Insulin

1. Nature & Source: Insulin is a 51-amino acid peptide hormone (molecular weight ~5.8 kDa) synthesized by β-cells of pancreatic islets as proinsulin, which is cleaved to active insulin and C‑peptide.

2. Main Action: Lowers blood glucose by stimulating glucose uptake (via GLUT4) in muscle & adipose tissue and promoting glycogenesis, lipogenesis, and protein synthesis while inhibiting gluconeogenesis.