Nick Menhart , Ph.D.

My major research interest concern the use of spectroscopic techniques to study multi-domain proteins. In many cases high resolutions structural data is available for only individual domains of proteins, while their arrangement within the context of the entire molecule is unknown. This arrangement is crucial to the function in certain cases.

One of the systems currently under study, plasminogen, is such a case. This is a key enzyme in the haemostatic system, being responsible for the dissolution of blood clots. Treatments for heart attack and stroke, the first and third leading causes of mortality in affluent societies, involve activating plasminogen to its active form, plasmin. However, plasminogen normally circulates in a compact from that is not easily activated. It is known that it can adopt a more open form, for instance on the surface of a fibrin clot, that is much more readily activated.

This open form - closed form transition has been studies by a wide variety of techniques, however detailed structural information has not been forthcoming, since the intact molecule is too large for current NMR instruments, and has so far been refractory to crystallization. However, plasminogen is composed of several constituent units; in addition to its protease domains (~25kDa), plasminogen contains in five other compact domains known as kringles (~10kDa). The structures of most of these domains are known in isolation by either X-ray or NMR studies, or in some cases both. We are attempting to study this transition by placing spectroscopic probes (paramagnetic or fluorescent) on the various domains of plasminogen to try to localize these specific domains and determine their proximity to each other during this transition. By combining distance measurement between these domains, solvent accessibility data for the various surfaces, with the known high resolution structures of each isolated domain, we hope be able to assemble a model of the structure of the intact molecule in each of its conformational states.