Binding of a substrate analogue can induce co-operative structure in the plasmin serine-proteinase domain

Human miniplasminogen and miniplasmin were studied by n.m.r. spectroscopy and differential scanning calorimetry (d.s.c.) in order to investigate the structural properties of the serine-proteinase domain. The d.s.c. thermograms of both miniplasminogen and non-inactivated miniplasmin at pH 4.0 can be closely fitted to two transitions, at 62 +/- 2 and 72 +/- 2 degrees C, corresponding to unfolding of the kringle 5 and proteinase domains respectively. No evidence was found, under these conditions, for non-co-operative unfolding of the proteinase domain. On inactivation of miniplasmin with an affinity label, a number of additional resonances arising from residues of the proteinase domain are observed in resolved regions of the n.m.r. spectrum. A combination of variable-temperature n.m.r. and d.s.c. has shown that part of the proteinase domain undergoes a major conformational transition on heating which is distinct from the unfolding of the remainder of the proteinase domain or the kringle 5 domain. This additional transition occurs at a temperature that depends on the nature of the affinity label present and is not observed in the absence of an inactivating agent. These results provide direct evidence for the existence of a region of the proteinase domain which, under these conditions, becomes structured only in the presence of a bound substrate.