CONFORMATIONS AND STABILITIES OF HUMAN GLU(1)-PLASMINOGEN AND LYS(78)-PLASMINOGEN AND OF THE FRAGMENTS MINIPLASMINOGEN AND MICROPLASMINOGEN, ANALYZED BY CIRCULAR-DICHROISM AND DIFFERENTIAL SCANNING CALORIMETRY

The conformations and stabilities of two forms of human plasminogen, G lu(1)-plasminogen (Glu(1)-HPg, Glu(1)-Asn(791)) and Lys(78)-plasminoge n (Lys(78)-HPg, Lys(78)-Asn(791)), and two enzymatically derived plasm inogen fragments, miniplasminogen (mini-HPg, Val(443)-Asn(791)) and mi croplasminogen (micro-HPg, Lys(531)-Asn(791)) were analysed by circula r dichroism and differential scanning calorimetry. The two plasminogen forms differ by the lack of 77 N-terminal amino acids in Lys(78)-HPg in comparison to Glu(1)-HPg. Mini-HPg is composed of kringle 5 and the protease domain of HPg whereas micro-H Pg is built from the protease domain of HPg and a stretch of about 15 amino acids from kringle 5. Di fferential scanning calorimetric measurements of Glu(1)-HPg and Lys(78 )-HPg reveal seven thermal transitions for both plasminogen forms. The results obtained for Lys(78)-HPg largely agree with recently publishe d data (Novokhatny, V. V., Kudinov, S. A. and Privalov, P. L. J. Mol. Biol. 1984, 179, 215). Three thermal transitions corresponding to krin gle 5 and to two subdomains of the C-terminal protease region were ide ntified for mini-HPg. In micro-H Pg, the two thermal transitions of th e protease region were found but one of the protease subdomains was mo dified and its stability was much higher than in any of the other stud ied proteins. According to the microcalorimetric data obtained for min i-HPg and micro-HPg, transitions 5 and 6 of Glu(1)-HPg and Lys(78)-HPg were reassigned to kringle 5 and to a subdomain of the protease regio n, respectively, in contrast to literature data. From circular dichroi sm measurements, mini-HPg and micro-HPg can be classified as beta-II p roteins which are characterized by distorted beta-strands and short ir regular strands. Circular dichroism melting experiments yield lower tr ansition temperatures (midpoint at 70 degrees C) for micro-HPg than th e microcalorimetric measurements (transition temperatures trs(6) = 73. 3 degrees C and trs(7) = 73.8 degrees C), but, in agreement with the m icrocalorimetric data, show a higher thermostability of micro-HPg in c omparison with mini-HPg.