide of 110 amino acids. Preproinsulin is a precursor processed by proteases to proinsulin by removal of the signal peptide and then to insulin by removal of the connecting peptide (C-peptide) in order to be biologically active.

Almost no preproinsulin exists in the cell, since removal of the signal peptide is not a separate step but closely linked to translation into the endoplasmic reticulum (ER). For the same reason, preproinsulin is rarely used unlike insulin, the mature product, and proinsulin, a stable ER intermediate.

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