The 3C-like protease (3CL^pro) or main protease (M^pro), formally known as C30 endopeptidase or 3-chymotrypsin-like protease, is the main

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...

found in

coronavirus Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the co ...

es. It cleaves the coronavirus polyprotein at eleven conserved sites. It is a

cysteine protease Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad. Discovered by Gopal ...

and a member of the PA clan of proteases. It has a cysteine-histidine catalytic dyad at its

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate ( binding site) ...

and cleaves a

Gln Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral, ...

–( Ser/ Ala/ Gly)

peptide bond In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another, along a peptide or protein cha ...

. The

Enzyme Commission The International Union of Biochemistry and Molecular Biology (IUBMB) is an international non-governmental organisation concerned with biochemistry and molecular biology. Formed in 1955 as the International Union of Biochemistry (IUB), the union ...

refers to this family as SARS coronavirus main proteinase (M^pro; ). The 3CL protease corresponds to coronavirus nonstructural protein 5 (nsp5). The "3C" in the common name refers to the 3C protease (3C^pro) which is a homologous protease found in

picornavirus Picornaviruses are a group of related nonenveloped RNA viruses which infect vertebrates including fish, mammals, and birds. They are viruses that represent a large family of small, positive-sense, single-stranded RNA viruses with a 30 nm ...

es.

Function

The 3C-like protease is able to catalytically cleave a peptide bond between a

glutamine Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral ...

at position P1 and a small

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

(

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − for ...

alanine Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side ...

, or

glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...

) at position P1'. The SARS coronavirus 3CLpro can for instance self-cleave the following

peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

s: The protease is important in the processing of the coronavirus replicase polyprotein (). It is the main protease in coronaviruses and corresponds to nonstructural protein 5 (nsp5). It cleaves the coronavirus polyprotein at 11 conserved sites. The 3CL protease has a cysteine-histidine catalytic dyad at its active site. The

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

of the

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, some ...

acts as a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they ar ...

and the imidazole ring of the

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the ...

as a general base.

Nomenclature

Alternative names provided by the EC include ''3CLpro'', ''3C-like protease'', ''coronavirus 3C-like protease'', ''Mpro'', ''SARS 3C-like protease'', ''SARS coronavirus 3CL protease'', ''SARS coronavirus main peptidase'', ''SARS coronavirus main protease'', ''SARS-CoV 3CLpro enzyme'', ''SARS-CoV main protease'', ''SARS-CoV Mpro'' and ''severe acute respiratory syndrome coronavirus main protease''.

As a treatment target

The protease 3CLpro is a potential

drug target A biological target is anything within a living organism to which some other entity (like an endogenous ligand or a drug) is directed and/or binds, resulting in a change in its behavior or function. Examples of common classes of biological targets ...

for coronavirus infections due to its essential role in processing the polyproteins that are translated from the

viral RNA An RNA virus is a virusother than a retrovirusthat has ribonucleic acid (RNA) as its genetic material. The nucleic acid is usually single-stranded RNA ( ssRNA) but it may be double-stranded (dsRNA). Notable human diseases caused by RNA viruses ...

. The

X-ray structure X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

s of the unliganded SARS-CoV-2 protease 3CLpro and its complex with an α-ketoamide inhibitor provides a basis for design of α-ketoamide inhibitors for a treatment of

SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19 (coronavirus disease 2019), the respiratory illness responsible for the ongoing COVID-19 pandemic. The virus previously had a ...

infection. A number of

protease inhibitors Protease inhibitors (PIs) are medications that act by interfering with enzymes that cleave proteins. Some of the most well known are antiviral drugs widely used to treat HIV/AIDS and hepatitis C. These protease inhibitors prevent viral replicat ...

being developed targeting 3CLpro and homologous 3Cpro, including CLpro-1, GC376, rupintrivir, lufotrelvir, PF-07321332, and AG7404. The

intravenous Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutrie ...

administered

prodrug A prodrug is a medication or compound that, after intake, is metabolized (i.e., converted within the body) into a pharmacologically active drug. Instead of administering a drug directly, a corresponding prodrug can be used to improve how the dru ...

PF-07304814 (lufotrelvir) entered

clinical trial Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...

s in September 2020. The orally-active follow-up drug PF-07321332 is in phase II/III clinical trials as a

combination drug A combination drug or a fixed-dose combination (FDC) is a medicine that includes two or more active ingredients combined in a single dosage form. Terms like "combination drug" or "combination drug product" can be common shorthand for a FDC prod ...

with

ritonavir Ritonavir, sold under the brand name Norvir, is an antiretroviral drug used along with other medications to treat HIV/AIDS. This combination treatment is known as highly active antiretroviral therapy (HAART). Ritonavir is a protease inhibitor ...

