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, Glu(1)-plasminogen (Glu(1)-HPg, Glu(1)-Asn(791)) and Lys(78)-plasminogen (Lys(78)-HPg, Lys(78)-Asn(791)), and two enzymatically derived plasminogen fragments, miniplasminogen (mini-HPg, Val(443)-Asn(791)) and microplasminogen (micro-HPg, Lys(531)-Asn(791)) were analysed by circular 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. Differential 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 published data (Novokhatny, V. V., Kudinov, S. A. and Privalov, P. L. J. Mol. Biol. 1984, 179, 215). Three thermal transitions corresponding to kringle 5 and to two subdomains of the C-terminal protease region were identified for mini-HPg. In micro-H Pg, the two thermal transitions of the protease region were found but one of the protease subdomains was modified and its stability was much higher than in any of the other studied proteins. According to the microcalorimetric data obtained for mini-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 region, respectively, in contrast to literature data. From circular dichroism measurements, mini-HPg and micro-HPg can be classified as beta-II proteins which are characterized by distorted beta-strands and short irregular strands. Circular dichroism melting experiments yield lower transition temperatures (midpoint at 70 degrees C) for micro-HPg than the microcalorimetric measurements (transition temperatures trs(6) = 73.3 degrees C and trs(7) = 73.8 degrees C), but, in agreement with the microcalorimetric data, show a higher thermostability of micro-HPg in comparison with mini-HPg.