Renin stimulates production of angiotensin and thus affects blood pressure.Renin, also known as angiotensinogenase, is a circulating enzyme that participates in the renin-angiotensin system that mediates extracellular volume, arterial vasoconstriction, and consequently mean arterial blood pressure. The enzyme is secreted by the kidneys from specialized juxtaglomerular cells in response to decreases in glomerular filtration rate (a consequence of low blood volume), diminished filtered sodium chloride and sympathetic nervous system innervation. The enzyme circulates in the blood stream and hydrolyzes angiotensinogen secreted from the liver into the peptide angiotensin I. Angiotensin I is further cleaved in the lungs by endothelial bound angiotensin converting enzyme (ACE) into angiotensin II, the final active peptide. Renin is a member of the aspartic protease family. Structurally, aspartic proteases are bilobal enzymes, each lobe contributing a catalytic Aspartate residue, with an extended active site cleft localized between the two lobes of the molecule. The N- and C-terminal domains, although structurally related by a 2-fold axis, have only limited sequence homology except the vicinity of the active site. This suggests that the enzymes evolved by an ancient duplication event. The active site is located at the groove formed by the two lobes, with an extended loop projecting over the cleft to form an 11-residue flap, which encloses substrates and inhibitors in the active site. Specificity is determined by nearest-neighbor hydrophobic residues surrounding the catalytic aspartates, and by three residues in the flap. The enzymes are mostly secreted from cells as inactive proenzymes that activate autocatalytically at acidic pH. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).