Cardiac disorders such as acute myocardial infarction, embolism and stroke are primarily attributed to excessive fibrin accumulation in the blood vessels, usually consequential in thrombosis. Numerous methodologies including the use of anti-coagulants, anti-platelet drugs, surgical operations and fibrinolytic enzymes are employed for the dissolution of fibrin clots and hence ameliorate thrombosis. Microbial fibrinolytic enzymes have attracted much more attention in the management of cardiovascular disorders than typical anti-thrombotic strategies because of the undesirable after-effects and high expense of the latter. Fibrinolytic enzymes such as plasminogen activators and plasmin-like proteins hydrolyse thrombi with high efficacy with no significant after-effects and can be cost effectively produced on a large scale with a short generation time. However, the hunt for novel fibrinolytic enzymes necessitates complex purification stages, physiochemical and structural-functional attributes, which provide an insight into their mechanism of action. Besides, strain improvement and molecular technologies such as cloning, overexpression and the construction of genetically modified strains for the enhanced production of fibrinolytic enzymes significantly improve their thrombolytic potential. In addition, the unconventional applicability of some fibrinolytic enzymes paves their way for protein hydrolysis in addition to fibrin/thrombi, blood pressure regulation, anti-microbials, detergent additives for blood stain removal, preventing dental caries, anti-inflammatory and mucolytic expectorant agents. Therefore, this review article encompasses the production, biochemical/structure-function properties, thrombolytic potential and other surplus applications of microbial fibrinolytic enzymes.

The present study demonstrates the production and thrombolytic potential of a novel thermostable thiol-dependent fibrinolytic protease by Bacillus cereus RSA1. Statistical optimization of different parameters was accomplished with Plackett–Burman design and validated further by central composite design with 30.75 U/mL protease production. Precipitation and chromatographic approaches resulted in 33.11% recovery with 2.32-fold purification. The molecular weight of fibrinolytic protease was 40 KDa and it exhibited a broad temperature and pH stability range of 20–80 °C and pH 5–10 with utmost activity at 50 °C and pH 8, respectively. The protease retained its fibrinolytic activity in organic solvents and enhanced the activity in solutions with divalent cations (Mn²⁺, Zn²⁺, and Cu²⁺). The enzyme kinetics revealed K_m and V_max values of 1.093 mg/mL and 52.39 µg/mL/min, respectively, indicating higher affinity of fibrinolytic activity towards fibrin. Also, complete inhibition of fibrinolytic activity with DFP and a 2-fold increase with DTT and β-mercaptoethanol indicates its thiol-dependent serine protease nature. MALDI–TOF analysis showed 56% amino acid sequence homology with Subtilisin NAT OS = Bacillus subtilis subsp. natto. The fibrinolysis activity was compared with a commercial thrombolytic agent for its therapeutic applicability, and fibrinolytic protease was found highly significant with absolute blood clot dissolution within 4 h in in vitro conditions. The isolated fibrinolytic protease of Bacillus cereus RSA1 is novel and different from other known fibrinolytic proteases with high stability and efficacy, which might have wide medicinal and industrial application as a thrombolytic agent and in blood stain removal, respectively.

Blood clot formation in blood vessels (thrombosis) is a major cause of life-threatening cardiovascular diseases. These clots are formed by αA-, βB-, and ϒ-peptide chains of fibrinogen joined together by isopeptide bonds with the help of blood coagulation factor XIIIa. These clot structures are altered by various factors such as thrombin, platelets, transglutaminase, DNA, histones, and red blood cells. Various factors are used to dissolve the blood clot, such as anticoagulant agents, antiplatelets drugs, fibrinolytic enzymes, and surgical operations. Fibrinolytic enzymes are produced by microorganisms (bacteria, fungi, etc.): streptokinase of Streptococcus hemolyticus, nattokinase of Bacillus subtilis YF 38, bafibrinase of Bacillus sp. AS-S20-I, longolytin of Arthrobotrys longa, versiase of Aspergillus versicolor ZLH-1, etc. They act as a thrombolytic agent by either enhancing the production of plasminogen activators (tissue or urokinase types), which convert inactive plasminogen to active plasmin, or acting as plasmin-like proteins themselves, forming fibrin degradation products which cause normal blood flow again in blood vessels. Fibrinolytic enzymes may be classified in two groups, as serine proteases and metalloproteases, based on their catalytic properties, consisting of a catalytic triad responsible for their fibrinolytic activity having different physiochemical properties (such as molecular weight, pH, and temperature). The analysis of fibrinolysis helps to detect hyperfibrinolysis (menorrhagia, renal failure, etc.) and hypofibrinolysis (diabetes, obesity, etc.) with the help of various fibrinolytic assays such as a fibrin plate assay, fibrin microplate assay, the viscoelastic method, etc. These fibrinolytic activities serve as a key aspect in the recognition of numerous cardiovascular diseases and can be easily produced on a large scale with a short generation time by microbes and are less expensive.