Welcome to our e-Learning talk. Today, we will be discussing the role enzymes play in the lifecycle of the SARS-CoV-2 virus and the subsequent COVID-19 disease.

Enzymes are a category of proteins, which possess the ability to catalyze a biochemical reaction. Enzymes support a variety of biological functions including cellular signaling, breaking down macromolecules, or producing larger molecules. In these ways, enzymes play critical roles in the healthy, normal functioning of the human body.

Viruses use enzymes in a number of ways to ensure their continued survival. In some cases, viruses possess enzymes, which allows for more efficient replication. In other cases, such as in SARS-CoV-2, viruses co-opt enzymes that already exist in the organism they are infecting to support their survival.

In order for viruses to infect a host cell, they first must recognize a protein on the host cell surface. This process is called viral recognition. In the case of SARS-CoV-2, the virus recognizes the enzyme angiotensin-converting enzyme 2 or ACE2 on the surface of lung alveolar cells. Specifically, the S1 spike protein of SARS-CoV-2 recognizes the enzymatic domain of ACE2.
 
ACE2 works to counterbalance the function of a separate enzyme, ACE, by converting the vasoconstrictor angiotensin II into the vasodilator angiotensin. In this role, ACE2 is found throughout the body including the lungs, heart, and olfactory system. While SARS-CoV-2 primarily affects a patient’s lung tissue due to its mode of transmission, the presence of ACE2 in most organs suggests that it is likely capable of infecting other tissues as well. Several reported side effects including the loss of smell (or anosmia) and cardiac injuries, are perhaps due to the presence of ACE2 in both olfactory sensory neurons and the myocardium.

Once a virus recognizes its host cell, it needs to enter the host cell in order to be replicated. The S proteins on the surface of SARS-CoV-2 are class I viral fusion proteins, meaning they require protease cleavage for activation and fusion to the host membrane and virus entry. Coronaviruses can use various host proteases to facilitate this cleavage including furin, trypsin, cathepsins, or serine proteases, among others.
 
Preliminary studies have shown that similar to SARS-Associated Coronavirus, or SARS-CoV, TMPRSS2 is a critical protease in the ability of SARS-CoV-2 to infect target cells for viral spreading in an infected host. The biological function of TMPRSS2 is currently unknown. In addition to TMPRSS2, both cathepsins B and L play smaller roles and have some ability to support protease cleavage and fusion of SARS-CoV-2.

Once the virus attaches to the host cell through the spike protein/ACE2 interaction and the spike protein is cleaved via TMPRSS2 enzymatic activity, the innate surveillance mechanisms start to be activated.

Some groups have suggested inhibiting the function of TMPRSS2 to minimize viral entry since TMPRSS2 inhibitors exist today and the enzyme has not shown any indispensable function.

Pattern recognition receptors, or PRRs, are proteins in cells, which recognize non-native proteins from viruses and other pathogens. PRRs play a critical role in the initiation of a specific immune response known as the inflammasome response. One such PRR, known as NLRP3, is commonly found in macrophages and has been identified as a critical player in the immune response to SARS-CoV.
 
Studies have shown that in SARS-CoV, the viral protein Viroporin 3a triggered the activation of the NLRP3 inflammasome leading to the cleavage of pro-caspase-1 and subsequent activation of caspase-1. Caspase-1 is then able to activate pro-IL-1β, resulting in its secretion and triggering of additional immune responses. The observation of increased IL-1β levels in patients suffering from COVID-19 suggests that this mechanism may play a crucial role in the immune response to SARS-CoV-2 as well. The role of IL-1β will be discussed in more detail in a separate part of our video series.

Researchers and pharmaceutical companies are exploring multiple mechanisms to minimize SARS-CoV-2 from recognizing host ACE2. The first is to introduce recombinant ACE2 which in turn will bind the virus. This will block the spike proteins of the virus from binding to the ACE2 on the cell surface and minimize viral entry.

A second method being explored is the use of anti-ACE2 antibodies or peptides to block the binding site on ACE2 that is recognized by the virus. These molecules would compete with the virus for the opportunity to bind to ACE2 receptors on the cell surface and also minimize viral recognition, and thus, entry.

A final method being suggested is the use of ACE2 siRNA to reduce the presence of ACE2 on the cells most at risk for exposure to the virus. By eliminating the surface receptor for the virus, there would be no way for the virus to recognize the host cell and invade.

While many methods for treatment are being explored, it will likely be some time before the most effective treatment is identified.

As we have shown, enzymes play a critical role in the lifecycle of viruses and the host response. SARS-CoV-2 recognizes host cells by the presence of the enzyme ACE2 on the cellular surface. It then utilizes a host protease, TMPRSS2, in order to fuse with the host cell and proceed to enter the cell for viral replication.

Meanwhile, the host uses enzymes in its response to the virus. In particular, the inflammasome response is critical in initiating immune signaling and uses the protease Caspase-1 in order to mount an effective immune response.

Researchers are exploring a number of different ways in which they can manipulate these enzymes in an effort to effectively treat COVID-19.

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