BackgroundCoagulation factor XII is a serine protease that is important for kinin generation and blood coagulation, cleaving the substrates plasma kallikrein and FXI.ObjectiveTo investigate FXII zymogen activation and substrate recognition by determining the crystal structure of the FXII protease domain.Methods and resultsA series of recombinant FXII protease constructs were characterized by measurement of cleavage of chromogenic peptide and plasma kallikrein protein substrates. This revealed that the FXII protease construct spanning the light chain has unexpectedly weak proteolytic activity compared to β-FXIIa, which has an additional nine amino acid remnant of the heavy chain present. Consistent with these data, the crystal structure of the light chain protease reveals a zymogen conformation for active site residues Gly193 and Ser195, where the oxyanion hole is absent. The Asp194 side chain salt bridge to Arg73 constitutes an atypical conformation of the 70-loop. In one crystal form, the S1 pocket loops are partially flexible, which is typical of a zymogen. In a second crystal form of the deglycosylated light chain, the S1 pocket loops are ordered, and a short α-helix in the 180-loop of the structure results in an enlarged and distorted S1 pocket with a buried conformation of Asp189, which is critical for P1 Arg substrate recognition. The FXII structures define patches of negative charge surrounding the active site cleft that may be critical for interactions with inhibitors and substrates.ConclusionsThese data provide the first structural basis for understanding FXII substrate recognition and zymogen activation.
The emergence of the novel coronavirus and then pandemic outbreak was coined 2019- nCoV or COVID-19 (or SARS-CoV-2 disease 2019). This disease has a mortality rate of about 3·7 percent, and successful therapy is desperately needed to combat it. The exact cellular mechanisms of COVID-19 need to be illustrated in detail. This study aimed to evaluate serum cytokines in COVID-19 patients. In this study, serum was collected from volunteer individuals, moderate COVID-19 patients, severe cases of COVID-19 patients, and patients who recovered from COVID-19 (n = 122). The serum concentrations of interleukins such as IL-1, IL-4, IL-6, IL-8, IL-10, and tumor necrosis factor-alpha (TNF-α), were measured by enzyme-linked immunosorbent assays (ELISA). The concentrations of IL-1 and TNF-α were did not differ significantly among groups. However, the concentration of IL-6 was significantly higher in moderate COVID-19 and severe cases of COVID-19 groups compared to control and recovered groups indicating it to be an independent predictor in the coronavirus disease. The levels of IFN-γ and IL-4 were significantly lower in the recovery group than the severe case of the COVID-19 group. In contrast, the level of IL-10 in recovered COVID-19 patients was significantly higher in compare to severe cases, COVID-19 patients. Varying levels of cytokines were detected in COVID-19 group than control group suggesting distinct immunoregulatory mechanisms involved in COVID-19 pathogenesis. However, additional investigations are needed to be to be performed to understand the exact cellular mechanism of this disease.
SARS-CoV-2 or Coronavirus disease 2019 (COVID-19) outbreak which caused by the severe acute respiratory syndrome, has rapidly spread over the world. The exact mechanism how this virus will affect the liver remained elusive. The aim of this study was to evaluate the liver function in patients with severe acute respiratory syndrome coronavirus 2 and potential causes of hepatic enzymes disease in these patients. Clinical characteristics and laboratory findings were collected from patients with COVID-19 who were admitted to the corona center in Erbil city/Kurdistan region of Iraq, from March 10 to July 10, 2020. Serum was collected from patients with COVID-19 and liver enzyme tests were measured. Liver alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and total bilirubin (TBIL) were analyzed in these patients. Of the 74 patients, 25 (34.7%) had abnormal ALT activity, 28 (40%) had abnormal AST activity, 12 (20.3%) had abnormal ALP activity, and 39 (52.7%) had abnormal total bilirubin P -value <0.05. The inflammatory biomarkers CRP and IL-6 in COVID-19 patients with abnormal liver function test (4.9±1.0 mg/dl) and (231.2±35.7 pg/ml) respectively. The levels of both biomarkers were statistically significantly higher than COVID-19 patients with normal liver function test (2.1±0.5 mg/dl) and (2.1±0.5 mg/dl) respectively, P -value <0.05. However, CRP and IL-6 were not statistically significant different between male and female COVID-19 patients P -value <0.05. In conclusion, we found that most of the patients with SARS-CoV-2 have abnormal hepatic enzyme activities and that is might related to virus replication in the liver.
