The trap hypothesis: α2 and protease inhibition

In the 1970s, the Strangeways Laboratory in Cambridge consisted of a small number of groups collectively focused on the mechanisms of pathological connective-tissue damage. One of these groups, headed by Alan Barrett, was breaking ground on the destruction of the protein components of the matrix and was therefore heavily involved in identifying and categorizing newly emerging types of tissue-degrading enzymes. These enzymes, which Alan Barrett urges scientists to call peptidases, are also commonly called proteases or proteinases*. In the early 1970s, there were about 100 described human peptidases, a reasonable sampling of the 500–600 now known in humans in the post-genomic age. Approximately 2% of the human genome encodes peptidases, and roughly 1% encodes proteins with the ability to inhibit these enzymes. As the peptidases developed different catalytic mechanisms to solve the problem of cleaving the notoriously stable peptide bond, so the families of protease inhibitors acquired distinct strategies to regulate peptidase action. The strategies are usually directed towards blocking the peptidase active site directly or, less commonly, by allosteric mechanisms. But perhaps the most bizarre mechanism is that performed by members of the protein clan exemplified by the human protein 2-macroglobulin (α2).