title: Cooperative roles of factor V(a) and phosphatidylserine-containing membranes as cofactors in prothrombin activation.
abstract: Activation of prothrombin, as catalyzed by the prothrombinase complex (factor X(a), enzyme; factor V(a) and phosphatidylserine (PS)-containing membranes, cofactors), involves production and subsequent proteolysis of two possible intermediates, meizothrombin (MzII(a)) and prethrombin 2 plus fragment 1.2 (Pre2 & F1.2). V(max), K(m), or V(max)/K(m) for all four proteolytic steps was determined as a function of membrane-phospholipid concentration. Proteolysis was monitored using a fluorescent thrombin inhibitor, a chromogenic substrate, and SDS-PAGE. The kinetic constants for the conversion of MzII(a) and Pre2 & F1.2 to thrombin were determined directly. Pre2 & F1.2 conversion was linear in substrate concentration up to 4 microm, whereas MzII(a) proteolysis was saturable. First order rate constants for formation of MzII(a) and Pre2 & F1.2 could not be determined directly and were determined from global fitting of the data to a parallel, sequential model, each step of which was treated by the Michaelis-Menten formalism. The rate of direct conversion to thrombin without release of intermediates from the membrane-V(a)-X(a) complex (i.e. "channeling") also was adjusted because both the membranes and factor V(a) have been shown to cause channeling. k(cat), K(m), or k(cat)/K(m) values were reported for one lipid concentration, for which all X(a) was likely incorporated into a X(a)-V(a) complex on a PS membrane. Comparing previous results, which were obtained either with factor V(a) (Boskovic, D. S., Bajzar, L. S., and Nesheim, M. E. (2001) J. Biol. Chem. 276, 28686-28693) or with membranes individually (Wu, J. R., Zhou, C., Majumder, R., Powers, D. D., Weinreb, G., and Lentz, B. R. (2002) Biochemistry 41, 935-949), with results presented here we conclude that both factor V(a) and PS-containing membranes induce similar rate increases and pathway changes. Moreover, we have determined: 1) factor V(a) has the greatest effect in enhancing rates of individual proteolytic events; 2) PS-containing membranes have the greatest role in increasing the preference for the MzII(a) versus Pre2 pathway; and 3) PS membranes cause approximately 50% of the substrate to be activated via channeling at 50 microm membrane concentration, but factor V(a) extends the range of efficient channeling to much lower or higher membrane concentrations.
authors.affiliation: Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7260, USA.
authors.forename: Gabriel E
authors.initials: GE
authors.lastname: Weinreb
authors.forename: Kasturi
authors.initials: K
authors.lastname: Mukhopadhyay
authors.forename: Rinku
authors.initials: R
authors.lastname: Majumder
authors.forename: Barry R
authors.initials: BR
authors.lastname: Lentz
mesh_terms: D001665:Binding Sites; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000378:metabolism; D015943:Factor Va / Q000378:metabolism*; D007700:Kinetics; D008563:Membrane Lipids / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000378:metabolism; D010718:Phosphatidylserines / Q000378:metabolism*; D011516:Prothrombin / Q000378:metabolism*; D013917:Thrombin / Q000378:metabolism*
chemical_list: D004792:Enzyme Precursors; D008563:Membrane Lipids; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D015943:Factor Va; D011516:Prothrombin; D013917:Thrombin; C026812:meizothrombin; C017251:1,2-oleoylphosphatidylcholine