Coagulation factor XII (FXII) is a key initiator of the contact pathway, which contributes to inflammatory pathways. FXII circulates as a zymogen, which when auto-activated forms factor XIIa (FXIIa). Here, the production of the recombinant FXIIa protease domain (FXIIa His ) with yields of $1-2 mg per litre of insect-cell culture is reported. A second construct utilized an N-terminal maltose-binding protein (MBP) fusion (MBP-FXIIa His ). Crystal structures were determined of MBP-FXIIa His in complex with the inhibitor d-Phe-Pro-Arg chloromethyl ketone (PPACK) and of FXIIa His in isolation. The FXIIa His structure revealed that the S2 and S1 pockets were occupied by Thr and Arg residues, respectively, from an adjacent molecule in the crystal. The Thr-Arg sequence mimics the P2-P1 FXIIa cleavage-site residues present in the natural substrates prekallikrein and FXII, and Pro-Arg (from PPACK) mimics the factor XI cleavage site. A comparison of the FXIIa His structure with the available crystal structure of the zymogen-like FXII protease revealed large conformational changes centred around the S1 pocket and an alternate conformation for the 99-loop, Tyr99 and the S2 pocket. Further comparison with activated protease structures of factors IXa and Xa, which also have the Tyr99 residue, reveals that a more open form of the S2 pocket only occurs in the presence of a substrate mimetic. The FXIIa inhibitors EcTI and infestin-4 have Pro-Arg and Phe-Arg P2-P1 sequences, respectively, and the interactions that these inhibitors make with FXIIa are also described. These structural studies of FXIIa provide insight into substrate and inhibitor recognition and establish a scaffold for the structure-guided drug design of novel antithrombotic and antiinflammatory agents.
Essentials Corn Trypsin Inhibitor (CTI) is a selective inhibitor of coagulation Factor XII (FXII).Molecular modelling of the CTI‐FXIIa complex suggested a canonical inhibitor binding mode.Mutagenesis revealed the CTI inhibitory loop and helices α1 and α2 mediate the interaction.This confirms that CTI inhibits FXII in canonical fashion and validates the molecular model. SummaryBackgroundCorn trypsin inhibitor (CTI) has selectivity for the serine proteases coagulation factor XII and trypsin. CTI is in widespread use as a reagent that specifically inhibits the intrinsic pathway of blood coagulation but not the extrinsic pathway.ObjectivesTo investigate the molecular basis of FXII inhibition by CTI.MethodsWe performed molecular docking of CTI, using its known crystal structure, with a model of the activated FXII (FXIIa) protease domain. The interaction model was verified by use of a panel of recombinant CTI variants tested for their ability to inhibit FXIIa enzymatic activity in a substrate cleavage assay.ResultsThe docking predicted that: (i) the CTI central inhibitory loop P1 Arg34 side chain forms a salt bridge with the FXIIa S1 pocket Asp189 side chain; (ii) Trp22 from CTI helix α1 interacts with the FXIIa S3 pocket; and (iii) Arg43 from CTI helix α2 forms a salt bridge with FXIIa H1 pocket Asp60A. CTI amino acid substitution R34A negated all inhibitory activity, whereas the G32W, L35A, W22A and R42A/R43A substitutions reduced activity by large degrees of 108‐fold, 41‐fold, 158‐fold, and 100‐fold, respectively; the R27A, W37A, W39A and R42A substitutions had no effect. Synthetic peptides spanning CTI residues 20–44 had inhibitory activity that was three‐fold to 4000‐fold less than that of full‐length CTI.ConclusionsThe data confirm the validity of a canonical model of the FXIIa–CTI interaction, with helix α1 (Trp22), central inhibitory loop (Arg34) and helix α2 (Arg43) of CTI being required for effective binding by contacting the S1, S3 and H1 pockets of FXIIa, respectively.
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