, and in November 2021, results were announced including 89% reduction in hospitalizations when given within three days after

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019. The disease quick ...

symptom onset. An ultralarge virtual screening campaign of 235 million molecules was able to identify a novel broad-spectrum inhibitor targeting the main protease of several coronaviruses. On 25 May 2022,

Insilico Medicine Insilico Medicine is a biotechnology company based in Pak Shek Kok, Hong Kong in Hong Kong Science Park near the Chinese University of Hong Kong and in New York, at The Cure by Deerfield. The company combines genomics, big data analysis, and de ...

explored use of 3CL protease inhibitor for COVID-19 treatment. The preclinical candidate has a specialized structure designed by

Artificial Intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech ...

to target 3C-like proteases. It acts as an inhibitor and binds to 3C-like proteases of SARS-COV-2 and MERS viruses, disabling them and inhibiting viral replication. The preclinical candidate is used for treating people who are already infected by preventing the coronavirus from replicating. Doi

Other 3C(-like) proteases

3C-like proteases (3C(L)pro) are widely found in (+)ssRNA viruses. All of them are cysteine proteases with a

chymotrypsin Chymotrypsin (, chymotrypsins A and B, alpha-chymar ophth, avazyme, chymar, chymotest, enzeon, quimar, quimotrase, alpha-chymar, alpha-chymotrypsin A, alpha-chymotrypsin) is a digestive enzyme component of pancreatic juice acting in the duod ...

-like fold (PA clan), using a catalytic dyad or

triad Triad or triade may refer to: * a group of three Businesses and organisations * Triad (American fraternities), certain historic groupings of seminal college fraternities in North America * Triad (organized crime), a Chinese transnational orga ...

. They share some general similarities on

substrate specificity Chemical specificity is the ability of binding site of a macromolecule (such as a protein) to bind specific ligands. The fewer ligands a protein can bind, the greater its specificity. Specificity describes the strength of binding between a giv ...

and inhibitor effectiveness. They are divided into subfamilies by sequence similarity, corresponding to the family of viruses they are found in: * This entry is the coronavirus 3CL^pro. * ''

Picornaviridae Picornaviruses are a group of related nonenveloped RNA viruses which infect vertebrates including fish, mammals, and birds. They are viruses that represent a large family of small, positive-sense, single-stranded RNA viruses with a 30 ...

'' have a picornavirus 3C^pro (; ; MEROPS C03). This is the earliest-studied family. Examples include the ones found in

poliovirus A poliovirus, the causative agent of polio (also known as poliomyelitis), is a serotype of the species '' Enterovirus C'', in the family of '' Picornaviridae''. There are three poliovirus serotypes: types 1, 2, and 3. Poliovirus is composed of ...

and in rhinovirus (both are members of genus ''

Enterovirus ''Enterovirus'' is a genus of positive-sense single-stranded RNA viruses associated with several human and mammalian diseases. Enteroviruses are named by their transmission-route through the intestine ('enteric' meaning intestinal). Serologic ...

''). * ''

Caliciviridae The ''Caliciviridae'' are a family of "small round structured" viruses, members of Class IV of the Baltimore scheme. Caliciviridae bear resemblance to enlarged picornavirus and was formerly a separate genus within the picornaviridae. They are p ...

'' have a 3CL^pro (; MEROPS C37). Examples include the one found in Norwalk virus. Additional members are known from ''

Potyviridae ''Potyviridae'' is a family of positive-strand RNA viruses that encompasses more than 30% of known plant viruses, many of which are of great agricultural significance. The family has 12 genera and 235 species, three of which are unassigned to ...

'' and non-''Coronaviridae'' ''

Nidovirales ''Nidovirales'' is an order of enveloped, positive-strand RNA viruses which infect vertebrates and invertebrates. Host organisms include mammals, birds, reptiles, amphibians, fish, arthropods, molluscs, and helminths. The order includes the ...

''.

References

External links

* * Peptidase C30/C16 in coronavirus, . The MEROPS C16 one is the "papain-like" PL-PRO. {{Portal bar, Biology, border=no EC 3.4.22 Coronavirus proteins Viral nonstructural proteins