Kininogens are precursor proteins for kinins, biologically active polypeptides involved in blood coagulation, vasodilation, smooth muscle contraction, inflammatory regulation, and the regulation of the cardiovascular and renal systems.

There are two main types of kininogen (KNG), high-molecular-weight-kininogen and low-molecular-weight-kininogen, with a third type – T-kininogen – only found in rats but not humans.

High-molecular-weight-kininogen (HK) is a non-enzymatic cofactor involved in the kinin-kallikrein system, which plays a role in blood coagulation, blood pressure regulation, and inflammation. It is synthesized in endothelial cells and is produced mostly by the liver. It is also a precursor protein for bradykinin.

Low-molecular-weight-kininogen (LK) is mainly a precursor protein for kallidin. LK, however, is not actively involved in blood coagulation, but its byproducts can be later converted and introduced to the coagulation pathway.

T-kininogen (TK) is only found in rats and is a protein whose function is still being researched. TK is believed to be a biological indicator of senescence in rats, which can be measured by the level of endothelial cell production during the aging process.

HK consists of 644 amino acid residues, which are separated into six different domains. Domains 1, 2, and 3 are called the "heavy chain" with Domains 2 and 3 having cysteine protease activity. Domains 5 and 6 are called the "light chain," both of which bind specific molecules: Domain 5 binds heparin and zinc and selectively binds to anionic surfaces while Domain 6 binds prekallikrein, the protease precursor to plasma kallikrein. Domain 4 connects the heavy chain and light chain together, and its cleavage at this site releases bradykinin.

LK consists of 427 amino acid residues, which can also be separated into a "heavy chain" and a "light chain."

T-kininogen consists of 430 amino acid residues.