Functional mapping of charged residues of the 82-116 sequence in factor Xa: evidence that lysine 96 is a factor Va independent recognition site for prothrombin in the prothrombinase complex.
Research article published in Biochemistry (2005)
Abstract
It has been hypothesized that two antiparallel structures comprised of residues 82-91 and 102-116 in factor Xa (fXa) may harbor a factor Va- (fVa-) dependent prothrombin recognition site in the prothrombinase complex. There are 11 charged residues in the 82-116 loop of human fXa (Glu-84, Glu-86, Lys-90, Arg-93, Lys-96, Glu-97, Asp-100, Asp-102, Arg-107, Lys-109, and Arg-115). With the exception of Glu-84, which did not express, and Asp-102, which is a catalytic residue, we expressed the Ala substitution mutants of all other residues and evaluated their proteolytic and amidolytic activities in both the absence and presence of fVa. K96A and K109A activated prothrombin with 5-10-fold impaired catalytic efficiency in the absence of fVa. All mutants, however, exhibited normal activity toward the substrate in the presence of fVa. K109A also exhibited impaired amidolytic activity and affinity for Na(+); however, both fVa and higher Na(+) restored the catalytic defect caused by the mutation. Analysis of the X-ray crystal structure of fXa indicated that Glu-84 may interact by a salt bridge with Lys-109, explaining the lack of expression of E84A and the lower activity of K109A in the absence of fVa. These results suggest that none of the residues under study is a fVa-dependent recognition site for prothrombin in the prothrombinase complex; however, Lys-96 is a recognition site for the substrate independent of the cofactor. Moreover, the 82-116 loop is energetically linked to fVa and Na(+) binding sites of the protease.
Abstract sourced from PubMed (NCBI) for the cited record. See the original publication for the authoritative version.
Summary
It has been hypothesized that two antiparallel structures comprised of residues 82-91 and 102-116 in factor Xa (fXa) may harbor a factor Va- (fVa-) dependent prothrombin recognition site in the prothrombinase complex. There are 11 charged residues in the 82-116 loop of human fXa (Glu-84, Glu-86,...
Why This Matters for Hirudotherapy
This study used alanine substitution mutagenesis to investigate charged residues in the 82–116 loop of human factor Xa, testing whether they form a factor Va-dependent prothrombin recognition site in the prothrombinase complex. The authors found that K96A and K109A showed 5–10-fold impaired catalytic efficiency for prothrombin activation in the absence of factor Va, yet all mutants exhibited normal activity in its presence, concluding that Lys-96 functions as a cofactor-independent substrate recognition site while none of the tested residues serves a factor Va-dependent role. This work holds relevance for hirudotherapy because factor Xa generates thrombin—the enzyme directly inhibited by leech-derived hirudin—and elucidating prothrombinase assembly advances understanding of the coagulation cascade that medicinal leech anticoagulants are used to modulate. The key limitation is that this basic biochemistry study involves no leech-derived substances, and its connection to clinical leech therapy is entirely indirect.
Citation
Functional mapping of charged residues of the 82-116 sequence in factor Xa: evidence that lysine 96 is a factor Va independent recognition site for prothrombin in the prothrombinase complex.
Manithody et al. · Biochemistry, 2005
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