A protease is an enzyme that catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products. They do this by cleaving the peptide bonds within proteins by hydrolysis, a reaction where water breaks bonds. Proteases are involved in numerous biological pathways, including digestion of ingested proteins, protein catabolism, and cell signaling.
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, the binding site, and residues that catalyse a reaction of that substrate, the catalytic site. Although the active site occupies only ~10–20% of the volume of an enzyme, it is the most important part as it directly catalyzes the chemical reaction. It usually consists of three to four amino acids, while other amino acids within the protein are required to maintain the tertiary structure of the enzymes.
In biology and biochemistry, protease inhibitors, or antiproteases, are molecules that inhibit the function of proteases. Many naturally occurring protease inhibitors are proteins.
Serine proteases are enzymes that cleave peptide bonds in proteins. Serine serves as the nucleophilic amino acid at the (enzyme's) active site. They are found ubiquitously in both eukaryotes and prokaryotes. Serine proteases fall into two broad categories based on their structure: chymotrypsin-like (trypsin-like) or subtilisin-like.
A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes. An acid-base-nucleophile triad is a common motif for generating a nucleophilic residue for covalent catalysis. The residues form a charge-relay network to polarise and activate the nucleophile, which attacks the substrate, forming a covalent intermediate which is then hydrolysed to release the product and regenerate free enzyme. The nucleophile is most commonly a serine or cysteine amino acid, but occasionally threonine or even selenocysteine. The 3D structure of the enzyme brings together the triad residues in a precise orientation, even though they may be far apart in the sequence.
Diisopropyl fluorophosphate (DFP) or Isoflurophate is an oily, colorless liquid with the chemical formula C6H14FO3P. It is used in medicine and as an organophosphorus insecticide. It is stable, but undergoes hydrolysis when subjected to moisture.
The drug aprotinin, is a small protein bovine pancreatic trypsin inhibitor (BPTI), or basic trypsin inhibitor of bovine pancreas, which is an antifibrinolytic molecule that inhibits trypsin and related proteolytic enzymes. Under the trade name Trasylol, aprotinin was used as a medication administered by injection to reduce bleeding during complex surgery, such as heart and liver surgery. Its main effect is the slowing down of fibrinolysis, the process that leads to the breakdown of blood clots. The aim in its use was to decrease the need for blood transfusions during surgery, as well as end-organ damage due to hypotension as a result of marked blood loss. The drug was temporarily withdrawn worldwide in 2007 after studies suggested that its use increased the risk of complications or death; this was confirmed by follow-up studies. Trasylol sales were suspended in May 2008, except for very restricted research use. In February 2012 the European Medicines Agency (EMA) scientific committee reverted its previous standpoint regarding aprotinin, and has recommended that the suspension be lifted. Nordic became distributor of aprotinin in 2012.
AEBSF or 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride is a water-soluble, irreversible serine protease inhibitor with a molecular weight of 239.5 Da. It inhibits proteases like chymotrypsin, kallikrein, plasmin, thrombin, and trypsin. The specificity is similar to the inhibitor PMSF, nevertheless AEBSF is more stable at low pH values. Typical usage is 0.1 - 1.0 mM. AEBSF was first reported for use in biochemistry in 1993, and came into common use for the inhibition serine proteases and of non-protease enzymes such as acetylhydrolases in the mid 1990s.
Activity-based proteomics, or activity-based protein profiling (ABPP) is a functional proteomic technology that uses chemical probes that react with mechanistically related classes of enzymes.
Serine hydrolases are one of the largest known enzyme classes comprising approximately ~200 enzymes or 1% of the genes in the human proteome. A defining characteristic of these enzymes is the presence of a particular serine at the active site, which is used for the hydrolysis of substrates. The hydrolysis of the ester or peptide bond proceeds in two steps. First, the acyl part of the substrate is transferred to the serine, making a new ester or amide bond and releasing the other part of the substrate is released. Later, in a slower step, the bond between the serine and the acyl group is hydrolyzed by water or hydroxide ion, regenerating free enzyme. Unlike other, non-catalytic, serines, the reactive serine of these hydrolases is typically activated by a proton relay involving a catalytic triad consisting of the serine, an acidic residue and a basic residue, although variations on this mechanism exist.
In molecular biology, Proteinase K is a broad-spectrum serine protease. The enzyme was discovered in 1974 in extracts of the fungus Parengyodontium album. Proteinase K is able to digest hair (keratin), hence, the name "Proteinase K". The predominant site of cleavage is the peptide bond adjacent to the carboxyl group of aliphatic and aromatic amino acids with blocked alpha amino groups. It is commonly used for its broad specificity. This enzyme belongs to Peptidase family S8 (subtilisin). The molecular weight of Proteinase K is 28,900 daltons.
Palmitoyl protein hydrolase/thioesterases is an enzyme (EC 3.1.2.22) that removes thioester-linked fatty acyl groups such as palmitate from modified cysteine residues in proteins or peptides during lysosomal degradation. It catalyzes the reaction
Carboxylesterase, type B is a family of evolutionarily related proteins that belongs to the superfamily of proteins with the Alpha/beta hydrolase fold.
The alpha/beta hydrolase superfamily is a superfamily of hydrolytic enzymes of widely differing phylogenetic origin and catalytic function that share a common fold. The core of each enzyme is an alpha/beta-sheet, containing 8 beta strands connected by 6 alpha helices. The enzymes are believed to have diverged from a common ancestor, retaining little obvious sequence similarity, but preserving the arrangement of the catalytic residues. All have a catalytic triad, the elements of which are borne on loops, which are the best-conserved structural features of the fold.
Liver carboxylesterase 1 also known as carboxylesterase 1 is an enzyme that in humans is encoded by the CES1 gene. The protein is also historically known as serine esterase 1 (SES1), monocyte esterase and cholesterol ester hydrolase (CEH). Three transcript variants encoding three different isoforms have been found for this gene. The various protein products from isoform a, b and c range in size from 568, 567 and 566 amino acids long, respectively.
Kallikrein-6 is a protein that in humans is encoded by the KLK6 gene. Kallikrein-6 is also referred to as neurosin, protease M, hK6, or zyme. It is a 223 amino acid sequence, derived from its 244 original form, which contains a 16 residue presignal and 5 residue activation peptide.
Palmitoyl-protein thioesterase 1 (PPT-1), also known as palmitoyl-protein hydrolase 1, is an enzyme that in humans is encoded by the PPT1 gene.
The Kazal domain is an evolutionary conserved protein domain usually indicative of serine protease inhibitors. However, kazal-like domains are also seen in the extracellular part of agrins, which are not known to be protease inhibitors.
A protein superfamily is the largest grouping (clade) of proteins for which common ancestry can be inferred. Usually this common ancestry is inferred from structural alignment and mechanistic similarity, even if no sequence similarity is evident. Sequence homology can then be deduced even if not apparent. Superfamilies typically contain several protein families which show sequence similarity within each family. The term protein clan is commonly used for protease and glycosyl hydrolases superfamilies based on the MEROPS and CAZy classification systems.
Acyl-protein thioesterases are enzymes that cleave off lipid modifications on proteins, located on the sulfur atom of cysteine residues linked via a thioester bond. Acyl-protein thioesterases are part of the α/β hydrolase superfamily of proteins and have a conserved catalytic triad. For that reason, acyl-protein thioesterases are also able to hydrolyze oxygen-linked ester bonds.
