Antithrombin - Cleaved and Latent Antithrombin

Cleaved and Latent Antithrombin

Cleavage at the reactive site results in entrapment of the thrombin protease, with movement of the cleaved reactive site loop together with the bound protease, such that the loop forms an extra sixth strand in the middle of beta sheet A. This movement of the reactive site loop can also be induced without cleavage, with the resulting crystallographic structure being identical to that of the physiologically latent conformation of plasminogen activator inhibitor-1 (PAI-1). For this reason the conformation of antithrombin in which the reactive site loop is incorporated uncleaved into the main body of the protein is referred to as latent antithrombin. In contrast to PAI-1 the transition for antithrombin from a normal or native conformation to a latent conformation is irreversible.

Native antithrombin can be converted to latent antithrombin (L-antithrombin) by heating alone or heating in the presence of citrate. However, without extreme heating and at 37°C (body temperature) 10% of all antithrombin circulating in the blood is converted to the L-antithrombin over a 24 hour period. The structure of L-antithrombin is shown in Figure 6.

The 3-dimensional structure of native antithrombin was first determined in 1994. Unexpectedly the protein crystallized as a heterodimer composed of one molecule of native antithrombin and one molecule of latent antithrombin. Latent antithrombin on formation immediately links to a molecule of native antithrombin to form the heterodimer, and it is not until the concentration of latent antithrombin exceeds 50% of the total antithrombin that it can be detected analytically. Not only is the latent form of antithrombin inactive against its target coagulation proteases, but its dimerisation with an otherwise active native antithrombin molecule also results in the native molecules inactivation. The physiological impact of the loss of antithrombin activity either through latent antithrombin formation or through subsequent dimer formation is exacerbated by the preference for dimerisation to occur between heparin activated β-antithrombin and latent antithrombin as opposed to α-antithrombin.

A form of antithrombin that is an intermediate in the conversion between native and latent forms of antithrombin has also been isolated and this has been termed prelatent antithrombin.