Converting blood coagulation factor IXa into factor Xa: dramatic increase in amidolytic activity identifies important active site determinants

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Abstract
The coagulation factors IXa (fIXa) and Xa (fXa) share extensive structural and functional homology; both cleave natural substrates effectively only with a cofactor at a phospholipid surface. However, the amidolytic activity of fIXa is 104‐fold lower than that of fXa. To identify determinants of this poor reactivity, we expressed variants of truncated fIXa (rf9a) and fXa (rf10a) in Escherichia coli. The crystal structures of fIXa and fXa revealed four characteristic active site components which were subsequently exchanged between rf9a and rf10a. Exchanging Glu219 by Gly or exchanging the 148 loop did not increase activity of rf9a, whereas corresponding mutations abolished reactivity of rf10a. Exchanging Ile213 by Val only moderately increased reactivity of rf9a. Exchanging the 99 loop, however, dramatically increased reactivity. Furthermore, combining all four mutations essentially introduced fXa properties into rf9a: the amidolytic activity was increased 130‐fold with fXa substrate selectivity. The results suggest a 2‐fold origin of fIXa's poor reactivity. A narrowed S3/S4 subsite disfavours interaction with substrate P3/P4 residues, while a distorted S1 subsite disfavours effective cleavage of the scissile bond. Both defects could be repaired by introducing fXa residues. Such engineered coagulation enzymes will be useful in diagnostics and in the development of therapeutics.