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Ribosomes feed the growing amino acid chain (preproinsulin) directly into the ER where the signal peptide (red) is immediately cleaved 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 translational product of the INS gene. It is a peptide that is 110 amino acids in length. Preproinsulin is a proinsulin molecule with a signal peptide attached to its N-terminus.

Insulin synthesis pathway[edit]

Preproinsulin is a biologically inactive precursor to the biologically active endocrine hormone insulin. Preproinsulin is converted into proinsulin by signal peptidases, which remove its signal peptide from its N-terminus. Finally, proinsulin is converted into the bioactive hormone insulin by removal of the C-peptide.

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

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