Tissue Plasminogen Activator

Tissue plasminogen activator (tPA), also known as alteplase, is the most widely used treatment for ischemic stroke.¹ Ischemic stroke is one of the two types of stroke (the other being hemorrhagic), a common medical condition when a particular region of the brain lacks adequate blood supply either by means of a blood clot (ischemic) or a ruptured blood vessel (hemorrhagic).¹ The lack of blood supply and oxygen results in the death of brain cells and functional impairment.¹ tPA (brand name: Activase®) was the first FDA-approved treatment for ischemic stroke that dissolves blood clots and restores blood flow to the impaired region of the brain.⁶ The development of tPA as a treatment has led to quick and successful treatments and drastically increased the rate of survival for victims of stroke.¹

Figure 1. Diagram outlining the conversion of plasminogen to plasmin^​1

tPA is a serine proteolytic enzyme (protease) that catalyzes the conversion of plasminogen to plasmin (as shown in Figure 1), the enzyme that is primarily involved in the dissolution of blood clots, allowing blood to continue flowing into the affected region of the brain.² For this conversion to happen, the enzyme cleaves the inactive plasminogen, or a zymogen, at the peptide bond between arginine and valine to form a tertiary protein with a double polypeptide chain linked together with disulfide bonds.² Once the plasmin enzyme has been activated, it is able to break up the fibrin molecules, a protein involved in blood clots, and dissolve the blood clot, blocking blood supply to the brain.²

Figure 2. The primary structure of tPA with highlighted domains. ⁵

A key component of tPA’s mechanism of action is its structure. As mentioned before, tPA is a serine protease, specifically falling under the category of trypsin-like serine proteases, meaning that the amino acid serine acts as the nucleophilic amino acid at the enzyme’s active site. ³ In addition, the protease consists of 527 amino acids, several cysteine bridges, and cleavage sites. ⁶ tPA’s functions are largely determined by the structures and properties of its five distinct domains: the finger domain, the epidermal growth factor (EGF)-like domain, two kringle domains, K1 & K2, and a serine protease proteolytic domain (Fig. 2). ⁴

Figure 3. The cleavage site in plasminogen between the amino acids Arginine and Valine, made with Microsoft OneNote

These domains each uniquely allow tPA to interact with various binding proteins and receptors in the brain’s tissue.² The finger domain interacts with certain proteins to support the crossing of the infamous blood-brain barrier.² The EGF-like domain mediates trophic and mitogenic functions of tPA.² Moreover, while the K1 domain’s role is poorly investigated, the K2 domain is known to bind with various proteins in the blood and brain parenchyma (the brain’s functional tissue).² Lastly, the catalytic domain supports the tPA in its function to cleave the aforementioned Arg-Val peptide bond, forming the plasmin enzyme required to break down the blood clot (Fig. 3).²

Without the incredible mechanisms of the thrombolytic agent of tPA, the 800,000 people per year (in the US) who experience a stroke would likely not live much longer.⁵ Thanks to quick and effective treatment methods like tPA, stroke patients are far more likely to survive and recover from their condition now than a few decades ago. Along with the increased rate of survival, tPA has drastically reduced the cost of stroke treatment, saving $4 million dollars for every 1000 patients treated with tPA.⁵ However, medical research has a long way to go as artificial intelligence and machine learning, for better or for worse, will bring completely new and unexpected methods in the treatment of patients with stroke or any other medical condition.

Work Cited

1. Tissue Plasminogen Activator for Acute Ischemic Stroke (Alteplase, Activase®) | National Institute of Neurological Disorders and Stroke. www.ninds.nih.gov. https://www.ninds.nih.gov/about-ninds/impact/ninds-contributions-approved-therapies/tissue-plasminogen-activator-acute-ischemic-stroke-alteplase-activaser (accessed 2023-07-31).
2. Hedstrom, L. Serine Protease Mechanism and Specificity. Chemical Reviews 2002, 102 (12), 4501–4524. https://doi.org/10.1021/cr000033x.
3. del Zoppo, G. J. Plasminogen Activators in Ischemic Stroke: Introduction. Stroke 2010, 41 (10, Supplement 1), S39–S41. https://doi.org/10.1161/strokeaha.110.595769.
4. Docagne, F.; Parcq, J.; Lijnen, R.; Ali, C.; Vivien, D. Understanding the Functions of Endogenous and Exogenous Tissue-Type Plasminogen Activator during Stroke. Stroke 2015, 46 (1), 314–320. https://doi.org/10.1161/strokeaha.114.006698.
5. Jilani, T. N.; Siddiqui, A. H. Tissue Plasminogen Activator. Nih.gov. https://www.ncbi.nlm.nih.gov/books/NBK507917/.
6. Mican, J.; Toul, M.; Bednar, D.; Damborsky, J. Structural Biology and Protein Engineering of Thrombolytics. Computational and Structural Biotechnology Journal 2019, 17, 917–938. https://doi.org/10.1016/j.csbj.2019.06.023.